Anim. Behav., 1991, 41,819-828

'Shooting' springtaiis with a sticky rod: the flexible hunting behaviour of Stenus

comma (Coleoptera; Staphylinidae) and the counter-strategies of its prey

T H O M A S B A U E R & M A R T I N P F E I F F E R Tier6kologie [, Universitiit, D-8580 Bayreuth, Germany

(Received20 October 1989; initial acceptance 30 November 1989; final acceptance 8 October 1990; MS. number: 3476)

Abstract. The behaviour of the beetle Stenus comma hunting three species of springtails was investigated. In almost half the attacks on prey, the beetle used its protrusible labium to catch the springtail. However, attacks using the labium were less successful when the springtail was large or was covered in scales or setae, as the prey item was more likely to become detached from the sticky tip of the labium. Thus the beetle was more likely to attack large springtails without using its labium.

Many arthropod predators, particularly those species that orient visually, wait in ambush or approach their prey slowly but strike rapidly and often with organs specialized for fast capture (e.g. Stomatopoda: Schaller 1953; Burrows 1969; Mantodea: Rossel 1983: Ranatra sp.: Bailey 1986; dragonfly larvae: Tanaka & Hisada 1980; Mantispa sp.: Ulrich 1965). A surprise attack on the prey is often the only way to overcome it if its reaction time is short and it is capable of fast take off.

Among the insects, one of the most sophisticated adaptations for hunting fast-fleeing prey is demon- strated by the modified labium of adults of the beetle Stenus sp. (Staphylinidae) (Schmitz 1943; Weinreich 1968; Fig. 1).

During attack on the prey the labium is pro- truded by haemolymph pressure. If the sticky cushions on its tip hit the prey, glue is apparently squeezed onto their surface from glands in the head, which serves to hold the prey in place. The labium is retracted immediately to pull the prey into the range of the mandibles.

Springtails (Collembola) are the main prey of Stenus species that inhabit the soil. However, they possess a very effective escape mechanism. When they are touched, the instantaneous action of their furca causes them to shoot into the air within 10-50 ms (Christian 1979; Bauer 1982a; Bauer & Christian 1987). This escape mechanism operates at a similar speed in all surface-dwelling springtails. Predatory insects that specialize in this group as prey are equipped with special behavioural and morphological adaptations by which the springtails are surprised and fixed at the moment of contact (for a review and examples see Bauer & Kredler 1988).

The labial apparatus of Stenus sp. seems to be ideal for attacking Collembola, provided that the sticky cushions hit and secure them. We hypothe- sized that the success of attack might vary with the size, movements and surface structures of the prey, which differ between springtail species.

To test this hypothesis we confronted Stenus comma Leconte 1863 (Steninae) with three spring- tail species which differ considerably with respect to these features (cf. Table I). Two of these, the riparian species Isotomuruspalustris Mfiller 1778 and Podura aquatica Linn6 1758, are common and frequent in the habitat of Stenus comma; Heteromurus nitidus Templeton 1835, on the other hand, is ubiquitous in the upper layer of various soils. In this paper we analyse the attack process and the rate of cap- ture with respect to special features of the prey species.

METHODS

The beetles were caught on the edge of ponds at the bottom of sand pits in northern Bavaria, kept indi- vidually in vessels on moist plaster of Paris at 20~ and fed with Collembola from a laboratory culture. Before experiments were performed the beetles were starved for 4-5 days.

The Collembola were caught in the habitat of the beetles or taken from a laboratory culture, kept on moist plaster and fed with soybean flakes.

High Speed Photography

We filmed 15 attacks by the beetles at rates of 1000-1050 frames/s using a Hycam camera and a

0003-3472/91/050819 + 10 $03.00/0 �9 1991 The Association for the Study of Animal Behaviour 819

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Table I. Differences in locomotor behaviour, surface structure and size of the springtail species

821

Podura Heteromurus lsotomurus Springtail species aquatica nitidus palustris

Active (in % of the observation time) Duration of the pauses (s;X_ so) Running speed (cm/min, X+sn) Changes of course per min (X_+ so) 45-90 ~ angles > 90 ~ angles Body surface

Body length of the attacked springtails (cf. Table II) (mm; X+__ so)

Body weight (mg; .Y+ SD)

90.1% 96.2% 39.1% 25.0+19.4 12-3___11.6 52.0_+80.1 6.91_+0.59 31.72__+3.56 43.41__+4.62

1.0___0.996 13.88_+2.92 7.15+3.31

0.40__+0.08 8.39___2.13 4.24_+0.24 Covered with Covered with Covered with

small tubercles scales setae 0.787 _ 0.14 1.32_ 0.24 1.95_+ 0.31

0.075+0.014 0.16_+0.029 0.45__+0.072

Strobokin flash lamp (Impulsphysik, Hamburg) as the light source. A beetle and a single springtail (H. nitidus) were placed together in a cuvette (ground space 25 x 20 mm) with a floor of frosted glass. The light of the flash lamp was directed from below so that the films show silhouettes of the beetles and their prey. The recordings were made in the Institute fiir Wissenschaftlicher Film (IWF) G6ttingen, Germany.

Video Recordings A Panasonic NV-180 recorder, a GXN8E

camera and a TV-screen (56 x 44 cm) were used. The behaviour of the animals was recorded in cylin- drical glass cuvettes with circular floors. The floors were covered with moist plaster and the inside sur- face of the walls was painted with dull black paint to reduce reflection.

To analyse the movements of the springtails we used a cuvette 9 cm in diameter and 5 cm in height. The light intensity was 1500 lx at the bottom of the vessel and the temperature 20~ We filmed five individuals of each species, each for 11 min, and recorded the duration of activity periods and pauses. To determine the running speed and changes in course the path of five springtails per species which moved continuously for 2 min was drawn on transparent paper mounted on the video screen. The length of the path was measured with an odometer, the changes of the course with a goniometer. Only abrupt changes of greater than 45 ~ were included in the evaluation. The results are summarized in Table I.

The recognition distance of S. comma was evalu- ated in the same vessel under the same conditions

confronting individual beetles with three specimens of H. nitidus at a time, each 1.5 mm long, or with three specimens ofP. aquatica 0-8 mm in length. We used 18 beetles for these recordings, and evaluated 160 recognition events per prey species, eight or nine per beetle.

We recorded the attacks on springtails using the same conditions but smaller vessels (diameter: 3 cm, height: 6 cm) to obtain better resolution. These recordings were made with 117 beetles, and 8-12 attacks per beetle were evaluated (Fig. 2).

The body length of the springtails was deter- mined under a binocular microscope and the weight on a microbalance (Sartorius 2004 MP). Length/weight calibration curves were plotted for the three springtail species so that the body mass of each individual that was attacked could be estimated from its length.

The dependence of failure or success of attack with respect to prey species and size was tested for significance with the F-test, t-test (Sachs 1982) and chi-squared test of independence (SPSS/PC+, SPSS-Inc., Chicago).

Scanning Electron Microscopy

Specimens of the beetles and springtails were fixed in alcohol, dried to the critical point (CPD---020 Critical Point Apparatus, Balzers, Liechtenstein), glued to stubs with silver paint, coated with gold (Sputter Coater S 150 B, Edwards, Crawley, West Sussex, U.K.) and viewed in a Stereoscan ($90, Cambridge Instruments, Cambridge, U.K.).

822 Animal Behaviour, 41 ,5

Attacks

/ \ Attacks with Attacks without labial apparatus labial apparatus

Prey hit by the Prey missed (no / \ sticky cushions contact with the /

sticky cushions)

/ \ Prey seized with Prey detached from Prey seized with the mandibles the sticky cushions the mandibles

Prey missed (touched but not seized)

Figure 2. Scheme of evaluation of the attacks of S. comma.

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Figure 3. Frequency distribution of recognition distance of S. comma confronted with ft. nitidus (Q) and P. aquatica ( � 9 as prey.

R E S U L T S

Predatory Tactics While searching for prey S. comma moves in

jerks. Moving prey animals are mainly recognized during the short pauses. The first reaction of the beetle is then to turn to the moving object.

In our experimental arrangement the mean (___ so) recognition distance (distance between head

of the beetle and the closest point of the prey immediately before the beetle first turns towards it) was 10.65_+6-36mm with H. nitidus as prey and 10.14_+6.01 with P. aquatica. More than 60% of the turns were released from between 5 and 15 mm and less than 1% from more than 30 mm (Fig. 3).

The approach is continued in a jerking fashion if the target moves. However, if a springtail remains absolutely motionless, which was often observed

Bauer & Pfeiffer: Beetle predation on springtails 823

with L palustris as prey (Table I), S. comma turns away after a while and continues its search else- where. Surprisingly, our recordings revealed that S. comma attacks both with and without making use of its labium.

The range of the extended labial apparatus lies between 1-2 and 1.4 mm which is about 24% of the body length. (The values were derived from high speed film recordings of five attacks where the fully extended labium missed the prey.) In (successful) attacks with the labium the beetles approached to within 0-817 • 0.375 mm (N= 30) of the prey. From this distance the labium is protruded while the beetle either maintains the position of its body or lunges less than 1 mm forward (Fig. 4). The protru- sion of the labial apparatus takes 1-3 ms. Immedi- ately afterwards the beetle withdraws for 30-40 ms. After this movement it takes another 100 ms until the labium is completely retracted.

In (successful) attacks without the labium the beetles approached to within 0"789+0.194mm (N= 30). Then the body is hurled forward over a distance of up to one-half of the body length but without protrusion of the labium (Fig. 4). This fast forward lunge takes about 30 ms. A short back- ward movement of the body with simultaneous lift- ing of the head was also observed in this type of attack. This sudden retreat, which has also been observed with other visually hunting beetles (Bauer 1981, 1985a), serves to snatch the seized prey away from the ground and to reduce its chances of freeing itself by jumping.

The labium was used in 42.7% of all attacks; 33.9% of all the attacks were successful. The success rate was greatest for P. aquatica and least for H. nitidus (Table II).

Counterstrategies of Prey

Locomotion

Podura aquatica walks slowly but continuously and does not change its course very often. The running speed of H. nitidus, which is also con- tinuously active (Table I) is 4.6 times greater. In addition this species changes its course about 16 times per unit time more often. Isotomuruspalustris is less active than the other two prey species. When it moves, it runs even faster than H. nitidus but with fewer deviations. These differences should affect the success of labial attack because an unpredict- ably moving target is more difficult to hit. This was indeed observed (Table II). Labial attacks on

H. nitidus were more likely to miss than on the two other species (P<0.001) while attacks on L palustris and P. aquatica were equally successful.

Surface structure

Scales or setae on the prey (Fig. 5) should reduce the adhesion of the sticky cushions and affect the rate of success of labial attacks relative to those without the labium. With P. aquatica which has a smooth surface both types of attack were equally successful (Table II). Labial attack was signifi- cantly less successful with H. nitidus (P < 0'01); and I. palustris (P < 0-05).

Further evidence of the effectiveness of scales and setae comes from comparing attacks on P. aquatica and specimens of the other two species with a similar body mass: 12-2% of the attacked H. nitidus and L palustris specimens were between 0.5 and 1 mm and weighed between 0.007 and 0'06 mg. In 65.4% of the hits these small specimens became detached from the sticky cushions. This is significantly more (P < 0-001) than with P. aqua- tica, where only 10.5% of the 133 specimens that had been hit by the labium became detached. The success of attacks with the labium on H. nitidus and L palustris individuals of similar size did not differ. Scales and setae are obviously of similar protective value.

Body Mass

The influence of the prey's body mass is clearly shown with individuals of H. nitidus and L palustris that were hit by the sticky cushions and then became detached (Table II): more of the larger L palustris became detached (P < 0.01). With L palus- tris the individuals that were hit and seized were significantly (P< 0.001) smaller ( ,~ . so = 1-68 • 0-490mm; 0.307+0'090mg) than those that became detached 2-00+ 0.289 mm; 0.419 • 0.061 rag).

Prey Size and Type of Attack

Larger and heavier springtails are more likely to become detached from the labium. Thus we expect S. comma to attack larger prey without using the labium. Indeed, the percentage of attacks with the labium decreased with increasing size of the prey (Table II): 60.1% with P. aquatica (mean size 0'79mm), 49.2% with H. nitidus ( l '32mm) and 22-1% with I. palustris (1.95 mm). These differences are significant (P < 0-001).

824 Animal Behaviour, 41, 5

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Table II. Stenus comma attacking different collembolan species

825

Podura Heteromurus Isotomurus Springtail species aquatica nitidus palustris

Observed attacks (N) 276 516 399 Successful attacks (%) 75.0 19.4 24.3 Attacks with the labial apparatus (N) 166 254 88 Prey missed (no contact with the sticky 19.9 55.5 27.3

cushions, %) Prey hit by the labial tip (%) 80.1 44.5 72.7 Prey seized with the mandibles (%) 71.7 13.4 14.8 Prey hit but detached from the sticky 8.4 31. I 57.9

cushions (%) Attacks without labial apparatus (N) 110 262 311 Prey seized with the mandibles (%) 80.0 25.2 27.0 Prey missed (only touched but not seized, 20.0 74.8 73.0

%)

Figure 5. The body surface of the springtail species used in the experiments. (A) Isotomurus palustris; (B) Heteromurus nitidus; (C,D) Podura aquatica.

This is also observed if we examine the behaviour of S. comma in hunting differently sized specimens of the same prey species. In our experiments 50.4%

of the specimens of H. nitidus had a size range of 0 .5-1 .2mm and 49.6% between 1-2 and 2 .1mm. The attacks on the smaller specimens were carried

826 Animal Behaviour, 41, 5

out with the labium significantly more often (57%) than those on the larger specimens (41.4%; P < 0-004).

D I S C U S S I O N

Counterstrategies of Prey

The main defence mechanism of the majority of springtails is their jumping ability, which enables them to leave the range of a predator in a few milli- seconds. The problem with this flight mechanism, however, is that it is released mainly by mechanical stimuli. No species is known that can detect visually an approaching predator of its own size and evade it at a safe distance by jumping. Flight jumps are normally released by touching, and it always takes between 10 and 50 ms for the springtails to leave the ground after mechanical stimulation (Bauer & Christian 1987). Nearly all species that feed on springtails surprise their prey with a rapid attack and are able to fix them at the moment of contact so that escape by jumping is impossible (Bauer & Kredler 1988).

Both types of attack of S. comma work in this way. During labial attack the labial apparatus is protruded rapidly (1-3ms), and if the beetle's sticky cushions meet and attach to the springtail, it has no chance to escape.

In attacks without the labium the behaviour of S. comma corresponds to that of some ground beetles that feed on Collembola (Bauer 1981, 1985a). The body is hurled forward from a short distance to the prey with open mouthparts that are snapped shut at the end of this lunge, the extent of which is always somewhat greater than the final approach distance (the prey is always touched, cf. Table II). The springtail can escape only if the mouthparts fail to get a good grasp on it.

The present study shows that, in addition to the main escape mechanism of Collembola, which operates in the same way in the three species used in our experiments, certain species-specific features exist that are of protective value.

In the field I. palustris normally moves very little or only very slowly while feeding and is well pro- tected by its cryptic colour. Only disturbed animals flee by fast running or jumping and then disappear into soil cavities. Avoiding continuous and there- fore conspicuous motion is a common strategy of avoiding attack by a visual hunter, and it is wide- spread among springtails (Bauer & Christian 1987)

and other shore insects. The larvae of Saldidae (shore bugs), for example, have well developed compound eyes and recognize the approach of S. comma visually. However, unlike the adults, which escape by jumping and by flight, the larvae freeze and thus avoid triggering the attack (Griesinger & Bauer 1990).

Podura aquatica lives in aggregations close to or on the water. This species has no protective colour. It is dark blue and can often be seen walking about even if it is not disturbed. So it has nearly 'ideal' releaser qualities for the attack of a visual hunter. However, this species also has a chemical defence mechanism (Larsen 1939; Weinreich 1968). Although these springtails were caught in 75% of all attacks, 88 % of the seized animals were released within a few seconds and nearly 50% of them started to move around immediately afterwards, indicating that they were not seriously hurt by the capture. After attempted capture the beetles cleaned their mouthparts by rubbing them on the soil, and it normally took a while until a renewed attack could be released. Conspicuousness together with an effective chemical defence secretion ensures that its distastefulness is quickly learned by preda- tors. Although we do not know whether S. comma is able to learn (as, for example, a praying mantis can, Gelperin 1968) P. aquatica was refused as prey for some time after a contact.

The special features that allow H. nitidus to avoid predation resemble those of other litter species. They all run fast and unpredictably if disturbed (Bauer & Christian 1987).

Scales and setae also helped to protect H. nitidus and L palustris. Scales are found in very different insect groups and may have rather different func- tions (cf. Larink 1976). In springtails they occur in only a few families and are homologous to the long and distally thickened or fringed setae that are common in this group. Scales and setae are easily removed and always found sticking to the mouth- parts of arthropods that feed on these insects. Probably one of the main functiovs of scales is to increase the chances of escape of insects that possess them, because they reduce the friction and the adhesion of the hunting equipment (mouth- parts, adhesive structures, webs etc., cf. Eisner et al. 1964; Nentwig 1982) of the predators.

Prey Size and Type of Attack The importance of prey size for visual hunters

has been demonstrated for many species (e.g.

Bauer & Pfeiffer: Beetle predation on springtails 827

for Asilidae: Lavigne & Holland 1969; Dennis & Lavigne 1975; Shelley & Pearson 1978; Hespenheide & Rubke 1977; Mantodea: Holting 1964; Loxton & Nicholls 1979; larvae of Odonata: Baldus 1926; Mokrushov & Zolotov 1973; Thompson 1978; Ranatra sp.: Bailey 1986) and the present study shows this to be so for S. comma as well. Surprisingly, S. comma attacks particularly large prey without using the sticky labium. This behavioural peculiarity compensates for a special deficiency of the labial attack mechanism: the adhe- sive strength of the sticky cushions is limited, and larger prey items may become more easily detached.

The majority of other insects with organs special- ized for attack seem always to use these organs during attack. Mantodea or Heteroptera with specialized forelegs never seem to attack without using them (see Bailey 1986). For dragonfly larvae the approach and handling time vary considerably with respect to the prey species (Blois 1985), but the attack itself always seems to be made with the protrusible labium.

However, we suggest that the plasticity of the attack behaviour observed here is more common than other observations suggest. It is only difficult to observe. Weinreich (1968), who worked intensively with Stenus sp., did not recognize that they also attack without using the labium, and also we did not observe this ourselves until we analysed the high speed film recordings of attacks.

Another case in which the type of attack varies according to the size of the prey was reported by Griesinger & Bauer (1990). Shore bugs, Saldula saltatoria, normally attack by extending their beak and trying to sting moving objects after a visually controlled approach. Large prey items such as Diptera, which are nearly as large as thebug, how- ever, are attacked by jumping on their back from various distances.

We do not yet understand the biological advan- tage of the highly specialized labial apparatus. We thought that it must be the ideal tool for overcom- ing the rapid escape response of Collembola. How- ever, S. comma's rate of successful attacks is far below that of other specialized springtail hunters. With H. nitidus as prey, the ground beetles (Carabidae) Loricera pilicornis (7.5mm) and Leistus rufomarginatus (9 mm), for example, which attack with specially arranged setae on head and antennae, respectively (Bauer 1982b, 1985b), have a success rate of nearly 80% (S. comma: mean

34%, with H. nitidus as prey 19.4%, Table II). The success rate ofNotiophilus biguttatus (5 mm), which has better optical resolution than S. comma but 'normal' mouth-parts, is about 53% (Bauer 1981). Moreover S. comma is able to catch springtails of any size even without its labium, so it does not seem to be essential for survival. However, the results cannot be automatically applied to the situation in the field and to other species. For example, the labial apparatus may be well suited to catching small prey specimens in narrow cavities of the soil, into which the beetles cannot pursue their prey, and it is prob- ably also an advantage for Stenus species that mount plants and hunt in the three-dimensional space of vegetation where approaching prey is more difficult than on the soil.

A C K N O W L E D G M E N T S

We thank Werner Arens for support in performing the REM analysis, Mechthild Kredler for tech- nical assistance and Patricia Nevers for help in correcting the English of this manuscript. This paper is dedicated to F. Schaller, Vienna, on the occasion of his 70th birthday.

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