227

The sensory neurones and sensilla in the abdomen andthorax of the blowfly larva

ByM. P. OSBORNE(From the Department of Zoology and Comparative Physiology, The University,

Birmingham, 15)

With 2 plates (figs. 10 and 11)

SummaryThe peripheral nervous system in the Phormia larva consists of bipolar neurones thatend in specialized sensilla, and bipolar or multipolar neurones with ramifying distalprocesses. The former are associated with the cuticular sensilla and chordotonalorgans. The latter (a) constitute a sub-epidermal nerve-plexus with processes thatoccasionally run over skeletal muscles, (b) innervate specifically orientated strands ofconnective tissue, the longitudinal, vertical, and ventral stretch receptors, and (c)innervate the tracheal epithelium, imaginal buds and associated pedicels, and thenerve-sheaths.

IntroductionA SUB-EPIDERMAL nerve-plexus of multipolar neurones was first describedin insects by Viallanes (1882 a, b) in the dipteran larvae Stratiomys, Eristalis,and Musca. A similar plexus has been described in cerambycid beetles(Monti, 1893, 1894), in the caterpillar Sphinx lingustri (Holmgren, 1896), inthe silkworm larva (Bombyx mori) (Hilton, 1902), and in the larva of Melolonthavulgaris (Zawarzin, 19126). Multipolar neurones associated with the body-wall, muscles, and joint membranes of various insects have been reported byother workers (Cajal, 1890; Zawarzin, 1912a; Rogosina, 1928; Tonner, 1936;Josting, 1942; Meyer, 1955; Barbier, 1961; Hamon, 1961). Zawarzin (1916)and Orlov (1924) have described multipolar neurones associated with themuscles and epithelia of the insect gut.

Zawarzin (19126) and Monti (1893, 1894) are confident that the dendritesfrom adjacent neurones do not anastomose, but other authors, for instanceViallanes (1882 a, b), Holmgren (1896), and Hilton (1902), believe that thedendrites from different neurones fuse to form a 'nerve-net'.

The structure and function of the sub-epidermal nerve-plexus in insects hasbeen reviewed by Snodgrass (1926). The terminal branches of the sub-epidermal plexus are believed to end freely on the basement membrane. It issuggested that these neurones are sensitive to mechanical stimuli or perhapschanges in temperature, although they may receive general sensations such aspain.

Welsh (1937) reported two clusters of multipolar neurones in the prothoraxof dipteran larvae and thought that they were photoreceptors. Bolwig (1946)proved the dipteran photoreceptors to be located on the head, but statedemphatically that there were no multipolar neurones associated with the[Quart. J. micr. Sci., Vol. 104, pt. 2, pp. 227-41, 1963.]

228 Osborne—The sensory neurones and sensilla in the

epidermis, and suggested that the multipolar neurones described by Viallanesand Welsh were tracheal end-cells. Hertweck (1931) described in detail thenervous system and sense organs of Drosophila melanogaster but did not referto any multipolar neurones innervating the epidermis. He even saw the longi-tudinal stretch receptor strands (Seitenstrdnge) but did not describe theirinnervation by sensory neurones.

Thus there appears to be considerable controversy in the literature aboutthe occurrence of a multipolar nerve-plexus beneath the epidermis of dipteranlarvae, and whether the dendrites from different nerve-cells anastomose. Itwas noticed during the investigation of dipteran stretch receptors that manymultipolar nerve cells were associated with the epidermis of the blowflylarva. In view of the difference of opinion which exists between earlierworkers, it was decided to reinvestigate the sub-epidermal nerve-plexus in thedipteran larva.

Material and methodsLarge larvae of Phormia terrae-novae were used for this work. The peri-

pheral nervous system was stained supravitally with methylene blue by thetechnique described in a previous paper (Osborne and Finlayson, 1962).After staining and fixing, the skeletal muscles were carefully removed so as toexpose the nerves innervating the epidermis. The material was then dehy-drated and mounted in dammar. Photomicrographs of preparations weretaken on Ilford Pan F 3 5-mm film with a Zeiss photomicroscope.

ResultsThroughout this work only the thoracic and abdominal segments 1 to 7 were

investigated. The topography of the peripheral neurones and sense-organs isdifferent in each thoracic segment (figs. 1, 2) but similar in all abdominalsegments (fig. 3). For the sake of clarity only the main dendrites of themultipolar neurones are shown. The peripheral nerve-cells are classifiedaccording to Zawarzin (1912a).

Type I cells are always bipolar and are associated with the cuticular sensillaand chordotonal organs. Type II cells are multipolar or bipolar, and includethe stretch receptor neurones, the neurones innervating the epidermis, andcertain cells to be described later which have a type of innervation notpreviously reported.

Type I cells

Two types of epidermal sensilla were found in the thorax and abdomen.The more numerous are campaniform sensilla (figs. 4, A; 10, B, E), of symmet-rical form. Twenty-two have been found in each abdominal segment, 20 inthe meso- and metathorax, and 22 in the prothorax. Their positions on thecuticle are remarkably constant. In the mesothorax, metathorax, and abdominalsegments they are arranged roughly in an annulus round the centre of the

abdomen and thorax of the blowfly larva 229

segment (figs. 1, 2, 3). In the prothorax (fig. 1) they are arranged in a ring onthe ventral and lateral sides, but in the dorsal region they form a cluster.

The second type of sensillum consists of 3 neurones, the dendrites of whichextend into a fine channel running through the cuticle and terminate beneatha small pore on the surface of the cuticle (figs. 4, B; 10, F). Only 4 of this typeof sensillum are found in each segment. In the prothorax 2 are found in

FIG. 1. Lateral view (from inside) of the right half of the pro- and mesothorax of a Phormialarva, to show the topography of the sensory neurones and sensilla. cam, campaniformsensillum; cs, cuticular sensillum (possibly a chemoreceptor); co, chordotonal organ; Ir,

longitudinal stretch receptor; mn, multipolar neurone; sp, spiracle.

the dorsal region and 1 on each ventro-lateral area, while in the meso- andmetathorax they are found on the lateral and ventral surfaces. In theabdominal segments they are arranged in similar manner to those in theprothorax.

The topography and structure of the chordotonal organs in the abdomen ofPhormia are exactly the same as those described by Hertweck (1931) inDrosophila larva. There are, however, fewer chordotonal organs in the thoraxof Phormia than in Drosophila.

There are 8 chordotonal organs in abdominal segments 1 to 7. In eachsegment there are 2 on each side of the ventral mid-line, 1 in each lateralregion, and 1 on each side of the dorsal mid-line (fig. 3). The ven-tral chordotonal organs all have a single scolopidium and are orientated

230 Osborne—The sensory neurones and sensilla in the

transversely. Each lateral organ has 5 scolopidia (fig. io, G) and is orientatedobliquely. The dorsal chordotonal organs are orientated transversely and eachhas a single scolopidium. In both the meso- and metathorax (figs. 1, 2) thereare only 2 chordotonal organs, one on each side of the dorsal mid-line. Each is

ObmrnFIG. 2. Lateral view (from inside) of the right half of the metathorax of a Phormia larva toshow the topography of the sensory neurones and sensilla. cam, campaniform sensillum; cs,cuticular sensillum; co, chordotonal organ; Ir, longitudinal stretch receptor; mn, multipolar

neurone.

orientated transversely and has 3 scolopidia. Four chordotonal organs arefound in the prothorax (fig. 1). There is 1 on each side of the dorsal mid-lineand 1 in each lateral region. All have 2 scolopidia. The axons from the lateralchordotonal organ join the main segmental nerve of the mesothorax, unlikethe other chordotonal organs whose axons join the segmental nerves of thesegments in which they are located.

abdomen and thorax of the blowfly larva 231

Type II cells

The sub-epidermal nerve-plexus is composed of multipolar neurones (figs.10, A; 11, A). The neurones and their main dendrites are surrounded by aneurilemma sheath (fig. 10, c, j). Often dendrites or dendrites and axons

O S /7J/7J

FIG. 3. Lateral view (from inside) of the righthalf of the fourth abdominal segment of aPhormialarva to show the topography of the sensory neurones and sensilla. cam, campaniform sensillum;cs, cuticular sensillum; co, chordotonal organ; Ir, longitudinal stretch receptor; mn, multipolar

neurone; ven, ventral stretch receptor; vr, vertical stretch receptor.

from different neurones run in common sheaths (fig. 5). Connective-tissuefibres sometimes join adjacent neural sheaths together, or anchor them to theepidermis (figs. 5; 10, j). The main dendrites branch profusely, giving rise toa plexus of exceedingly fine neural processes which have a characteristicbeaded appearance and are apparently without a neurilemma sheath (figs.

232 Osborne—The sensory neurones and sensilla in the

5; 11, A). The plexus forms a delicate interwoven meshwork that runs overthe basement membrane of the epidermal cells. I believe that this plexuscovers practically the entire epidermis of the blowfly larva. No dendrites wereseen to penetrate the basement membrane between the epidermal cells orramify beneath the cuticle. The vast majority of dendrites appeared to end onthe basement membrane of the epidermal cells although a few were found to

FIG. 4. Cuticular sensilla from the abdomen and thorax of the Phormia larva. A, campaniformsensillum. B, possibly a chemoreceptor.

terminate on skeletal muscles. In no case has any anastomosis been seenbetween the dendrites of different neurones, although processes from differentcells may run very close together. The number of neurones in each segmentis always the same, and there is little variation in their topography. There are24 neurones constituting the nerve-plexus of the prothorax, 28 in both themeso- and metathorax, and 30 in each abdominal segment (figs. 1, 2, 3).

Typically the neurones are arranged approximately in 6 main groups in allsegments. Two are located dorsally, 2 ventrally, and 1 in each lateral region.The topography of the neurones in the dorsal groups of the mesothorax,metathorax, and abdominal segments is similar. One of the neurones in thedorsal group always has a cell-body that is about twice the diameter of that ofthe other neurones (fig. 11, A). In the prothorax the anterior neurones of the

abdomen and thorax of the blowfly larva 233

dorsal plexus are grouped more closely together and are situated nearer to theintersegmental fold than they are in other segments. A comparison of thelateral groups shows that the neurones are distributed anteriorly in the pro-thorax and abdomen, and in the middle of the segment in the meso- and meta-thorax. The neurones constituting the ventral groups are arranged similarly

FIG. 5. Drawing of part of the multipolar nerve-plexus of a Phormia larva to show dendritesand a dendrite and axon from different neurones running in common neurilemma sheaths, ax,

axon; con, connective tissue; d, dendrite; ns, neurilemma sheath.

in the thoracic segments. The neurones of the ventral groups in the abdomenare more widely distributed laterally and anteriorly than they are in the thorax.

A number of neurones in the body-cavity are associated with strands ofconnective tissue. These are the stretch receptors (Finlayson and Lowenstein,1955; Slifer and Finlayson, 1956; Finlayson and Lowenstein, 1958; Osborneand Finlayson, 1962).

In the larva of Phormia 3 pairs of stretch receptors have been located inabdominal segments 1 to 7. Two are present above the dorsal musculature,the longitudinal and vertical receptors, but the third is located amongst theventral musculature (fig. 6). These paired ventrally located receptors are con-sequently termed 'ventral receptors'. The longitudinal and vertical receptors

234 Osborne—The sensory neurones and sensilla in the

lie above 2 layers of dorsal longitudinal muscles which run obliquely acrossthe segment.

The longitudinal receptor is suspended between the intersegmental folds and

07S/n/7?

FIG. 6. Diagram of the right half of the sixth abdominal segment of a Phormia larva withvarious muscles removed to show the location of the 3 stretch receptors. Ir, longitudinal

receptor; ven, ventral receptor; vr, vertical receptor.

consists of a multipolar neurone associated with a strand of connective tissue(fig. 10, H). The neurone, which is contained in a fibrous capsule, is situatednear the centre of the strand. The connective-tissue fibres of the strand areorganized to form a tube, inside which run the dendritic processes (fig. 7, A).These have been seen to extend as far as the regions where the connective-tissue fibres fan out at the ends of the strand to make the attachments to the

abdomen and thorax of the blowfly larva 235

intersegmental folds (figs. 7, A; 10,1). Fine connective-tissue fibres anchor thereceptor to the main branch of the tergal nerve which runs between the secondand third bands of both layers of dorsal muscles to provide sensory innervationfor the tergal epidermis. The axon from the neurone joins this nerve.

The vertical receptor (fig. 8) consists of a bipolar neurone associated with astrand of connective tissue. The strand is anchored at one end to the tergalepidermis (fig. 6), and at the other to the motor branch of the tergal nerve

50//

FIG. 7. Diagram to show the structure and posterior attachments of the longitudinal, A, andventral, B, stretch receptors of the Phormia larva.

where it runs below the second and third dorsal muscle-bands. Some con-nective-tissue fibres are also attached to these two muscles. The connective-tissue fibres are more diffusely arranged than those of the longitudinalreceptor, with the result that the tubular structure is not so compact or welldefined. The neurone is situated on the epidermis and is enclosed within aneurilemma capsule which is continuous with that of one of the epidermalsensilla that have 3 neurones (fig. 8). The distal process runs inside theconnective-tissue strand as far as its attachment to the tergal nerve. The axonruns with those of the other 3 neurones to join the main nerve innervating thedorsal epidermis.

The ventral receptor is orientated longitudinally and has essentially thesame structure as the longitudinal receptor, consisting of a multipolar neuronewhose dendrites run inside a tube of connective tissue. It differs, however,in its mode of anchorage. Posteriorly it is attached to the edge of a ventraloblique muscle; anteriorly it is attached at two points to the ventral epidermisand also to the edge of another ventral oblique muscle (fig. 6). At each attach-ment to a ventral oblique muscle, a single muscle-fibre is seen to enterthe strand of connective tissue (figs. 7, B; 10, K). The receptor strand is

236 Osborne—The sensory neurones and sensilla in the

anchored to adjacent muscles, nerves and tracheae by connective-tissuefibres.

The thorax has also been examined for stretch receptors. In the thoraxstretch receptors are restricted to the meso- and metathorax, but even in thesesegments only the longitudinal receptors were found (figs. 1, 2).

A number of multipolar nerve-cells have been found in the lateral region of

FIG. 8. Schematic drawing of the structure of the vertical stretch receptor of the Phormialarva. The neurones drawn in dotted lines represent those of a cuticular sensillum.

the abdomen which have a type of innervation not previously described (fig. 9).The most anterior of these neurones (mm) is located in a capsule attachedto the posterior edge of the transverse intersegmental muscle. It is a tripolar(fig. 11, E). One of its processes runs in a fine nerve which ultimately joinsone of the ventral unpaired nerves; another runs along the edge of the transverseintersegmental muscle and enters a strand of connective tissue that anchors themain longitudinal tracheal trunk to the body-wall. The third process, probablythe axon, runs into a nerve-branch supplying the ventro-lateral muscles. Thisnerve-branch ultimately joins the main segmental nerve. The next neurone(mm) is situated in a capsule adjoining the motor-nerve which the thirdprocess from cell mm enters (fig. 11, D, E, G). One dendrite innervates thepedicel and associated imaginal bud that is attached to a trachea. A secondinnervates the epithelium of a trachea (fig. 11, B, C), a third runs into the

abdomen and thorax of the blowfly larva 237

motor-branch adjoining the capsule of the cell-body, a fourth runs towardscell mm, and a fifth, which is most likely the axon, enters the main segmentalnerve. Cell mnT, is situated within the nerve-trunk that innervates the lateraltransverse muscles (figs. 9; n , 1). Sometimes a fourth cell, mn\, is situatedadjacent to m«3 (fig. 11, j). One process from each of these cells always runsin the nerve towards the central nervous system. These are considered to be

0 3mmFIG. 9. Drawing of the musculature of the lateral region of the second abdominal segmentof a Phormia larva to show the topography of neurones mm, mm, and mn^ and the distribu-tion of their processes, con, connective-tissue strand; ib, imaginal bud; mm, mnz, mn2, Multi-

polar neurones; t, trachea; uvn, unpaired ventral nerve.

the axons. One process from cell mn^ leaves the main nerve-trunk and runsover the epithelium of a trachea. The other processes apparently innervatethe nerve-trunk and its branches. Cell mn\ often appears to be associated witha strand of connective tissue that tags the nerve to adjacent muscles (fig. 11, j).In the first abdominal segment mn^ and m«4 are always present. In this casethe nerve-cells are situated in a capsule that is attached to the nerve-trunk.The capsule is often anchored by a strand of connective tissue to a nearbynerve. The axons leave the capsule and enter the nerve-trunk. The dendriticprocesses always innervate the epithelium of a trachea (fig. 11, F, H).

DiscussionIn the larva of Phormia the cuticle of the abdomen and thorax is completely

free of hair sensilla, unlike other cyclorrhaphan larvae such as Drosophila

238 Osborne—The sensory neurones and sensilla in the

and Eristalis, which have numerous hairs (Viallanes, 1882 a, b; Hertweck,1931). This may have led Bolwig (1946) to believe that the abdomen andthorax of Musca larva were completely free of any sensilla. Viallanes, con-trary to Bolwig's belief, found campaniform organs in the Musca larva whichresemble those of Phormia. These he termed 'sensory buttons'.

The other type of cuticular sensillum found in Phormia, which consists of 3neurones associated with a cuticular pore, does not appear to have beendescribed before on the abdomen and thorax of dipteran larvae. In view ofits structure it is probably a chemoreceptor (Snodgrass, 1926; Dethier andChadwick, 1948).

In cyclorrhaphan larvae (Keilin, 1915) cutaneous sensory organs are said tobe present where the imaginal bud pedicels join the cuticle. No sensory in-nervation of these regions has been found in Phormia.

Bolwig's theory that tracheal end-cells were mistaken for multipolarneurones is completely disproved by the present work, as the 2 types of cellcan be clearly distinguished (fig. 10, c, D).

The dendrites of the neurones constituting the sub-epidermal nerve-plexusdo not anastomose to form a 'nerve-net'. It was noticed during this work thatmethylene blue often stained the nerve-sheaths and not the nervous elements.As has been previously mentioned, the sheaths enveloping the processes ofdifferent neurones often fuse to form common nervous pathways for variousdendrites, or dendrites and axons. In addition connective-tissue fibres oftenjoin adjacent nerve-sheaths together. It seems fairly clear from Viallanes'sand Hilton's drawings that they drew anastomoses between the nerve-sheathsand not the nerve-fibres. Thus this may have lead previous authors to concludethat the nerve-cell processes anastomose. Barbier (1961) and Hamon (1961)found sensory fibres innervating epidermal cells which they were unable totrace to nerve-cell bodies. However, no such fibres were found in Phormia.

Various functions have been attributed to the sub-epidermal nerve-plexusin insects such as mechanoreception and thermoreception (Snodgrass, 1926).On the other hand, Alexandrowicz (1957) suggests that the sub-epidermal

FIG. 10 (plate). Photomicrographs of preparations of the Phormia larva stained with methy-lene blue.

A, sub-epidermal nerve-plexus from the dorsal region of an abdominal segment.B, bipolar neurone from a companiform sensillum.C, multipolar neurone enclosed within a neurilemma capsule.D, tracheal end-cell.E, cuticular dome of a campaniform sensillum.F, cuticular pore of a sensillum, possibly a chemoreceptor.c, the oblique chordotonal organ from an abdominal segment.H, neurone and connective-tissue strand of a longitudinal stretch receptor.1, posterior attachment of a longitudinal stretch receptor.J, two multipolar neurones from the dorsal nerve-plexus. Note connective-tissue strands

connecting their neurilemma sheaths together.K, posterior attachment of a ventral stretch receptor to an oblique skeletal muscle-band.

Note the muscle-fibre which leaves the muscle-band and enters the connective-tissue strand.ax, axon; bn, bipolar neurone; co, chordotonal organ; con, connective tissue; mn, multipolar

neurone; n, neurone.

abdomen and thorax of the blowfly larva 239

nerve-plexus in Crustacea may furnish sensory information on the stateof the cuticle prior to and during ecdysis. Multipolar neurones are foundin the largest numbers in soft skin larvae and are also associated with thearticular membranes of adult insects, that is to say they are situated in areaswhere the cuticle is subjected to greatest flexion. This suggests that theneurones respond to cuticular movement. If this is so, then in Phormia it ispossible that the multipolar neurones respond to cuticular bending, whereasthe campaniform sensilla respond to strains set up in the cuticle during move-ment (Pringle,i938). As the dendrites terminate beneath the epidermal cellsand not beneath the cuticle it is highly unlikely that they are contact thermo-receptors. One cannot, however, dismiss the fact that the nerve-endings may bestimulated directly by radiant heat energy. Alexandrowicz's suggestion thatthese neurones relay information during ecdysis is an interesting possibility,but at least in insects this cannot be their only function, since they are found ininsects which have undergone the final moult. However, before any seriousconclusions are drawn concerning the function of the sub-epidermal nerve-plexus, electron microscopic studies and electrophysiological experimentsshould be attempted in order to determine the site of the dendritic terminationsand to furnish clues as to their function. Whatever the function of the peri-pheral nerve-plexus, it is certainly not clear how the central nervous systeminterprets the afferent responses from these neurones because of the consider-able overlap of the dendritic fields from adjacent neurones and the apparentrandomness of the dendritic distribution. A similar situation is found in thefree nerve-endings of the vertebrate skin. The one constant feature of theperipheral nerve-plexus is the topographical position of the cell-bodies. Thusit may be that the positions of the cell-bodies are the spatial frames of reference

•onesF I G . 11 (plate). Preparations from Phormia larvae stained with methylene blue.A, 3 multipolar neurones from the dorsal sub-epidermal nerve-plexus. One of the neur

has a cell-body that is about twice the diameter of the other 2 neurones.B, c, photomicrographs of the same preparation at two focal planes to show the dendrite

from cell mm that runs over the epithelium of a trachea.D, cell mnz in its capsule. Note the process that innervates the imaginal bud pedicel.E, topography of cells mm and mm. The former is attached to the edge of the transverse

intersegmental muscle; 2 of its dendrites run along the edge of this muscle, and a third process,possibly the axon, runs towards cell mm. Note that the axon of cell vim enters the segmentalnerve and also that one of its processes runs to an imaginal bud pedicel.

F, cells m«3 and m«4 in the first abdominal segment. They are situated in a neurilemmacapsule adjoining the segmental nerve. The dendrites run to a trachea.

G, cell mm in its neurilemma capsule. Note the distribution of its processes. Cell mm isfaintly stained.

H, cell mti3 from the first abdominal segment. Note that the distal process runs to a tracheawhere it branches and subdivides and runs over the tracheal epithelium.

1, cell m«3 from the fourth abdominal segment is situated in the nerve-trunk. Its processesapparently innervate the neurilemma of the nerve-trunk and its branches.

j , cells m«3 and TO«4 in the nerve-trunk. Cell mti4 is associated with a strand of connectivetissue.

K, photomicrograph showing connective-tissue fibres connecting adjacent muscle-bandstogether.

ax, axon; con, connective tissue; d, dendrite; ibp, imaginal bud pedicel; mm, mnz, mni,mn\, multipolar neurones; 'mot, motor nerve; t, trachea.

240 Osborne—The sensory neurones and sensilla in the

that the central nervous system utilizes when decoding the input from thesecells.

The blowfly larva has longitudinal, vertical, and ventral stretch receptors.The longitudinal and ventral receptors are similar to the hymenopteran longi-tudinal receptor (Finlayson and Lowenstein, 1958), comprising a tube ofconnective-tissue fibres in which run the dendrites of the sensory neurone.The vertical receptor has a structure reminiscent of the vertical receptor foundin the more primitive orders of insects, especially the Ephemeroptera andPlecoptera (Osborne and Finlayson, 1962). The ventral receptor appears to beunique, since receptors associated with the body musculature of arthropodswere thought to be restricted to the dorsal muscles. The grub-like locomotionof the dipteran larva may have influenced the development of the ventralreceptor. Perhaps more efficient co-ordination of the body segments is neces-sary for this type of locomotion than in insects, where locomotion is performedby the thoracic appendages. Thus the ventral receptor may be a larval special-ization. However, a more thorough survey of the ventral muscles in insects andother arthropods should be undertaken before speculations are made con-cerning the evolution of the dipteran ventral receptor.

The physiological significance of cells mm, mnz, mn^, and mn\ is obscure.In all abdominal segments investigated, other than the first, the dendrites fromcells WZW3 and mn\ appear to innervate the nerve-sheaths, and perhaps they arestimulated by movement of the nerve trunk and its branches. Zawarzin (1916)reported neurones apparently innervating the neurilemma sheaths of thesympathetic nervous system of the gut, and Alexandrowicz (1953, 1957)described neurones in the motor-nerve trunks of Crustacea, but made nosuggestion as to their function. Cells mn$ and mnq. in the first abdominalsegment innervate the epithelium of a trachea. Perhaps these furnish infor-mation on tracheal flexion or may provide the central nervous system withinformation on the shedding of the tracheal lining during ecdysis.

Cells mm and mnz have an even more puzzling innervation. However,although cell mnz innervates the imaginal bud and its pedicel as well as thetracheal epithelium, it must be remembered that the imaginal pedicel andtracheal epithelium are extensions of the epidermis, so that in reality cells mtf$and mn\ in the first abdominal segment and cell mnz innervate epidermalderivatives. Therefore they may simply be extensions of the sub-epidermalnerve-plexus.

Cell mm innervates a strand of connective tissue anchoring the trachealtrunk to the body-wall, and perhaps its processes also extend to the trachealepithelium. It also has connexions with the somatic and ventral unpairednervous systems.

Perhaps all the above cells furnish information on pressure changes ormovement in the internal structures that they innervate, although in view oftheir diverse innervation it is difficult to see how the central nervous systemcould detect which structures were stimulating the nerve-cells. Further workis obviously necessary to determine whether these cells are present in other

abdomen and thorax of the blowfly larva 241

insects. A more detailed knowledge of the distribution of these types of nerve-cell would very probably provide useful clues concerning their function.

Meyer (1955) and Wigglesworth (1956) describe abundant connective tissuein insects, and the present work confirms their findings. Connective tissue isdistributed throughout the insect body, anchoring tracheae and tracheoles tothe epidermis, nerve-trunks to the epidermis and muscles, and connectingadjacent muscle-bands together (fig. 11, K). Connective-tissue fibres are alsogenerally distributed in the nerve-sheaths of the central and peripheralnervous systems. It is surprising that Edwards (i960) indicated that recentelectron microscopical studies failed to show a widespread distribution ofconnective-tissue fibres in insects, although collagen-like fibrils have beendescribed in the perilemma of insect nerves (Hess, 1958; Smith and Wiggles-worth, 1959).

I wish to thank Dr. L. H. Finlayson for his generous help and encourage-ment throughout this work.

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1894. Ibid., 16, 6.ORLOV, J., 1924. Z. wiss. Zool., 12a, 425.OSBORNE, M. P., and FINLAYSON, L. H., 1962. Quart. J. micr. Sci., 103, 227.PRINGLE, J. W. S., 1938. J. exp. Biol., 15, 101.ROGOSINA, M., 1928. Z. Zellforsch., 6, 732.SLIFER, E. H., and FINLAYSON, L. H., 1956. Quart. J. micr. Sci., 97, 617.SMITH, D. S., and WIGGLESWORTH, V. B., 1959. Nature, Lond., 183, 127.SNODGRASS, R. E., 1926. Smithson. misc. Coll., 77, 1.TONNER, F., 1936. Zool. Anz., 113, 125.VIALLANES, H., 1882a. Bull. Soc. philom. Paris, 6, 94.

18826. Ann. Sci. nat., 4, 1.WIGGLESWORTH, V. B., 1956. Quart. J. micr. Sci., 97, 89.ZAWARZIN, A., 1912a. Z. wiss. Zool., 100, 245.

19126. Ibid., 100, 447.1916. Rev. zool. russe., 1, 176.