Optimal Range of Prey Size for Antlions
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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’.