Optimal Range of Prey Size for Antlions

Ecological Entomology (2015), 40, 776–781 DOI: 10.1111/een.12254

Optimal range of prey size for antlions

ANTOINE HUMEAU,1 JUSTINE ROUGÉ1 and , JÉRÔME CASAS1 2 1Institut de Recherche sur la Biologie de l’Insecte, UMR 7261 CNRS - Université François-Rabelais, Tours, France and 2Institut Universitaire de France, IUF, Paris, France

Abstract. 1. Antlions are opportunistic trap building predators that cannot control prey encounter. Their trap should ideally retain a great diversity of prey. However, building a single trap that captures many prey with varying characteristics can be challenging. 2. A series of five different ant species ranging from thin to large, of sizes ranging from 2.75 to 6.5 mm, and a mean weight ranging from 0.54 to 6.00 mg were offered in a random succession to antlions. The state of satiation of the antlions was controlled, and their mass and the depth of their pit were recorded. The reaction of antlion to the prey, the probability of capture as well as the time to escape were recorded. 3. The probability of an antlion reaction is an increasing function of the pit depth and a decreasing function of antlion mass. The probability of capture is highest for intermediate prey mass and is an increasing function of pit depth. The time to escape is a declining function of prey mass and an increasing function of pit depth. 4. There is an upper limit to prey mass given that large prey escape out of the pit. There is a lower limit to prey mass given the difficulty to apprehend the smallest, thin species. Consequently, there is a range of prey mass, corresponding to a medium-sized ant of 2 mg, for which the pit functions best. The physics of insect locomotion on sandy slopes was identified as the key to understanding the functioning of antlion pits. Key words. Ants, granular medium, pit building, predator–prey interaction, sand.

Introduction prey (Hansell, 2005). Traps can also expand the search area by increasing the accuracy of prey detection or the distance at Predation strategies vary from highly active approaches to pas- which prey can be detected. Many spiders build tubular traps sive ambush strategies (Griffiths, 1980a; Huey & Pianka, 1981; with horizontal extensions, helping them to detect the vibrations Pietruszka, 1986; Mansell, 1996; Perry, 1999; Cooper, 2005; generated by the prey (Coyle, 1986). Traps can also retain prey Elimelech & Pinshow, 2008). Ambush predators invest their within the range of predator, making it easier for the predator energy principally in subduing their prey, rather than search- to subdue it. The commonest method involves the use of glued ing for prey (Griffiths, 1980a). Some ambush predators build threads, such as those produced by Araneoidea spiders (Hansell, traps to enhance predation (Scharf et al., 2011; Klokocovnikˇ & 2005). Antlion and wormlion pits also retain prey within the Devetak, 2014). The trap-building activity is generally the range of the predator, but they do not make use of glued material costlier one, at least among the feeding activities, even if it (Wheeler, 1930). Their pit consists of a cone dug out of sand; the requires less than 1 day of maintenance activity for antlions larva sits at the bottom, waiting for prey to fall down. (Lucas, 1985; Tanaka, 1989; Elimelech & Pinshow, 2008). Trap The low mobility of trap building predators constraints them building predators account for less than 1% of all animal species to be dependent on the prey movements for the encounter (Ruxton & Hansell, 2009). Trap-based capture methods are (Scharf et al., 2006). The probability of prey encounter for dependent on the interaction between the predator, the trap, and a trap building predator is less controllable than for active the prey. Traps can intercept prey efficiently, even in water; predator (Elimelech & Pinshow, 2008; Tsao & Okuyama, some caddisflies, for example, build nets to filter their planktonic 2012). Thus, because of this prey dependence, trap building predators are opportunistic. The trap should ideally retain a Correspondence: Antoine Humeau, Institut de Recherche sur la great diversity of prey, i.e. a trap should capture prey with a Biologie de l’Insecte, UMR 7261 CNRS - Université François-Rabelais, large diversity of morphological or behavioural characteristics. Avenue Monge, 37200 Tours, France. E-mail: [email protected] Building such an ideal trap can, however, be challenging, owing

776 © 2015 The Royal Entomological Society Optimal range of prey size mass for antlions 777 to the one-to-many mapping. Can a predator capture all types of Overview of the experimental procedure prey equally? More specifically, is there an optimal size of prey? Because of the geometrical simplicity of the pit of antlion, Antlions were kept under standardised laboratory conditions this construction is a good model to study the interactions for 2 weeks to control for their state of satiation (Scharf et al., between the predator, the prey, and the trap (Fertin & Casas, 2009). The experiments also lasted for 2 weeks. Antlions had 2006). The prey trapped by antlions belong to at least 14 orders first 2 days to build a pit. Each antlion encountered then one of insects and also include arachnids, woodlice, earth worms, ant every day for 5 days. This phase of 5 days is named and millipedes (Réaumur, 1742; Heinrich & Heinrich, 1984; thereafter a ‘predation experiment’. The sequence ‘pit building Devetak, 1985; Lucas, 1986; Matsura, 1986, 1987; Mencinger, and predation experiment’ was repeated twice so that each 1998; Morrison, 2004). Walking arthropods in general account antlion encountered two ants of each species. We measured the for 66–85% of all prey, the remainder including notably flying depth of the pit every morning with two tubes. A tube was placed insects and insects living in trees (Lucas, 1986; Matsura, 1986, above the pit centre and the other was placed above and outside 1987; Griffiths, 1993). Beyond selecting walking prey, does the pit. The distance between the two ends was measured with a the pit select preferentially prey on other criteria? Ants are the rule to the nearest mm (Fig. 1a). We offered an ant to the antlion main taxon, accounting for between 35% and 70% of all antlion every afternoon during the predation experiment. Each antlion prey (Wilson, 1974; Heinrich & Heinrich, 1984; Lucas, 1986; was weighted to the nearest 0.1 mg the morning of the first day Matsura, 1986, 1987). A list of species identity cannot, however, of experiment and after the 2 weeks of experiments. We used the alone solve that question. An equivalent list of available prey mean of these two masses in the analyses. This procedure was around the pit is needed to assess the variability of capture repeated with two independent sets of antlions, during sessions success. This list will most likely be very sensitive to pit position, 1 and 2, beginning on 8 and 23 July 2013, respectively. at different spatial and temporal scales. An alternative way to solve that question is instead by identifying some of the prey variables determining capture success. We do so by focusing Standardisation before the experiment on ants, because they are the main prey, ant diversity is high and their morphology presents a wide variability (Pie & Tschá, Sixty-six and 53 antlions were captured for sessions 1 and 2, 2013). ‘Size’ refers, thereafter, to a global characteristic of the respectively, and maintained in the laboratory for 2 weeks. Each organism, including both mass and length. We took into account antlion was placed in a cylindrical plastic box (79 mm diameter both the antlion mass and the pit geometry as they can impact and 50 mm height) filled with Fontainebleau sand at a height of the entire capture process. around 40 mm. The grain size ranged from 0.100 to 0.315 mm. They were fed with one ant A. subterranea each day for 5 or 6 days for session 1 and 2, respectively. They were thereafter Materials and methods kept unfed for 7 days. At the end of the standardisation, we selected 40 antlions by session, based on their willingness to Materials build complete pits.

Antlion larvae Euroleon nostras (Geoffroy in Fourcroy) (Neuroptera: Myrmeleontidae) and the five ant species Ant assignment (Hymenoptera: Formicidae) came from Grandmont park in Tours, France (47∘21′N, 0∘42′E). Antlions were kept on an Ten ants of each species were captured each day during the LD 14:10 h cycle, beginning at 07.00 hours. The tempera- predation experiment and weighed to the nearest 0.1 mg. Each ∘ ture was kept constant at 22 C. We chose the species to antlion encountered one ant every afternoon and met a different test for the greatest possible range of mass of prey, with the species each day. Ant species were randomly assigned to an additional constraint of finding enough ants every day. The antlion for the first day. Then, each antlion met another ant ants were workers of Aphaenogaster subterranea (Latreille) species based on the alphabetical order of ant names. So, an (body length = 3.0–4.7 mm), Formica polyctena Foerster antlion meeting A. subterranea on the first day would meet (4.0–9.0 mm), Lasius brunneus (Latreille) (2.0–3.8 mm), F.polyctena on the second day. An antlion meeting T. erraticum Lasius emarginatus (Olivier) (2.4–3.9 mm), and Tapinoma on the first day would meet A. subterranea on the second day. erraticum (Latreille) (2.0–3.5 mm). The length of workers is from Bernard (1986). The colony of L. brunneus was the only one to live near an antlion zone. Statistical analyses The granular medium was made of microglass beads, type S (Sigmund Lindner Company, Warmensteinach, Germany) We recorded the reactions or the lack of reactions of antlions thereafter named ‘beads’. We used beads because their char- to the prey. The reactions were (i) movement of mandibles, (ii) acteristics are more standardised than natural sand. They are throwing off the sand when the prey was in the pit, and (iii) also commonly used by physicists so their properties are the capture of the prey. The probability of an antlion reaction well known (Daerr & Douady, 1999). The bead density was is defined as the number of antlions that reacted divided bythe 2.5 × 10−3 gmm−3, and the grain diameter ranged from 0.200 total number of ant–antlion interactions. The main observation to 0.300 mm. Experiments took place in beakers of borosilicate recorded was whether the antlion captured the ant or whether the glass of a 76 mm diameter. ant escaped out of the pit. The probability of capture is defined as

(a) (b) 100 50 20 10 glass tube 5 2

Mass (mg) 1 0.5 0.2 0.1

support depth pit 124 133 1435 13 132 75 Terr Lbru Lbema Asu Fpol Antlion Species

Fig. 1. (a) Measure of the depth of the pit. Two glass tubes were used. A tube was above the pit centre and the other was above and outside the pit. The distance between the two ends was measured with a rule to the nearest mm. (b) Masses of ant species and antlions in mg and logarithm scale. The ant species are Terr = Tapinoma erraticum,Lbru= Lasius brunneus,Lema= Lasius emarginatus,Asub= Aphaenogaster subterranea, Fpol = Formica polyctena. The number of data for each species is above the ant name. Shapes of ants and antlion larvae are scaled up two times. The three antlions represent the first (black), second (dark grey), and third (light grey) instar stages. The length of antlions is fromDevetak et al. (2005) and the length of ant workers is from Bernard (1986). the number of ants captured by antlions divided by the number of probabilities of antlion reaction and capture and with a Poisson ants offered to reactive antlions. Each ant could be captured only family for the time to escape. We used 80% nearest neighbour once, so the probabilities of capture from the antlion’s and the bandwidths and a local quadratic polynomial for the fits. We also ant’s points of view are equivalent. We also recorded the number checked the composite variable ‘mass of ant/mass of antlion’ of ants which were caught but then released by reactive antlions. representing an interaction between the predator and the prey. The time to capture, or time to escape, was also measured to the This ratio did not bring new information. nearest second. The two Lasius species, the most captured ants, ejected formic acid when they were bitten by antlions but we did not detect any positive effect for the ants. It was the only Results trait of aggressiveness we observed. Some 70% of ants were placed outside the pit. This position was not different between The probability of an antlion reaction was independent of the the reactive (72%) and non-reactive (73%) antlions, and between prey identity or their masses (Fig. 2a). It increased with the depth ants that were captured (72%) and that escaped (73%). of the pit (Fig. 2b). Light antlions reacted more often to the prey We also recorded the mass of the ant and the species identity, than heavy antlions (Fig. 2c). the depth of the pit and the mass of the antlion. The experiment The probability of capture differed between ant species, L. brunneus was designed for 800 ant–antlion interactions. However, some ranging from 15% to 74%. It was maximal for T. erraticum antlions did not build a pit for one or several day, corresponding and decreased both for the lighter species, ,and to 122 data. Few other data were missing for different reasons, for heavier species. The probability of capture was maximal between 1.75 and 2.25 mg (Fig. 3a). The probability of capture such as problems during weighing or errors in data transcription, increased somewhat with the depth of the pit (Fig. 3b). It was which were impossible to correct. Ant and antlion masses were independent of the antlion mass (Fig. 3c). uncorrelated (R = 0.02, CI =−0.06; 0.09, n = 647). Ant mass Reactive antlions caught, but then released 47% of and pit depth were also uncorrelated (R = 0.03, CI =−0.04; T. erraticum, 32% of A. subterranea,8%ofL. emarginatus, 0.11, n = 658). The depth of the pit was positively but weakly 4% of F.polyctena, and 4% of L. brunneus. correlated with the mass of the antlions (R = 0.15, CI = 0.08; The time to escape decreased with a mass up to about 3–4 mg 0.23, n = 667). The ant species had different masses, except and was constant thereafter (Fig. 4a). It decreased from the for L. emarginatus and A. subterranea (Fig. 1b). The number lightest species to the heaviest one. Aphaenogaster subterranea of data for each species varied between 122 and 136, also used more time to escape than L. emarginatus,andL. brunneus. depending on the variables. The time to escape increased with the depth of the pit (Fig. 4b) The aim of the present study was to understand the effects of and was independent of the antlion mass (Fig. 4c). the ant mass, pit depth, and antlion mass, on the probabilities of reaction and capture and the time to escape. It is not to predict any of the last three variables, nor to assess the relative Discussion weights of the first three ones. Thus, we refrained from carrying a multiple regression analysis, according to the reasoning of Variables influencing the behavioural sequence during attack Bookstein (2014) and Freedman (2009), and merely used a local regression method on bivariate relationships to visualise trends. Antlions may choose to react depending on their estimation of We used the locfit function on R with a binomial family forthe succeeding in capturing the prey. An antlion reacted more often

(a) 1.0 (b) 1.0 (c)1.0

0.8 0.8 0.8

0.6 0.6 0.6

0.4 0.4 0.4

0.2 0.2 0.2 Probability of antlion reaction Probability 0.0 0.0 0.0 02468 10 0 5 10 15 20 0 20406080 Ant mass (mg) Depth (mm) Antlion mass (mg)

Fig. 2. The probability of an antlion reaction as a function of ant mass (a), pit depth (b), and antlion mass (c). Solid lines are the expected probability estimated by local regression and dashed lines are their CI. Histograms are proportional to the number of reactive (top histograms) and non-reactive (bottom histograms) antlions.

(a) 1.0 (b) 1.0 (c)1.0

0.8 0.8 0.8

0.6 0.6 0.6

0.4 0.4 0.4

0.2 0.2 0.2 Probability of capture Probability 0.0 0.0 0.0 02468 10 0 5 10 15 20 020406080 Ant mass (mg) Depth (mm) Antlion mass (mg)

Fig. 3. The probability of capture by reactive antlions as a function of ant mass (a), depth of the pit (b), and antlion mass (c). Solid lines are the expected probability estimated by local regression and dashed lines are their CI. Histograms are proportional to the number of captured (top histograms) and escaped (bottom histograms) ants. to the prey if its pit was deep. This is in accordance with findings confirming results of Barkae et al. (2012). Finally, the proba- of Heinrich and Heinrich (1984) who measured pit diameter, as bility of capture decreased with the increasing mass of the prey pit depth and diameter are highly correlated (Scharf et al., 2009). identity, confirming previous results with other species (Wilson, Contrary to Heinrich and Heinrich (1984), we did not find simple 1974; Griffiths, 1980b; Gotelli, 1996; Missirian et al., 2006). effects of the species identity or prey mass on the reaction of We observed that the time to escape increased with the depth antlions. However, we found that light antlions reacted more of the pit. The pit is a conical structure (Fertin & Casas, 2006). often to prey than heavy antlions. Scharf et al. (2010) found a The distance that prey have to travel is proportional to pit depth, similar result, but antlion mass interacted with the size of prey thus explaining the positive relationship between the time to and the state of satiation. The first instar stage is also known escape and the pit depth. Lucas (1982) showed furthermore to react more often than the third instar stage (Nonato & Lima, that the time to escape an artificial pit increases with its slope. 2011). Our findings go against the hypothesis that antlions donot Ants are also more likely to struggle and fall more often on try to capture too small prey because of the small energy gain steeper slopes and finer sand (Botz et al., 2003). An increased (Heinrich & Heinrich, 1984). Distinguishing between antlion slope can render the pit more unstable to the perturbations made mass and pit depth enabled us to show that these two variables by struggling prey, hampering their locomotion and releasing abundant stimuli to the antlion. We also showed that the time act in opposition on the likelihood of the antlion reaction. Hence, to escape depends on the mass of the prey. Thus, the physics of using pit structure as a surrogate for antlion mass (Griffiths, the pit and its interaction with the locomotion of prey requires 1980b), instar or any property of the predator must be used dedicated attention irrespective of the predator. cautiously. We observed that the probability of capture increased with the depth of the pit, confirming previous results (Wilson, 1974; Grif- An optimal prey size fiths, 1980b, 1986; Heinrich & Heinrich, 1984). We conclude that a big pit, in diameter, depth or volume increases the proba- Several authors have studied antlion performance according bility of capture. We furthermore found no effect of the antlion to prey size (Wilson, 1974; Griffiths, 1980b; Gotelli, 1996; mass on the probability of capture and on the time to escape, Missirian et al., 2006). The work of Griffiths (1980b) is of

(a) 200 (b) 200 (c) 200 100 100 100 50 50 50

20 20 20 10 10 10 5 5 5 Time to escape (s) 2 2 2 1 1 1 02468 10 0 5 10 15 20 020406080 Ant mass (mg) Depth (mm) Antlion mass (mg)

Fig. 4. Time to escape as a function of ant mass (a), depth of the pit (b), and antlion mass (c). Time is in the second and logarithm scale. Solid lines are the expected probability estimated by local regression and dashed lines are their CI. Open points are the observed mean time to escape. The area of points is proportional to the number of data. particular relevance here as it also deals with different ant Acknowledgements species spanning a wide range of masses. He speculated on the existence of an optimal prey size but observed only a We thank Jérôme Crassous for the glass beads. We thank Alain monotonous negative relationship between the ant size and Lenoir for his help in the identification of ants. Two anonymous the capture success. We observed here for the first time a reviewers improved the manuscript. This work is part of the PhD positive relationship between prey mass and the probability thesis of A. H. under the supervision of J. C. This study was of capture, in the range of 0.1–2 mg. In particular, antlions supported by the Centre National de la Recherche Scientifique captured T. erraticum poorly, as they released around half of and by the Région Centre. A. H. contributed to experimental caught individuals. Antlions must break through the cuticle with design, collection and analyses of data, and writing of the paper. the tip of mandibles to eat prey. The probability of closing a J. R. contributed to experimental design, collection and analyses mandible in a successful configuration is low for small prey. of data. J. C. contributed to analyses of data and writing of the Hence, we identify an optimal prey mass range for antlions, as paper. predicted by Griffiths (1980b). More generally, the lack of prey smaller than 1 mm in nature also suggests a lower limit for prey of antlions (Wilson, 1974; References Heinrich & Heinrich, 1984; Lucas, 1986; Matsura, 1986). These small prey are most likely available around a pit (Siemann et al., Barkae, E.D., Scharf, I., Abramsky, Z. & Ovadia, O. (2012) Jack of 1996). So, either small prey does not fall into the pit, or they all trades, master of all: a positive association between habitat niche are not captured. There are thus lower and upper limits to the breadth and foraging performance in pit-building antlion larvae. PLoS prey size, and consequently an optimal range of prey size. The ONE, 7, e33506. existence of an optimal range of prey size would also explain Bernard, F. (1986) Les fourmis (Hymenoptera Formicidae) d’Europe that most of prey measure around 3 mm (Wilson, 1974; Lucas, occidentale et septentrionale. Masson, Paris, France. Bookstein, F.L. (2014) Measuring and Reasoning: Numerical Inference 1986; Matsura, 1986). in the Sciences. Cambridge University Press, Cambridge, U.K. Botz, J.T., Loudon, C., Barger, J.B., Olafsen, J.S. & Steeples, D.W. (2003) Effects of slope and particle size on ant locomotion: impli- Towards a mechanistic understanding of the physics of the pit cations for choice of substrate by antlions. Journal of the Kansas Entomological Society, 76, 426–435. We measured only the mass of ants. Many other aspects of Cooper, W.E. Jr. (2005) The foraging mode controversy: both continu- prey morphology might impact the capture efficiency, such as ous variation and clustering of foraging movements occur. Journal of the leg’s length or its shape. Dedicated studies will be necessary Zoology, 267, 179–190. to distinguish implications of differences in other morphological Coyle, F.A. (1986) The role of silk in prey capture by nonaraneomorph aspects. We also suggest studying the physical functioning of spiders. Spider: Webs, Behaviour and Evolution (ed. by Shear, W.A.) the pit in more depth. In particular, the mechanical properties of pp. 269–305. Stanford University Press, Stanford, California. the sand can explain the observed range of optimal sizes. Prey of Daerr, A. & Douady, S. (1999) Two types of avalanche behaviour in varying sizes imparts varying forces into the sand, with complex granular media. Nature, 399, 241–243. Devetak, D. (1985) Detection of substrate vibrations in the antlion larva, implications of signal propagation (Fertin & Casas, 2007) and Myrmeleon formicarius (Neuroptera: Myrmeleonidae). Bioloski Vest- locomotion stability (A. Humeau and J. Casas, unpublished). nik, 33, 11–22. Thus, the next step to understand the functioning of the pits as a Devetak, D., Špernjak, A. & Janžekovic, F. (2005) Substrate particle size successful animal construction is the study of insect locomotion affects pit building decision and pit size in the antlion larvae Euroleon on sandy slopes, in the spirit of the work carried out so far only nostras (Neuroptera: Myrmeleontidae). Physiological Entomology, on much larger animals (Li et al., 2013). 30, 158–163.

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