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The Influence of Moonlight on the Activity of Certain Nocturnal Insects, Particularly of theFamily Noctuidae, as Indicated by a Light TrapAuthor(s): C. B. WilliamsSource: Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, Vol. 226, No. 537 (Oct. 19, 1936), pp. 357-389Published by: The Royal SocietyStable URL: http://www.jstor.org/stable/92272 .Accessed: 26/10/2013 23:13

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IX-The Influence f Moonlight n the Activity f Certain

Nocturnal Insects, Particularly of the Family Noctuidae,as Indicated by a Light Trap

By C. B. WILLIAMS, Sc.D.,

Chief ntomologist,othamstedxperimentaltation, arpenden, ngland

(Communicated y SirJOHN usSELL.,F.R.S.-Received 24 January, Revised 16 March, Read

21 May, 1936)

CONTENTSPAGE

INTRODUCTION . . . . . . . .. . ... . ... .. ... .. .. ... .. .. 358

THE ROTHAMSTEDEXPERIMENTSThe Light Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . 358The Measurement f Night Cloud and Moonlight . . ... . . . . . . . . . . 359Note on the Apparent Movements f the Moon . . . ... . . . . . . . . . . 360Note on the Use of Logarithms n Analysis . . . . . . . . . . . . . . . . . 361

ANALYSISOF RESULTS IN THE FAMILY NOCTUIDAE (LEPIDOPTERA)Numbers, pecies and Time of Flight f Noctuidae . . . . . . . . . . . . . 362Comparison f Full Moon and No Moon Weeks . . . . . . . . . . . . . . . 363Varying ffect ue to the Height of the Full MoonI. . . . . . . . . . . . . 366Day-to-DayDepartures f Logarithms rom he 29-Day Mean . . . . . . . . . 367Discussion f the Asymmetry f the Lunar Effect . . . . . . . . . . . . . . 370Effect f the Phase of the Moon on Time of Flight f the Noctuidae . . . . . .374Interrelation f the nfluence f Cloud and Moonlight . . . . . . . . . . . . 375Correction f the 29-Day Mean Curve for Lunar Influence . . . . . . . . . . 379

LUNAR INFLUENCE ON OTHERGROUPS OF INSECTSComparison f Full Moon and No Moon Weeks n Various Groups . . . . . . . 381Effect n all Insects t Different eriods f the Night . . . . . . . . . . . .385Separate Effect f Moonlight nd Cloud on all Insects . . . . . . . . . . . . 386Effect f Moonlight nd Cloud on the Sub-family ipulinae (Diptera) . . . . . 386

GENERAL DISCUSSION . . . . ..... ......... ......... . 387

SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . .3 . . . . 388REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389

VOL. cCXXVI.-B 537 (Price s.) 3 D [Published9October,936.

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358 C. B. WILLIAMS

INTRODUCTION

For many years amateur entomologists ave considered hat on nights of full

moon t s of ittle se going ut to catch specimens, s insectswill be few n number.This belief pplies to all methods f collecting, ncluding ait (sugaring) nd light,and is supposed o apply particularly o the Lepidoptera.

Scattered through he literature n Agricultural ntomology ne finds occa-sionally eferenceso the use of ight raps or he destruction f pests, nd statements,usuallyfrom he tropics, hat the catches were ess at times of full moon; but sofar s I am aware no proper tatistical tudy f the questionhas ever been made.

One of the most triking eries f figures s that produced by PAGDEN (1932) bytrapping with a light trap the two Pyralid moths Diatraea uricilia nd Schoenobius

incertellus hich are pests of rice in Malaya. He found, between 18January nd29 June, 1931, six periods of maximum atch in both sexes of both speciescorre-spondingmore or less to the no moon periods, nd six periods of minimum atchcorresponding ven more definitely o the full moons. Scarcely any insects werecaptured t the time of full moon.

It has been known or many years hat ome animalshave periodicities f activitycorresponding o lunar months. Information bout these has been dealt with nrecent ears hiefly y Fox (1923). The animalsconcerned re mostlymarine, ndit s probable that n somecases the effect s produced hrough hetides; but DORR

(1932) has suggested hat there s a lunar influence n the dates of movements fmigrant irds. Specific eference o lunar nfluence n insects, ther han capturesby meansof traps, re rare. HORA1927) has suggested hat omespeciesof May-Flies Ephemeridae) end to emerge nd make their nuptial flights t definite hasesof the moon. The evidence hat he brings orward s, however, ooslight o be inany way conclusive, nd a series of regular observations hould be carried out insomesuitable ocality o seeif the theory s supported.

THE ROTHAMSTED EXPERIMENTS

TheLightTrapIn March, 1933, a light trap was started n one of the fields t Rothamsted

Experimental tation, bout twenty-five iles north f London, and it has been inpractically ontinuous se for nearly three years. The trap was similar to thatfirst esigned n Egypt WILLIAMS, 1924) except that an electric ight of about 300candle-powerwas used instead of acetylene, nd an arrangement as added whichdivided the catch nto eight qual periods during he night, o that t is possible o

estimate he time of entry f the nsects nto the trap and hence the time of activityof any group or species. A full description will be found n a more recent paper( WILLIAMS,1935)


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360 C. B. WILLIAMS

was 20F. or less, then the night was always cloudy. This relation was thereforeused retrospectively or1933.

(5) The light f the moon was measured directly uring most of 1934 and 1935by a photographic nstrument peciallydesigned or he purpose. The instrumenthas been recently ully escribed WILLIAMSand EMERY, 1935)and it is unnecessaryto go into detail here. The principle s that a line image of the moon, producedby a cylindrical ens, s focussed n to a fixed trip f photographic aper. As themoon moves hrough he sky the ens follows ts direction y means of a clockworkmechanism nd the ight from he moon thus forms band on the sensitive trip.When the moonlight s bright he strip s darkened nd when there s no moon thestrip s unaffected.

A comparison f the results f the star recorder nd the moonlight ecorder howsthat the former s much more sensitive o cloud. The clouds that obscure thepole-star till llow a considerable mount of ight o pass from he moon. In fact,measurable ight from the moon penetrates ll but the thickest loud. This issupported by the results below, which show that the lunar influence n certaininsects s detectable ven on cloudynights.

To summarize-the fivemethods sed for btaining measure f the night loudwere (1) personal observation, 2) Rothamsted tar camera (1934 and 1935), (3)Greenwich tar amera, 4) difference etween ir and grass minimum emperature,and (5) photographicmoonlight ecorder 1934and 1935).

Note ntheApparent ovementsf theMoonThe moon changes from ts highest osition n the sky at southing o its lowest

and back again during lunar month. In the summer t is low at full moon andhigh t no moon, while n the winter t is high at full moon and low at no moon.This results n the light of the full moon being much brighter n winter han insummer. The greatest ngular height f the moon at southing bove the horizonat London (latitude pprox. 51? N.) is 66? and the owest s 12?.

The moon ouths n an average 49 minutes ater ach night, ut within he course

of a single unar month he difference n time may vary from 1 to 66 minutes n asequencewhich has two maxima and two minima.The time of rising r of setting f the moon s later on an average each successive

day by the ameperiod 49 minutes) s that of the moonsouthing, ut the variationis much greater nd the extremes re approximately rom 2 minutes o I hour 33minutes. In each lunar month here s a cycle changing rom ong differences oshort differences nd then back again. In December the short differences ccurwhen he moon s rising r setting bout midday nd the ong differences hen t isrising r setting uring he night. In June the reverse ccurs seefig. 3). When

the short differences ccur during henight on one side of midnight with longerdifferences n the other ide, here san asymmetry f he unar nfluence n successivenights which s discussed more fully ater.


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ACTIVITY OF NOCTURNAL INSECTS 361

Any unar effect n the activity f nsects uring he night would be expected todepend n: (A) theduration f hemoonlight; nd (B) the ntensity f hemoonlight.The latter will depend on (1) the phase of the moon and (2) the angle of the moonabove the horizon, nd the atter gain willdepend on (a) the maximum height fthe moon above the horizon at southing) or he night nd (b) the particular hourbetween outhing nd moon-rise r moon-set.

Note n the Use ofLogarithmsn AnalysisIn,the tatistical reatment f number fvaluesfor aptures f nsects uchas that

which follows, t is frequently ecessary o combine ogether he captures n a seriesof dayswhich have somecondition n common, nd to compare he total or averagewith hat of a second eries f days with different onditions.

The catches,however, ary very considerably rom ay to day, and if the actualnumbers re added together nd the arithmetical mean used, there s danger of theresults eing swamped by one or two abnormally igh catches. For example, nthe full moonweekof October, 1933, the captures f Noctuidae on the sevennightswere 0 : 0: 1 : 62 : 0 : 0: 0; while in the corresponding o moon week thecaptures were 2 : 4 : 0 : 0 : 10 : 3 : 3. The higher total for the full moon weekis obviously nduly weighted y the single arge catch.

Further, hen the departures f series f values for atches from he arithmeticalmean are studied, t is found hat they onsist f a large number f small negativedepartures nd a few large positive departures. This gives a skew distributionwhich does not lend itself o statistical nvestigation y the normal formulae fstandard eviation, tc.

If the ogarithm f each value is taken nd these ummed nd averaged, measureis obtained f the geometricmean of the values. When this s done it is found hatthe swamping ffect f the single arge values is much reduced and if the logvalues are expressed s departures rom a mean, the distribution f departuresgives much morenormal urve.

This shows hat changes n numbers re equivalent t different evels f they re

in similar eometric nd not arithmetic roportion. For example, that the changefrom 100 to 150 insects s equivalent to that from 1000 to 1500 and not to thechangefrom 000 to 1050.

This result was to be expectedfrom priori easoning, s it is most probable thatvariations n climatic onditions ouldproduce imilar roportional hanges n differ-ent populations, ut t was necessary o confirm t by actual results efore doptingthe method f ogarithms or general nalysis.

A complication nsues f any of the values to be dealt with re zero, as the og ofzero is minus nfinity nd the geometricmean of any series nvolving ero is itself

zero. To overcome this t has been found possible to add onieunit to all thevalues before onverting nto ogs, nd then o subtract heunit ater when the ogis reconverted o an anti-log.


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362 C. B. WILLIAMS

The use of logarithms n the present eries of figures mphasizes ll differencesthat re consistent nd reduces ll that are not so. Thus in the two weeks' figuresfor he Noctuidae given above the sums of the numbers re 63 : 22, but sum of og(n + 1) are 2*10: 3*89.

The method has been used in the interpretation f figures howing he time offlight f nsects t night WILLIAMS, 1935), and always givesmore consistent esultsthan does the use of the actual numbers hemselves. This point s dealt with morefully n a separate publication Williams, 936, b.)

In the present tudy, therefore, he figures sed for comparisons re (unlessotherwise tated) og n + 1) for single alue or E log n + 1) for hevalue of series.

When the series to be examined s spread over a considerable ength of time,during which the total population s likely to have changed, a running mean ismade of the og (n + 1) and each night xpressed s a departure rom hat mean.The running mean here used is a 29-day mean, equal to the length of the lunarperiod, o as to eliminate ariations f a longer period than the moon, but to leavethe unar period unaffected.

ANALYSIS OF THE RESULTS IN THE FAMILY NOCTUIDAE (LEPIDOPTERA)

In the past, most upposed effects f moonlight n insects have been reported nthe Lepidoptera, o that the first nvestigation as made on this group. In order,however, hat any results hould be reliable statistically, t was necessary o obtainfigures hat would cover a number of lunar periods. No single species of Lepi-doptera whichwas common n the trap complied with hisdemand, most asting oronly one or two lunar periods or less. It was therefore ecided to study irst hepossible ffect n the total numbers f all speciesof the family Noctuidae s.l).

If all species so combined are affected imilarly y the moon then the effectshould be noticeable n the combined results. If some species are affected ndothers not, the combined results would then show a diluted effect. The onlydifficulty n interpretation ould arise f no effect as found n the group; as thismight mean that there was no effect n any species or alternatively hat omewere

affected ositively nd an equal number negatively, he two cancelling ut eachother n the combined otal. This, however, id not occur.There is no doubt that the most nteresting esults will be finally btained by

investigating ingle pecies r even each sex of species eparately, ut for his urposeeithermanyyears' onsecutive ork will be necessary ith raps n different ocalitiesor else some ong-or many-brooded peciesmust be chosen, robably n the tropics,such as the Rice borers nvestigated n Malaya by PAGDEN.

Numbers,pecies,nd Time fFlight f Noctuidae

In the course of the three years (1933, 1934, and 1935), 8712 Noctuidae werecaptured n the trap during he six summer unar months iscussed. The numbersin each year were 1582 n 1933, 1869 n 1934, and 5261 inl1935.


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About 110 specieswere represented n the captures, nd Table I showsthe 25species of which more than 50 specimens were captured, together with the totalnumbers of these n the three years. All these species occurred n each of thethree years. The nomenclature s, for convenience, hat used in South's " Mothsof the British sles .

These species account for 7838 individuals ut of the 8712 captured, eaving874 individuals istributed ver about 85 other pecies.

TABLE I

More Abundant Species and Numbers of Noctuidae Captured in the Trap inthe Three Years 1933-1935

Agrotis xclamationis . . . . . . . . 2074 Mianafasciuncula . . . . . . . . . 148Amathesychnidis . . . . . . . . . 781 Cerigomatura . . . . . . . . . . . 146Xylophasia onoglypha . . . . . . . 723 Taeniocampaothica . . . . . . . . 137Luperina estacea . . . 493 Rusina enebrosa . . . . . . . . . 124Noctua anthographa . . . . . . . . 483 Leucania onigera . . . . . . . . . 112Noctua -nigrum . . . . . . . . . . 469 Miana strigilis . . . . . . . . . . 102Anchocelisunosa . . . . . . . . . . 419 Leucaniaomma . . . . . . . . . 85Apameaecalis . . . . . . . . . . . 289 iNoctua rimulae . . . . . . . . . . 83Epineuroniaopularis . . . . . . . . 275 Grammesiarigrammica . . . . . . . 73Noctua ubi . . . . . . . . . . . . 227 Agrotis uta . . . . . . . . . . . . 55Leucania allens . . . . . . . . . . 206 Hydroecia icacea . . . . . . . . . 51Leucaniampura . . . . . . . . . . 184 Triphaena ronuba . . . . . . . . . 50Mianabicoloria . . . . . . . . . . 164

The Noctuidae in general are rather ate flyers. In the three years, he distri-,bution n the eight qual periods f the night, ourbefore nd four fter midnight,was as follows: 599: 826: 1013: 1454: 1759: 1429: 955: 309.

This givesa total of 3892 (470%) beforemidnight nd 4452 (530 ) after midnightwith hemaximum lightust after midnight. These figures re shown graphicallyin the vertical olumns n fig. together ith imilar nes prepared by a summationof the ogarithms.

Table II shows the Noctuidae captured each night n six lunar periods fromfull moon to full moon) in 1932and summaries f the captures n 1934and 1935,approximately rom he beginning f May to the end of October in each year,when the Noctuidaeweremostnumerous. This gives hepossibility f nvestigatingeighteen unar periods.

ComparisonfFull Moon ndNo Moon WeeksSince if any lunar effect was present t would be expected to be most obvious

near the two extremes f full nd no moon, a simplecomparison an first e madeby adding in each lunar month he captures for the first our nd last three days

(the " full moon week), nd comparing hemwith he figures or he middle evendays (the "'no moon " week). Table III showsthe results or the eighteen unarmonths, he larger figure f each pair being. n heavy type. The table shows first


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364 C. B. WILLIAMS

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the figures btained by adding the actual captures nd second those by addingthe ogarithms f the numbers.

It will be seen that for he actual numbers, n seventeen ut of the eighteen unarmonths he captures f the Noctuidae n the " no moon week were above thoseof the " full moon" week. The only exception s interesting s the 63 iinsectscaptured n that fullmoonweek October, 1933) wasmade up of no insects n eachof five nights, ne on one night, nd 62 insects on the remaining night. Thisexceptionalnight 6 October) gave the highest otal rapture of Noctuidae of anynight n the first woyears and actually was a night f heavy cloudand thereforeunlikely o be influenced y the moon. It will therefore e seen that the onlyexception o the rule of higher atches t no moon was statistically nd meteorologi-callyabnormal.

The second set of figures n Table III shows he same comparison ased on the

TABLE III

Comparison f the Captures of Noctuidae n the Full Moon and No Moon Weeksin the Six Lunar Months of the Summers f 1933-1935. (Maximum of eachpair in heavy type.) Also SimilarValues for. he Minimum Night Tempera-tures

Years All threeSum of numbers May June July Aug. Sept. Oct. Total years

1933-Week of full moon 15 55 58 64 29 63 284No moon 19 95 73 72 76 22 357

1934-Full moon 2 52 37 69 61 1 222 1242No moon 25 56 140 76 204 71 575 2853

1935-Full moon 2 15 477 168 64 10 736No moon 23 179 917 385 267 133 1904

Sum of ogarithms n + 1)1933-Weekoffullmoon 2.76 7.51 6*19 6*08 4*25 2.10 29-14

No moon 3 59 7.36 7 15 7 14 7 34 3 89 36 46

1934-Full moon 060 4.20 5.31 6.63 5.77 0K30 22-81 83*41No moon 3 64 5 86 9.01 7 11 10 20 6 95 42 77 133 20

1935-Full moon 060 2.75 10*87 8*99 6*32 1.93 31*46No moon 3 49 7 93 13-86 11[70 9 92 7 07 53 97

Mean minimum emperature1933-Week of full moon 48-3 53 0 58 7 56*6 52*3 44'1 52*2

Nomoon 45.9 49 5 56'0 51.0 504 42.1 49.2

1934-Full moon 41.1 48.3 51.4 51.7 48 0 48.0 48.1 49.1No moon 41-6 49 0 53 7 54 3 51.4 46.6 49.4 48.7

1935-Full moon 34.2 48.3 55*6 50*5 49*6 43.9 47 0No moon 45*45A 8 55*8 47.3 47 8 43A17I 5

VOL. CCXXVI.--B 3 E


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366 C. B. WILLIAMS

sum of the ogarithm f the catches ach night. It will be seen that once more 17out of the 18 comparisons ive values n favour f no moon, and only one (that ofJune, 1933)slightly n favour f full moon. The comparison f the weeks n thelunar month n October, 1933, which gave a number value strongly n favour ffull moonnow gives logvalue quite definitely n favour f full moon.

This ndicates hat hevalues n favour fno moon re due to consistent ifferencesand not to occasional arge and possibly ccidental divergences.

The mean difference er week for he 18 weeksfor henumbers s 87 6 i: 27 3which givesa " t " test of significance f 3 2. On the ogarithmic asis the meandifference s 2 77 ? 0 44 ; which gives " t " equal to 6v3, a value considerablymore ignificant han that obtained from henumbers.

Before ssuming hat this apparent effect s due to the moon it is necessary oconsider hepossibility f other factors. It has been found n other nvestigations,to be published ater, that the most mportant on-periodic inglefactor n deter-mining he catch is the minimum emperature f the night. The last division nTable III showsthe mean minimum emperatures or he same periods dealt within the amemanner s used for he ummation f heNoctuidae. From this mergesthe curiousfact that n 1933all of the six weeksof " no moon " were cooler thanthe corresponding eek of" full moon"; while n 1934the reverse ccurred ndfive successive no moon weeks were warmer than the corresponding fullmoon weeks. In 1935the temperatures ere more regularly istributed.

The two sets of figures aken n conjunction how that n 1933 the differencesin catch n favour f " no moon " were obtained n spite of an adverse emperatureeffect hich lone would have tended to produce a preponderance n the oppositedirection. The differences n 1934,are, on the contrary, n the same direction sthose that might be expectedfrom temperature ffect, ith the exception f thelast period which, curiously nough, gives the most striking xample of a largedifference n favour f no moon.

A combination f all the eighteen eriodsgivesa very light emperature ffectin favour of the full moon (49.10 F. ; 48.70 F.) with a very definite apture nfavour of the moonlessnights 1242: 2853 in total numbers nd 83 4: 133 2 inlogs).This preliminary nvestigation, eglecting he possible nfluence f cloud, whichwe have no reason to suppose s more prevalent t any particular phase of themoon, gives very definite upport o the dea that real lunar effect s present.

Varying ffect ue to theHeight f the ull MoonIt has been mentioned bove that hefull moons re high n the sky n December

and low in June. It would therefore e expected that the full moon would have

a greater ffect n winter han n summer.Unfortunately, ractically o Noctuidae are captured n the winter months, utthe Table IV shows verage for he three years of the difference etween he ogs


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of the captures at full moon and no moon weeks for each month from May toOctober.

TABLE IV

Variation n Difference f Lunar Effect n Different ummer Months Due toHeight of Full Moon Above the HorizonMay June July August Sept. Oct.

Difference n week's totals 2-25 2 23 2-45 1*43 3*71 4.53Mean difference er night 0 31 0 30 0-35 0.20 0 53 0 65

This sshowndiagrammatically nfig. , nd it will be seen that, with he xceptionof the month of August, he figures it ery loselyto the expected results f low difference n June, - -0 6

gradually increasing towards the autumn. The low _ -0-5value for August s undoubtedly ue to the factthat in two of the three years the full moons were - - OAwarmer han the no moon periods n this month. - - o03

A daily difference f 0'31 in the logs as in May l l - 02and June s equivalent o a doubling f the catch atno moon over full moon, while the daily difference - - 0of 0-65 in October is equivalent o an increase f M J JA pover four imes. May uie uly ug ept ct

FIG. 1-Diagram showing hediffer-ence in lunar effiect n theDay-to-DayDepartures f Logarithms rom he Nce inunreffe onthe

29-DayMean according o the height f the fullIn order oextend he analysis o include ll the moon.

daysof the unar period, he value for ach night fthe eighteen unar months has been expressed s a departure of log (n + 1) fromthe twenty-nine-day unning mean. This, as has been explained, reduces theswamping effect f high individual catches and eliminates, t least partially, heeffect f the rise and fall of the mean catch during heyear.

Taking for granted the calculations,which are unnecessary o reproduce here,the figures n Table II now become s shown n Table V. Thosefor 1933are givenfully, ut only the averagefigures or1934and 1935,and there s added a day-to-day mean departure or he three years together nd finally he same smoothed oa five-day unning mean. The results re shown diagrammatically n fig. 2 andto this has been added (dotted ine) the mean minimum emperature alculatedby the same method or omparison.

It will be seen that n each of the years there were arge negative departures ncaptures bout the time of full moon. In 1933 the positive epartures re chiefly

betweenno moon and the first uarter, while n 1934they re from he ast quarterto the first uarter i.e., the dark half of the cycle). In 1935 the highest ositivedepartures re a few days after o moon. When the figures or he three years re

3 E 2


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368 C. B. WILLIAMS

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Cd ~~~~~~~~~~~00

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o bO O b : > tzO C 0 C 0000C'


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Noctuidae _ _-_.[1933 Ii ~+0-20

520t 1i'.. I IIh.1 +0.10500 f[*x?- 0

48? *4rl t ti-l '1 ; 1 | |-0.10

1934

500 ~~~~~~~~ ~ ~~~ ~~~~~~~~~~~.........~480 - . 010

500 1935 +0?10

480 . 0

460 ..-. . -0j1v

440 - | * | Average departure of- 2log from 9-day mean

......'''Average minimumtemperature

1933+1934 1935 +6-20I~~~~~~~~,1 -~~~~+01500 .| 1 t.i l i, .. .l. .11 l. +490 0p; i ~ _ _ _ _ _ _ _0

48? i933+1934+193,5 5daymeanls -0-10

-Fulmon Lastqarter No;moonL Firstquarter Full moon 1o1 3 5 7 9 11 13 15 17 19 21 23 25 27 29

Daysfrom ullMoon

FIG. 2-Diagram showing average departures from he 29-day mean catch of Noctuidae for uccessivedays of the lunar month for six lunar months in the summers of 1933, 1934, and 1935, also (asdotted line) the mean departures of the minimum night temperatures.


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370 C. B. WILLIAMS

combined and smoothed a fairly regular curve is produced giving the lowestcaptures t full moon the change over from egative o positive ust after he astquarter; then steadily ncreasing ositivedepartures ill about half-way etweenno moon and the first uarter; and finally rapid fall and the change-over rompositive o negative etween he first uarter nd full moon.

The maximum ositive eparture n the five-day moothed urve s 0 16 and themaximum egative departures 0*28, giving total difference f 0 44 which sequivalent o a catch at no moon of approximately 75 when the full moon catchis considered s 100.

DiscussionntheAsymmetryf the unar ffectThe final urve n fig. is asymmetrical n two ways (a) the negative eparture

at the full moon period is shorter 11 days) and more extreme t its maximum(- 0.28) than the positive departure t the no moon period which asts for 17days and has a maximum eparture f + 0 16;

(b) the positive departure ontinues high for a number of days after no moon,and the cross-over rom ositive o negative ccurs nly four aysbefore ullmoon;while the reverse hange takes place 7-8 days after ullmoon.

The first symmetry s almost certainly ue to the fact that the intensity f thelight f the moon due to the proportion f ts urface lluminated) nd the durationof the moonlight hrough he night re both decreasing imultaneously s we passfrom full moon towards new moon; and at the first uarter not only has thereflected ight een reduced to about half, but the duration f ts effect uring henight has also been reduced to half on an average). This, witlh he reverse ffectbetween no moon and full moon, would tend to give the curve a flat top and anarrower nd more emphasizedminimum.

The second symmetry ppears to be due to the irregularity f the sequence ofthe hours of setting nd rising f the moon on successive ays in the lunar month,which has already been mentioned riefly.

Figs. 3A, B, and C show diagrammatically he hours of rising nd setting f themoon during a lunar month n May, in July, nd in December-January. The

figures re for heyear 1934-5, but the shapeof the curve s identical for he samedate in all years.The dark horizontal ines n the figure how the hours n successive ays (from

above and downwards)when the moon s below the horizon. Each dark ine endsat moonrise nd starts t moonset. The converging r diverging vertical linesshow the times f the eight uccessive eriods of the night nto which the capturesin the trap are divided, the centre ine being at midnight. The times of sunsetand sunrise re shown s broken ertical ines.

An examination hows hat n May (fig. A), owing o the onger nterval etween

successive isings eforemidnight, he first alfof the night s already dark (i.e., asregards moonlight) hree to four days after full moon and well before he "lastquarter ; but owing o the rapidly hortening ntervals etween uccessive isings


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ACTIVITY OF NOCTURNAL INSECTS 371Time 6pm 7 8 9 10 11Midnighta.m 3 f 5 6Period 1 2 3 4 5 6 7 8

A30

5

10andMay,

15- -- LI ;

193430 Sun-0 UArise

25

on~~~~~~~~n

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20 c in mid-winter.TFIG3b--A11Ciagra shwn th+olso iigadstigo h ona he ifeettmso h


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372 C. B. WILLIAMS

it takes bout ten days (the remainder f the. alf unar month) for he second halfof the night o become dark. A similar oursefollowswith he setting f the moonafter o moon within ourdays of no moon the first alf of the night s light, utten more days are required or hesecond half o become ight.

One might herefore xpect, under these onditions, nsects o be more abundantthan averageat the " last quarter and less abundant at the " first uarter .

InJuly fig. B), on the ontrary, hereverse ffect s found nd it snot until boutten daysafter uillmoonthat hefirst alf f the night s dark, nd not until endaysafter o moon that the first alf of the night s light. One might herefore xpectfewer nsects han normal t the ast quarter nd more at the first uarter.

Similar symmetry ontinues hroughout he utumn, but by mid-winter fig. C)conditions have become practically ymmetrical hroughout he night. In the

TABLE VIMean Log Departures rom he 29-DayMeans for Each Day of the Lunar Months,as Table V, but Separated nto the PeriodsBefore nd After mid-June o ShowDifferencesn Asymmetry. Figures re Smoothed o Five-DayRunning Means

Day of unar month May to Mid-June Mid-June oOctoberFull Moon 1 0X15 -034

2 -0X10 -0.353 -O008 -0334 -0 07 -0*255 -0 03 -0X166 0*00 0-117 - +005 -0098 + 0 06 -0*059 + 0.10 -0*01

10 + 0-10 +00411 +0X13 +0X0812 + 0X14 + 0X1213 + 0-13 + 0-1214 +0X13 + 0X12

No Moon 15 + 0X11 + 0X14

16 + 0415 + 041617 +0-08 +0-1718 + 0X13 + 0X1719 +0-08 +0X1920 +0-07 +0X1821 -0.01 + 041922 -0-08 + 0-1923 0X18 + 0X1824 -0X23 +0X1625 0X29 + 0X1126 -0-31 +00527 -0.26 -0*0228 -0.23 -0*1429 -or17 0-27


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spring n asymmetry imilar o that for May develops and persists ill about mid-June, when there s a rapid change over to that of the July-autumn ype.

The asymmetry hat we have found o exist n the Noctuid moth captures s ofthe "autumn " type, but the figures were based on a summation f captures rom

Days from ull Moon_ 3 s 7 9 11 13 15 171 Zp1 g l3 25 7 29Noctuidae Fiveday moothed1933+1934 1935 means

+0*1

MvAYom-id-JUNE

X1. I GidXT m-02~~~~~~~~~~~~~~~~~~+.

+0'2

+01

mid-JUNEFto OCTOBER

-0-2

Fullce

FullR oon Last quarter -Nomoon First uarter moo.

1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 2Days from ul Moon 4

FIG.4-Departures of the captures rom he 29-daymean in the successive ays of the unar monthseparated nto heperiods efore nd after mid-June o show he two opposite ypes f asymmetry.

May to October. If the above lunar movements re the true cause of the insectasymmetry e should expect the Noctuid curve to show an asymmetry n one

direction eforemid-June nd in the reverse irection fter his ime.Table VI and fig. 4 show the results btained when the captures of Noctuidaeare treated s previously, fter aving been separated nto two groups, a) captures

V7OL.CCXXVI. B 3 F


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374 C. B. WILLIAMS

from he beginning f May to mid-June; and (b) captures from mid-June o theend of October. It will be seen that the results upport hehypothesis, s the Maycurve hows higher atches t " last quarter and lower at " first uarter whilethe later curve showsthe reverse. The only nexplicablefeature t the momentis the very owcaptures bout four r five days before ullmoon in the May-Junecurve; but as this s based on a relatively mallnumber f figures t maybe accidental.

The original symmetry f the curve n fig. 2 is therefore nly due to the factthat t was based on a larger number f observations fter mid-June han before.

Effect f the haseof theMoononthe Time f Flight f theAoctuidae

Table VII and fig. 5 (histogram) howthe normal distribution f the Noctuidae

through he eight equal periodsof the night nto which the trap separates the

catches. This is based on all captures n the summers f 1933, 1934, nd 1935. Itwill be seen that the maximum light s ust after midnight n period 5, if calculatedon either number r ogarithmic asis,but that lmost qual numbers ome beforeand after midnight 4700 to 53%).

If the moon s inhibiting ctivity t would be expected hat the catch n the earlyportion f the night would be reduced when the moon was above the horizon arly(i.e.,before ull moon) and that the catch n the ater portion f the night wouldbe reducedwhen the moonwas late (i.e.,after he fullmoon). To test his he nightdistribution as calculated separately for the weeks before full moon and afterfull moon for the three summers nder discussion. Each set of figures n TableVII thus represents heresults f eighteen eeks'captures. The night f full moonis omitted.

TABLE VII

Numbers f Noctuidae Captured n Each Period of the Night for All Nights; forNights n the Week before ull Moon; and for Nights n the Week after FullMoon

Period of Night: 1 2 3 4 5 6 7 8Actual umbers

All captures 599 826 1013 1454 1759 1429 955 309Weeksbefore ullmoon 115 174 175 362 467 380 230 52Weeks fter ullmoon 75 136 150 234 190 152 70 35

Sum f ogarithm n + 1)All captures 99.4 140.8 161.7 182-7 185*6 168.4 121.4 55*8Weeksbefore ullmoon 21X1 27.8 30 7 38.9 43 0 39 3 30.0 13.1Weeks fter ullmoon 16.8 26.9 27-0 32>8 29.0 27.2 16-6 8.2

%;actual umbers) Beforemidnight After midnightAll captures 47 53Before ullmoon 42 58After ull moon 57 43


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Fig. 5 shows the results diagrammatically uperimposed n the normal distri-bution, he vertical cale for the separate weeksbeing four imes hat of the total(histogram).

It will be seen that the results re according to expectation. There is a slightbut distinct hift f the distribution ater, n the week before ull-moon, hen thereis still darkness n the second half of the night; and a corresponding hift arlier,in the week after ull moon when the darkness s gradually ncreasing n the firsthalf f the night.

In the weekbefore ull moon58% ofthe catches re after midnight nd 42 before,while n the week after o moon 57% are beforemidnight nd 43 after.

2000- 200

1800- -180

1600- -1601400 -140

1200- -120ooo0- -100800- so8ho00- 60

400- .7-40200- -20

Scale for numbers Scale forfor histogram. 234 8A6 - logarithms

Period. I 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8for histogram

Numbers Logarithms

FIG. 5-Diagram showing he normal distribution f the Noctuidae n the ight periods f the night(histogram) nd also the distribution n the week before ull moon continued ine) and in theweek after ullmoon dotted ine).

Interrelationf the nfluencef Cloud nd MoonlightUp till now the figures ave been analysed to show the presence f a lunar effect

irrespective f the state of the sky.To show the possible nfluence f cloud it was decided to divide the days n thelunar months nder review n two ways.

(1) As regards hestate of the moon, nto three divisions(a) nights uring he week of full moon(b) nights uring he weeksof ntermediate oon;(c) nights uring he week of no moon.

(2) As regards hestate of the sky, nto three divisions

(a) nightswith more than 90% of the sky lear (" clear ")(b) nights with 10-90% of the sky loudy " intermediate )(c) nights with more than0%rO cloudy " cloudy ).

3 F 2


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376 C. B. WILLIAMS

The interrelation f these two main divisions ivesnine possible ombinations sshown n Table VIII.

The next tage was to allot each night f the six lunar months n each year ntoits proper division. Table VIII shows heactual results or1933as an example ofthe method. It willbe seen that the numbers f nights n each division re differentt,depending ntirely n accidental weather onditions.

TABLEVIIINights n 1933 Divided Into Nine GroupsAccording o the Combination f Condi-

tions f Moon and CloudClear Intermediate Cloudy

Full moon June: 4, 5, 6, 7, 8. May: 10, 11, 12. May: 9.week July: 3, 4, 7. June: 9. June: 10.

August: 3, 4, 5, 7, 8. July: 6. July: 5, 9*.Sept.: 2, 4, 5, 6. August: 2, 3. August: 31.Oct.: 4. Oct.: 2, 3, 5, 6*. Sept.: 1, 3, 30.

Oct.: 1, 30.

First nd third May: 18, 19. May: 14, 15, 17, 20, 29, May: 13, 16, 28.quarters June: 1, 2, 3, 28. e 30, 31. June: 12, 15, 16, 29.

July: 2, 14, 16,25, 26, 27. June: 11, 13, 14, 17, 18, July: 13, 15, 17, 28, 29,August: 1, 12, 16, 26, 26, 27, 30. 31*.

27, 28. July: 1, 11, 12, 29. August: 11, 15.Sept.: 7, 8, 9, 10, 14. August: 9, 10, 13, 14, Sept.: 11, 12, 23, 24, 26,Oct. 11, 12, 14, 25*, 27. 24 25, 29, 30. 27, 28, 29.

Sept.: 13, 22, 25. Oct.: 7, 9*, 10*, 13, 22Oct.: 8, 23, 26*. 24*, 28, 29.

No moonweek May: 21, 22. May: 23, 26, 27. May: 24, 25.June: 19, 21. June: 22, 23. June: 20, 24, 25.July: 18, 19, 20. July: 21, 22. July: 23, 24.August: 23. August: 17, 18, 19, 20, August: 22.Sept.: 15. 21. Sept.: 17.Oct.: 15, 16*, 17, 19. Sept. 16, 18, 19, 20, 21. Oct.: 18, 21.

Oct.: 20.* = nightswith heavy wind.

Certain f thesedays,marked with n asterisk, ad captures onsiderably educedby heavy wind. These have been eliminated rom urther alculations.

In each of the subdivisions he departures f the ogs from the 29-day mean forthe correct ays were placed and these were summed and divided by the numberof days, thus giving n average departure or hat particular ombination f cloudand moon conditions.

Similarly verages were worked ut for each row and column, giving the effectof moon ndependent f cloud and cloud independent f moon respectively.Table IV shows the results hus obtained for the Noctuidae in the years 1933,1934,and 1935 (a, b, and c) separately nd also for he three ombined ogether g).


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ACTIVITY OF NOCTURNALINSECTS ;377

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378 C. B. WILLIAMS

It will be seen that n general there s a tendency or he " clear" column to belower han he cloudy , and for he full moon" to be lower han he no moonrow. The division fullmoon-clear sky gives n each case the greatest egativedeparture, nd the division " no moon-cloudy " gives the greatest positivedepartures.

Since the mean logarithms or he whole of the^ aptures f the series s known obe 0 882 (equal to a geometric mean catch of 6 62 insects) the mean departuresfor he three years Table IXg) can be subtracted rom r added to this mean anda true ogarithm h) and hence an average catch in numbers an be obtained foreach division. This is shown n Table IXi with the final addition of a Table JXjin which the figures re altered o bring he mean catch to 100, so that the figuresin each division re percentages f the mean catch.

This table is consistent xcept for the figure 133 in the " intermediate loud,intermediate oon division which s a little igher hanmight e expected. Thefigures or the rows and columns eparately re quite consistent nd show a ratioof 146: 120: 54 for nights with no moon, intermediate moon, and full moonrespectively and a ratio 140: 110: 80 for nights f full loud, ntermediate loud,and clear sky. The ratio of catches in the extreme onditions f " no moon-cloudy and " full moon-clear " is 184 : 42 or ust over 4 : 1.

Another oint worthy f notice s that the reduction f catch due to the clearnessof he ky sgreater n full moonnights 95 : 42) than on moonless ights 184 : 128);and similarly hereduction f catch by moonlight sgreater n clearnights 128 : 42)than on cloudynights 184 : 95).A three-dimensional odel of the nine main values in Table JXj is shown nfig. 6 with the full moon row in- ront nd the clear sky row to the right. Theheight of the vertical columns represents he catch under each combination ofconditions.

These figures must next be considered n connexion with possible temperaturevariations, nd for this purpose the minimum emperatures ave been treated nthe same way as the figures or captures except that they are not converted ologarithms) nd the results re shown n Table IXd, e,f, and k.

It will be seen that n each year there s a consistent emperature radient romcloudy nights o clear which s to be expected owing to the action of clouds inpreventing adiation. On the average of the three years the difference s 4- 03? F.,which s possibly ufficient oexplain the differences n captures n favour f cloudynights. A fuller nvestigation f this will be made later.

The temperature n relation omoonphaseshows as has alreadybeen mentioned)warmer onditions t full moon n 1933; at no moon n 1934 and at intermediatemoon n 1935.

On the three years ombined here s still bias of about 0. 3 F. in favour f the

full moon, so that the results btained are not due to any accidental temperatureeffect, nd, in fact, f temperature ad been normally istributed he effect f themoon might e expected o be slightly reater han that hown.


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It will also be seen that the temperature eparture n the division intermediatemoon-intermediate loud " is considerably ess than that in " no moon-inter-mediate loud ", so that the nconsistence f the catch figures or he former ivisionis at least partly xplained.

To sum up-the ratio of catches n the Noctuidae n the three years combinedis shown o be about 2-7 : 1 when no moon s compared to full moon (in spite of asmall temperature ffect n favour f full moon) ; and 1* 5 : 1 when cloudy nightsare compared with lear nights, his being associatedwith temperature ifferenceof about 40 F. in favour f the clotidy ights owing to reduction f radiation by theclouds), and possibly xplicable on this basis alone. Finally the ratio between

FIG. 6-Photograph of three-dimensional odel showing he average numbers f Noctuidaecapturedin the trap on nights f various ombinations f moon phase and cloud conditions.

" no moon-cloudy " and " full moon-clear " is ust over 4: 1,while n the cross-relation, lear nights with no moon give distinctly arger atches than cloudy nightswith ull moon 128 : 95). It must, owever, e recollected hat hegroup cloudynights ncludes ny night with ess than 10% of the sky clear, so that slight unarinfluence ould be expected on some of these nights, articularly s the ight of themoon can penetrate louds of considerable hickness.

Correctionf the 9-Day Mean Curve orLunar nfluenceIn fig. are shown s vertical olumns he og of the atches + 1)of he Noctuidae

each day during he ix unar months f the three yearsunder onsideration. There


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380 C. B. WILLIAMSQPp njDoN

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is also shown s a series f short horizontal ines the running 9-day mean of thesevalues. The difference etween the value for any one day and the 29-day meanfor he same day is due to the conditions f that day, ncluding uch non-periodicfactors s wind, temperature, loud, and humidity, hich change rapidly rom ayto day, and also to the phase of he moonwhich s not eliminated y taking hemeanover the same period as the ength f the unar months.

With the information e have gained above, however, t is now possible tocorrect his 29-day mean for the expected unar influence ased primarily n thecurves hown n figs. and 6 and the nformation n Table IV.

The factors hat have to be considered re(1) the mean + and - departures t no moon and full moon;(2) the fact that the differences re at their minimum n June and maximum n

October;(3) the alteration n asymmetry f the corrections rom he months efore mid-June to those fter.

With these facts s a basis, there has finally een added to fig. 7 a third urve(dotted) showing he 29-day mean corrected s nearly as possible for the lunarinfluence.

It will mmediately e seen how much more closely he curve fits he changes nthe log histogram, han did the 29-day mean; thereby howing what a high pro-portion f the variation s due to the unar nfluence.

For example, he og (n + 1) value for 25 July, 1934, s about 0 1 below the 29-day mean, but actually about 0 15 above the corrected mean. Thus the morerapidly changing weather conditions uch as temperature, tc., are actually re-sponsible or n increase n the captures bovenormal nd not a decrease s wouldappear if the unar effect ere neglected.

From his igure, herefore, t is possible ostart with new set of valuesfor achday showing epartures romwhich the unar periodicity asbeen practically limi-nated, and from which the influence f other factors an now be studied withconsiderable implification ue to the elimination f a periodic hange.

LUNAR INFLUENCE ON OTHER GROUPS OF INSECTS

ComparisonfFull Moon ndJNoMoonWeeksn Various roupsTable X shows the comparison f full and no moon week captures, calculated

in a way similar o that hown or he Noctuidae n Table III, for number f othergroups of nsects nto which the night atches were sorted. They are all based onthe ogarithm f the numbers oeliminate s far s possible hance high catches.

It will be seen first f all that n 1934 there re very few exceptions o the rulethat catches are highest n no moon weeks. In 1935, there re severalexceptions,particularly n the month f August; while n 1933 the xceptions re quite numerours

VOL. CCXXVTI.-B 3 G


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382 C. B. WILLIAMScq CS, V- Ct m Q0 Ct g 0o m b ot t

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ACTIVITY OFNOCTURNAL INSECTS 383g 0C 00 00 C g.0 c gDr ccc

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384 C. B. WILLIAMS

(although not outnumbering he others). This general result s due to the factalready explained that n 1934 the temperatures ere highest t no moon and in1933at full moon; while n 1935 Augustwas one of the months hat had a distincttemperature ifference n favour f full moon.

However, n spite of this there re distinct ifferences n behaviour between hegroups, nd these re summarized n Table Xl which howsthe groups with their

TABLE XI

Principal Groups of Insects Dealt with Arranged n Sequence of the DifferenceBetweenCaptures n Full and No Moon weeks

Mean log diff. Standardper week deviation t Significance

1 Noctuidae L) 2.77 { 0 44 6.30 certain2 All insects 2-11 0.94 2.24 probable3 All Lepidoptera 1 97 0-164 3 01 certain4 Psychodidae D) 1 62 1 -33 1 22 not sig.5 Ceratopogon D) 1 47 0 88 1.67 doubtful6 Geometridae L) 1.20 0*61 1 98 possible7 Mycetophilidae D) 1 07 0.95 1*14 not sig.8 Borboridae D) 0 95 0-88 1089 Crambidae L) 0 93 0 -61 1 52 doubtful

10 Cecidomyidae D) 0 92 0 89 1 03 not sig.11 Capsidae (R) -0 73 0-64 P1512 Chironomidae D) 0 43 0 94 0.4613 Coleoptera 0 24 040 0*60 ,14 Psocoptera 0.08 0.68 0 0115 Jassidae R) - 0.24 0-98 - 0.2416 Aphidae (R) -0'85 0-61 - 1.39

L = Lepidoptera. D = Diptera. R = Rhynchota.

mean weekly og difference; the standard deviation of that difference; nd the"t " test of significance otherwise he mean difference ivided.by the standarddeviation). The groups re placed in order of the value of the og difference.

The following actsmay be noted:(a) The Noctuidae are far above the other groups with a higher og difference,and a very much higher alue for .

(b) Out of the first ixgroups our re either epidoptera r include Lepidoptera(e.g., ll insects).

(c) Familiesof Diptera hold positions , 5, 7, 8, 10, and 12.(d) Five out of the last sixpositions re groups hat are neither epidoptera or

Diptera.It would appear from his that the effect as at its maximum n the Lepidoptera,

next n Diptera, nd lower n other groups. Sinceall the differences ealt with arelogarithmic r geometric atios he fact hat the Diptera make up a large proportionof the catch shouldnot affect heir osition.


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There is, however, ne other point of possible mportance. Work on the distri-bution of nsects uring he night WILLIAMS, 1935) has shown hat Noctuidaehavetheir maximum light ound about midnight in periods 4 and 5), the Geometridaea little arlier n period 3, and the Crambidae till arlier n period 2. On the otherhand, the Chironomidae ave their maximum light t dusk n period with sub-maximum t dawn (period 8); the Coleoptera and Psocoptera both have theirmaximum n period I ; the Jassidae resemble he Coleoptera, nd the Aphidae hada rather ndefinite maximum flight n period 1 in one year and period 2 in thenext.

Thus it appears that a possible explanation f the order of sequence in TableXI might be that the nsects which fly ate are most ffected, hile those that flyat dusk or dawn (12, 13, 14, 15, 16) are least affected.

In order to test this t is necessary o make a new comparison between full andno moon weeks f the nsects aught n each successive eriod f the night eparately.

Effect n ll Insects t Different eriods f theNightTable XII showsfor comparisonwith Table XI the mean differences, tandard

deviation, nd value of t for the 18 full and no moon weeks, for all the insectscaptured n each of the eight periods f the night eparately. It will be seen that(a) the standard deviation s (with one exception) remarkably onstant; (b) themean difference s always in favour of no moon-it is low in periods 1 and 8 but

higher n all the others (c) the test of significance t) shows that the results nperiods2-7 are quite significant nd approximately qually so.The probability f the results eing due to chance s about 1: 50. But in period

8 and particularly n period 1, the results lthough positive re not significant ndin the atter eriod the probability f the result eing due to chance s 0 4: I * .

TABLE XII

Difference etween aptures n Full and No moon Weeks, or ll Insects, onsideredAccording o the Time of Entry nto the Trap

Mean log StandardPeriod difference deviation t

1 0 71 0.85 0 83 not significant2 194 0 84 2 30 significant3 1*82 080 2*304 2l10 0*81 2*605 2*22 0*87 2*566 1 81 0 72 2*527 2*12 0 84 2*508 1 38 0*85 1*60 doubtfully ignificant

The lunar influence s therefore ery ow in the first eriod at dusk, significantand almost qually high n periods2-7, and low again at dawn in period 8.

3 G 3


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386 C. B. WILLIAMS

Further, he mean difference n the Noctuidae differs ery definitely rom hemean values of other nsects aught n the sameperiods 4 and 5), so that the unareffect n this group at least cannot be entirely ue to their ime of flight.

On the other hand, the ow position f the dawn and dusk flyers, he Coleoptera,Chironomidae, socoptera, assidae, nd Aphidae in Table XI isundoubtedly uein part to the ow lunar nfluence t the times f their light.

Separate4ffectf MoonlightndCloud nAll nsectsTable XIII showsthe results btained when the values for all insects aptured

each night n all three years ogether re treated imilarly o thosefor he Noctuidaein Table IX.

The logarithmic mean catch per night was 208 insects. Reducing the values

to percentages t will be seen from able XIII C that on nights f" full moon, nocloud the atch s 63; on "full moon with loud " the catch s115; on " no moon,no cloud" the catch is 81; while on " no moon, loudy the catch is 381. Allthe values n the table are consistent nd showquite definitely heseparate nfluenceof moon and cloud but the most remarkable ifference rom he Noctuid table isthe very high value obtained on the nights f no moon with cloud, which s almostthree imes hevalue of any of the adjoininggroups nd nearly ixtimes he numberof the opposite xtreme full moon, no cloud .

TABLE XIII

Effect f Various Combinations f Moon and Cloud Conditions n All Insects, orComparisonwith Table IX

A. Mean log B. Mean catch (anti-log - 1) C. 00 of mean catch2*12 2*16 2l38 2*19 131 144 239 154 63 69 115 742*17 2-34 2X43 2 31 147 218 268 203 70 105 129 972-23 2-46 2*90 2.50 169 287 793 315 81 138 381 1512-19 2*32 2*54 154 208 346 74 100 166

Another urious fact s the small difference etween the values for full moon

and no moon on nights when there s no cloud.If one were oattempt n explanation f the differences t the present arly tagein the nvestigation t might e suggested hat the nsects s a whole which ncludea very arge majority f small Diptera) are more sensitive o" optimum conditions,which are cloudy (and hence warm and damp) nights with no moon, and do notfly f hese onditions re not existing, hile the Noctuidae re more olerant f theirclimatic environment nd are only seriously educed by the poorest conditions,which re cool clear nights with full moon.

Effect f MoonlightndCloud n the ub-familyipulinaeDiptera)PINCHIN and ANDERSON(1936)have applied similar methods o the study of the

Tipulinae captured n the trap and have obtained the following alues (expressed


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as percentages) for the final moon-cloud diagram for the years 1933 and 1934combined. The results re once more consistent nd show a ration of about 4 : Ibetween heextremes f " full moon, no cloud " and " no moon, cloudy".

TABLE XIV-EFFECT OF VARIOUSCOMBINATIONSOF MOON ANDCLOUDCONDITIONSON THE FAMILY TIPULINAE

54 81 162 7773 127 162 116

100 139 200 13573 116 173

GENERAL DISCUSSION

It would appear from the above results that when certain groups of insects areattracted o a trap by means of a light, he captures how a lunar periodicity itha minimum t full moon and a maximum t or ust after no moon. This is parti-cularly definite n the familyNoctuidae of the Lepidoptera which have their maxi-mum activity oundmidnight, ut s very much essmarked n certain ther groups,such s Coleoptera nd Jassidaewhich ly hiefly t dusk nd dawn. The Noctuidae,however, differ significantly from other insects flying at the same time.

Certain asymmetries n the effect can be traced to similar asymmetries n the

apparent movements of the moon, and when the asymmetry f the moon changesthat of the captures does also.When the cloud effect s analysed eparately t is found hat cloudy nights have

larger captures than clear nights, ut these nights re, on an average, distinctlywarmer han he learnights, nd this may be sufficient o account for he differencesin catches.

The fullest ffect f the moon is noticeable on clear nights when the ratio ofcaptures n the Noctuidae between no moon and full moon i 128 : 42 or 3: 1;on cloudy nights he ratio s 184 : 95 or 2 : 1; while on all nights rrespective f

cloud conditions the ratio is 146 : 54 or about 2 7: 1.The reality f the reduction f catches by the moon may therefore e consideredto be definitely stablished.

This reduction may be due to one of two effects. First may be that themoonlight s reducing the activity f the insects, o that the active populationavailable to be sampled by the trap is smaller. Secondly, t is possiblethat thelight of the trap has to compete with the ight of the moon and so is less efficientand attracts nsects rom smaller rea.

It is not at present possibleto distinguish etween these two alternatives with

certainty, ut the fact that the Noctuidae differ onsiderably n their esponse romrother nsects flying t the same time suggests he effect may be partly at leastphysiological.


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388 C. B. WILLIAMS

Further nvestigation s proceeding o settle this point by the use of traps notdependent on light for their attractive ower. During 1935 a mechanical trapwas constructed nd tested WILLIAMS and MILNE, 1935), but while very uccessfulfor the smaller nsects t did not catch the larger Lepidoptera n any numbers.During 1936experiments re being started n the use of a bait trap.

If a reduction t full moon s shown by these traps, hen the effect f the moonmust be a general owering f activity f the insects oncerned, s is the popularbelief mong insect collectors. If on the contrary he mechanical and bait trapsshow no lunar periodicity, he effect f the moon must be merely o lower theefficiency f the ight s an attractant.

SUMMARY

The object of the nvestigation as to test he truth f a general belief hat nsectnight ctivity n certain roups, articularly epidoptera, s reduced at full moon.This beliefwas held to apply to insect ctivity n general, but in particular o thenumber f nsects ttracted o light.

A light rap was placed in a field t Rothamsted n March, 1933,and has beenin continuous use since that date. The captures of the Noctuidae during thesummers f 1933, 1934,and 1935 were selected or pecial study.

Three main methods f analysiswere used. (I) A comparison f total capturesand of the sum of the ogarithms f captures, n the full moon and no moon weeksof eighteen unar months, ix in each summer. (II) Averaging ver the eighteenlunar months he mean departures f the log of the catch from he 29-day meanfor ach of the 29 days of the unar cycle. (III) A calculation f the mean depar-tures f the og from he 29-day mean for ll days of the period grouped nto ninedivisions ccording o moon conditions full, ntermediate, nd no moon), and tocloud conditions clear, ntermediate, nd cloudy ky).

The amount f night loud was measured hiefly y an adaptation of the Green-wich pole-star amera; while the moonlight as measured by a speciallydesignedphotographic ecorder.

In the Noctuidae ll three methods ave a definite ndication f unar periodicity.The first omparison howed 17 of the 18 lunar months with a higher atch at nomoon than at full moon and a mean difference f six times he standard deviation.

The secondmethod avea curve with definiteminimum ery lose to full moonand a maximum lightly fter no moon. This curve s asymmetrical nd furtheranalysis howedthat t was compounded f two curves with opposite symmetries,each explicableby similar symmetries n the moon's apparent movements.

The third method hows a ratio of 3: 1 between no moon and full moon weekcaptureswhen the sky s clear, and 2: 1 when the sky s cloudy. The same method

shows hat the ratio of captures n cloudynights o clear nights s about I 75: 1,but this s associatedwith warmer onditions n cloudynights about 40 F. higheron minimum emperature) ue to the reduction f radiation by the clouds.


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A difference as been demonstrated etween he hours of flight uring he nightof the Noctuidae n the week before nd after ull moon, corresponding o the effectof the early or late portion f the night eing under the moonlight.

Other groups of nsects nalysed by the first method how varying unar effects,never, owever, o striking s that n the Noctuidae and in a few groups e.g.,Cole-optera,Jassidae, nd Aphidae) it s non-significant r slightly egative.

The groups howing east effect re shown to be those that fly hiefly t dusk ordawn, whereas he Noctuidae have their maximum ctivity t midnight; but aseparate nalysis f all insects, ccording o the different imes f the night t whichthey re captured, hows hat this time of flight s not sufficient o account entirelyfor he difference ound.

When the total insects aptured are analysed by the third method consistentresults re obtained, but with a very arge increase n captures on the days of nomoon-full cloud. It is suggested hat the explanation f this may be a narrowlylimited range of optimum onditions f moon, cloud, temperature, nd humidityin the smallerDiptera which make up most of the catch.

It is considered hat the lunar effect n the captures s definitely emonstrated,and that there s distinct vidence hat t differs n different roups part from nydifference n their time of flight. Therefore, t is probably physiological ffecton the activity f the insects nd not merely ue to reduction n the efficiency fthe light trap when the moon is shining. Further xperiments o test this morefully re beingcarried ut, by using raps not dependent n light or heir ttractingpower.

REFERENCES

ANON. (1931). "Greenwich Meteorological Observations, ntroduction," p. E8and 9.

DORR,J. N. (1932). 'S. B. Akad. Wiss. Wien.' Abt. 11 a, vol. 141,pp. 129-162.Fox, H. M. (1923). 'Proc. Roy. Soc.,' B, vol. 95, pp. 523-550.HORA, S. L. (1927). 'Proc. Asiatic Soc. Bengal,' vol. 23, pp. 339-341.PAGDEN, H. T. (1932). 'Malayan Agric.J.,' ol. 20, p. 122.PINCHIN, R. D., and ANDERSON,J. (1936). Proc. R. Ent. Soc. Lond., A, vol. 11,

pp. 75-78.WILLIAMS, C. B. (1924). Bull. Ent. Res.,' vol. 15, p. 57.

(1935). 'Trans. Roy. Ent. Soc. Lond.,' vol. 83, pp. 523-556.(1936a). 'J.Animal Ecol.' (In the ress.)(1936b). "Ann. App. Biology." (In the ress.)

WILLIAMS, C. B., and EMERY, G. A. (1935). 'J. Sci. Instr.,' vol. 12, pp. 111-115.WILLIAMS, C. B., and MILNE, P. S. (1935). 'Bull. Ent. Res.,' vol. 26, pp. 543-557.

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