Author's personal copy ARTICLE IN PRESS ZOOLOGY Zoology 111 (2008) 2–8 www.elsevier.de/zool Orientation of the pit-building antlion larva Euroleon (Neuroptera, Myrmeleontidae) to the direction of substrate vibrations caused by prey$ Bojana Mencinger-Vracˇko, Dusˇan Devetakà Department of Biology, Faculty of Natural Sciences and Mathematics, University of Maribor, Korosˇka 160, 2000 Maribor, Slovenia Received 13 March 2007; received in revised form 8 May 2007; accepted 10 May 2007 Abstract Pit-building antlion Euroleon nostras constructs efficient traps in sand to catch its prey. The predator is known to react to substrate vibrations produced by movements of its prey outside the pit with sand-tossing behaviour but it has not yet been ascertained if this reaction is directed towards the prey. The accuracy of the sand-tossing response in the presence of four prey species was measured using a video recording method. The sand-tossing angle was highly positively correlated with the prey angle. Sand tossing was most frequently elicited when prey was on the posterior sand surface. Covering the larval photoreceptors did not influence the antlion’s localizing behaviour. r 2007 Elsevier GmbH. All rights reserved. Keywords: Antlion; Myrmeleontidae; Predatory behaviour; Sand tossing Introduction avalanches typical of crater angles (Fertin and Casas 2006). The larvae of most antlion species (Neuroptera: When the prey arrives at the bottom of the trap, the Myrmeleontidae) are sand-dwelling insects, but only a larva rapidly grasps it with its mandibles. If the antlion few of them construct conical pits in dry, loose sand to does not succeed in catching the prey at its first attempt capture prey (for reviews see Gepp and Ho¨lzel 1989; or if the prey evades the antlion and tries to climb up the Scharf and Ovadia 2006; Devetak et al. 2007a). While walls of the trap, the larva tosses sand with violent flicks antlion larvae feed on a variety of arthropods, ants of its head and mandibles until the prey slides back to usually constitute the majority of prey items (Topoff the antlion’s jaws (Topoff 1977; Griffiths 1980). 1977; Griffiths 1980; Gepp and Ho¨lzel 1989). The pit The function of pit-building is obvious. The pit functions by conveying the prey towards the pit centre funnels prey to the antlion’s mandibles, and it also (Lucas 1982). The pit is an efficient trap, with slopes retards the escape of prey and consequently increases steep enough to guide prey to the mandibles without any the amount of time the prey is available for capture. attack, and shallow enough to avoid the likelihood of According to Mansell (1996, 1999), the advantages of constructing a pitfall trap as a predation strategy also include: (i) the need to hunt or pursue prey is reduced, $This paper is dedicated to Matija Gogala on the occasion of his 70th birthday. thereby conserving energy; (ii) the pit is a selective device ÃCorresponding author. Tel.: +386 22282160. for prey of a suitable size, because large prey would be E-mail address: [email protected] (D. Devetak). able to escape, and energy is not wasted on unsuccessful 0944-2006/$ - see front matter r 2007 Elsevier GmbH. All rights reserved. doi:10.1016/j.zool.2007.05.002 Author's personal copy ARTICLE IN PRESS B. Mencinger-Vracˇko, D. Devetak / Zoology 111 (2008) 2–8 3 attacks; (iii) it affords protection as large species falling adults of woodlice Trachelipus rathkei Brandt, ant into the pit cause the larva to retreat; (iv) fast-moving Formica sp., firebug Pyrrhocoris apterus (L.) and meal- prey can be intercepted by the pit; (v) prey can be worm beetle Tenebrio molitor L. rapidly subdued as it is disorientated upon falling into To reduce vibratory noise from the surroundings the the pit. A pit retards prey escape regardless of the plastic container with sand was placed on a sand layer presence or absence of an antlion (Devetak 2005). and this rested on cork, a mineral-wool layer and a The antlion detects its prey from a distance of a few concrete plate (75  50  4cm3, 37 kg) supported by a centimetres by sensing the vibrations that the prey mineral-wool layer. In addition, the whole setup was produces during locomotion (Devetak 1985; Mencinger placed on a vibration-free table. All measurements were 1998; Devetak et al. 2007b). It is known that sand- done in an anechoic chamber (130  90  160 cm3). tossing behaviour occurs during construction of the pit A prey item was carefully placed on the sand surface and when the prey is already trapped in the pit and tries 20 cm away from the centre of the pit and the activity of to evade the predator (Youthed and Moran 1969; the prey and the predator-prey interaction was video- Topoff 1977; Griffiths 1980). In this study sand-tossing taped at a distance of ca. 70 cm. Recording commenced behaviour of the European antlion species Euroleon with prey introduction and ended when the prey either nostras (Geoffroy in Fourcroy, 1785) is determined as a escaped from the area, or was consumed. Trials were response to prey prior to entering the pit. The larva evaluated only if the antlion responded to the presence reacts to the presence of prey approaching the pit rim of a prey item within 2 min. without seeing it. The aim of our study is to answer the The direction in which the head of the antlion was question whether this sand-tossing reaction is directed pointing is indicated as 01, and the direction of the end toward the prey or not. of the abdomen as 1801 (Fig. 1). At the moment when the larva reacted to the presence of a prey item with sand tossing, we measured two azimuth angles, the angle between the antlion’s long axis and the position of the Materials and methods prey (prey angle), and the angle between the antlion’s long axis and the end point of the sand toss (sand- Third-instar larvae of E. nostras collected in the tossing angle). The sand area in front of the head with surroundings of Maribor, Slovenia, were used in the jaws was defined as the anterior sand surface and the present study. Larval stages were determined by mea- sand area behind the caudal/dorsal part of the antlion’s suring head capsule width and body length (Devetak body as the posterior sand surface. Additionally, we et al. 2005). measured the distance between the midpoint of the prey Antlion larvae were kept in the laboratory at room and the centre of the pit. As larvae reacted at different temperature (22–26 1C) in sieved sand. In all experi- distances, only the maximum distance was evaluated ments fine-grained quartzite sand (Kema Puconci d.d.) during each trial. The depth and diameter of the pit were with the following grain size (gs) distribution was used: gs o0.1 mm, 15.5 weight% (wt%); gs 0.1–0.25 mm, 32.9 wt%; gs 0.25–0.315 mm, 15.2 wt%; gs 0.315– 0.5 mm, 29 wt%; gs 0.5–1 mm, 4.4 wt%; gs 1–1.5 mm, 3 wt% (mechanical analysis with sieves according to DIN 1170 and DIN 1171). In a previous study (Devetak et al. 2007b) it was shown that this kind of sand is most convenient for successful prey-catching behaviour in antlions. The moisture content of the sand was less than 2% by weight. Before experimental treatment the larvae were kept singly in plastic containers (25  20  5.5 cm3) filled with sand. Workers of the ant species Lasius fuliginosus (Latreille, 1798) and L. emarginatus (Olivier, 1791) were used as food source for the antlions. Feeding took place every day and one ant was delivered to each pit. Antlions used in the experiments were placed singly into a plastic container (40  30  5.5 cm3) filled with sand to a depth of 5 cm.Measurement started a day after the larva had constructed a pit in the centre of the Fig. 1. Sand-tossing behaviour of Euroleon nostras in the container. All prey animals were adults and laboratory- presence of prey. The antlion is positioned in the middle of the bred. The prey animals used in the experiments were bottom of the pit, and the prey is located outside the pit. Author's personal copy ARTICLE IN PRESS 4 B. Mencinger-Vracˇko, D. Devetak / Zoology 111 (2008) 2–8 also determined. From the data obtained we calculated from a PC monitor. Trials were not evaluated when a reaction distance, i.e. the distance between the prey and prey item was moving at the rim of the pit. The accuracy the antlion sitting at the bottom of the pit (see Devetak of the sand-tossing response was measured from selected 1985). frames using the computer program Mirror Video- Details of the antlion’s sand-tossing behaviour were Analysis for Windows. Selected frames were super- recorded on video tape and then traced frame-by-frame imposed on each other and prey and sand-tossing 0° 0° 0° 0° 270° 90° 270° 90° 270° 90° 270° 90° 180° 180° 180° 180° Tenebrio molitor Trachelipus rathkei 350 350 300 300 250 250 200 200 150 150 100 100 50 50 Sand tossing angle (degrees) 0 Sand tossing angle (degrees) 0 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 Prey angle (degrees) Prey angle (degrees) 0° 0° 0° 0° 270° 90° 270° 90° 270° 90° 270° 90° 180° 180° 180° 180° Formica sp.