Substrate Selection for Pit Making and Oviposition in an Antlion, Myrmeleon Bore Tjeder, in Terms of Sand Particle Size

Entomological Science (2005) 8, 347–353 ORIGINAL ARTICLE Substrate selection for pit making and oviposition in an antlion, Myrmeleon bore Tjeder, in terms of sand particle size Toshiaki MATSURA1, Yoshitaka YAMAGA2 and Madoka ITOH3 1Department of Biology, Kyoto University of Education, Fushimi-ku, 2Asia Kohsoku Corporation, Osaka and 3Doshisha Junior High School, Sakyo-ku, Kyoto, Japan Abstract The larvae of the pit-making antlion Myrmeleon bore Tjeder live in open sand in riverbeds with a substratum consisting of various particle sizes. We analyzed the spatial distribution of their pits in a sandy floodplain to determine their larval and adult responses to the heterogeneous substrate. The spatial distribution pattern of their pits had an aggregated distribution, and there was a significant positive correlation between pit density and the ratio of medium-size sand particles to total weight of sand. We examined the size of sand particles selected in the larval pit-building behavior and the oviposition behavior of the adult. Both larvae and adults selected medium-size sand particles. The larvae of M. bore are relatively sedentary predators and rarely move great distances. Thus, the present results suggest that habitat selection by adult females is a major factor causing the aggregative distribution of the pits. Key words: floodplain, habitat selection, Myrmeleontidae, Neuroptera, predator, spatial distribution. INTRODUCTION (Lucas 1982; Allen & Croft 1985). Pitfall traps built in fine sand grains have a steep slope (Allen & Croft 1985) Most predacious insects capture prey directly using their and prey (ants) take a long time to escape from the pit mandibles (e.g. dragonfly larvae and lacewing larvae) or (Lucas 1982; Allen & Croft 1985; Botz et al. 2003). their specialized forelegs (e.g. praying mantids and giant Some studies have revealed that antlion larvae select fine waterbugs), but a few are predators that adopt hunting sand when they are presented with fine and coarse sand tools such as pitfall traps and webs. Larvae of several at the same time (Morisita 1952; Loiterton & Magrath genera (e.g. Myrmeleon, Hagenomyia and Euroleon) in 1996; Farji-Brener 2003). Although there have been Myrmeleontidae (antlions) build an inverted conical pit many behavioral and ecological studies of antlions on the sand surface. The larval antlions wait for prey at in the laboratory in relation to the characteristics of the base of a conical pitfall trap, opening their mandi- sand, how antlions in the field respond to the hetero- bles. Unlike non-pit-building antlion larvae, pit-building geneity of the substratum where they live has been antlions can move only backward on the sand when poorly investigated. they construct their pits or relocate them. The larvae of the antlion Myrmeleon bore Tjeder In general, capture efficiency is important for preda- build a pit actively in open sand such as in seaside dunes tors to enhance their fitness. The capture efficiency of and in floodplains. The substratum of a floodplain is antlion larvae is said to be affected markedly by the size composed of particles of various sizes, while the sand of the sand particles where their pits are constructed of seaside dunes is composed of particles of a relatively uniform size. When they live in floodplains, the larvae should make their pits in areas with adequate sand Correspondence: Toshiaki Matsura, Department of Biology, particle size to increase their foraging efficiency. The Kyoto University of Education, Fishimi-ku, Kyoto, 612-8522 previous studies have revealed that the larvae rarely Japan. Email: [email protected] relocate their pits, even under severe fasting in the lab- Received 27 April 2005; accepted 7 July 2005. oratory (Matsura 1987; Matsura & Murao 1994). If © 2005 The Entomological Society of Japan T. Matsura et al. such sedentary behavior is adopted in nature, habitat Life history of Myrmeleon bore selection at oviposition by the female becomes an Myrmeleon bore is a univoltine species (Matsura et al. important factor governing larval spatial distribution. 1991). Females lay eggs during mid-summer. First instar In the present study, we analyzed the spatial distribution larvae begin to emerge from late summer. The larval of M. bore pits in a sandy floodplain in relation to sand development rate mainly depends on feeding rate, and particle size, and we examined in the laboratory sand first to third instar larvae are found before hibernation. particle size preference by the larvae for pit construction Overwintered larvae pupate from June to July and the and also by the adult females for oviposition. period of adult emergence is from mid-June to early August. MATERIALS AND METHODS Field survey Research area We counted the number of pits in each quadrat once a The Kizu River flows through southern Kyoto Prefec- month (except three times in May) from April to August ture in Japan and forms many sandy floodplains in the in 1998. We did not dig out the antlion larvae from the middle course of the river. The research area was located sand, to avoid disturbing them. Therefore, their precise within a bush growing on one of the floodplains age structure was unknown, but the population was ° ° (38.8 N, 135.9 E). We had found many pits constructed assumed to consist mainly of second or third (last) instar by the larvae of M. bore in an open sandy field sur- larvae during the period from spring to early summer, rounded by grass before starting this research. This open based on the pit diameters and previous reports (e.g. sand was located at the top of a mound with a shape Matsura et al. 1991). All the antlion larvae were dug × that was nearly rectangular (4 16 m). We established out on 21 August 1998 and the number of each instar × 39 quadrats (each 1 1 m) within this sandy area larvae per quadrat was recorded. (Fig. 1). The quadrats were classified into the following three groups based on the pit density observed on 28 May 1998: None, a quadrat with no pits; Low, a quadrat with one to nine pits; and High, a quadrat with 10 or more pits. We selected five quadrats arbitrarily from these three groups (15 samples) and collected surface sand (approximately 150 mL) at the center of each quadrat. The sand samples were separated with an elec- tric sieve into five particle size classes (fine, <0.25 mm in diameter; medium, 0.25–0.5 mm; coarse, 0.5–1 mm; very coarse, 1–2 mm; and gravel, >2 mm); this defini- tion of sand particle size was adopted also in the laboratory experiments. We weighed the sand after drying it at approximately 150°C for 24 h, and calculated the ratio of sand weight of each particle size class to the total sand weight. The proportion of medium-size sand particles was arcsine transformed and analyzed by a one-way analysis of variance (ANOVA) to determine the significance of difference among the three groups defined by the pit density. Sand particle size selected by larvae In May 1999, we examined the size of sand particles the larvae preferred as a pit-building site in a laboratory experiment under conditions of 25 ± 1°C, 50–60% rel- Figure 1 Monthly changes in the spatial distribution pattern ative humidity and 12 h light : 12 h dark (LD 12:12). A of the pits in the research area. “d”, number of pits per quadrat third instar antlion was put in a container containing (1 × 1 m). one of the above size classes of sand or non-sieved sand 348 Entomological Science (2005) 8, 347–353 © 2005 The Entomological Society of Japan Substrate selection in antlions for 1 week to adapt to the environment. The larva was oviposition experiment, we prepared 12, 13, 13 and 13 then placed in the center of a container (200 × cylindrical containers (90 mm in diameter and 50 mm 130 × 80 mm3) that contained two different kinds of high) and filled them with fine, medium-size, coarse and sand side by side to a depth of 40 mm: one was the sand very coarse sand, respectively, to a depth of 30 mm. One in which the larvae had been kept and the other was egg per container was placed on the sand surface and medium-size sand. covered with sand of the same particle size to approximately 5 mm. As the duration of egg in M. bore is Sand particle size selected by female approximately 2 weeks in midsummer (Matsura et al. adults in oviposition 2001), we examined 3 weeks after setting the experi- Females of M. bore flying into oviposition sites were ment whether the larvae had made pits or not. When caught at night and were brought to the laboratory. no pits were made, we observed the eggs under a ste- Because it is very difficult to find their eggs laid in sand, reoscopic microscope to determine whether they had we used the following method. We laid drawing paper hatched or not. This experiment was carried out under at the bottom of a container (200 × 130 × 80 mm3) and ambient room temperature. spread sand uniformly to a depth of 5 mm over it. The depth of eggs oviposited freely by M. bore is approximately 7.5 mm (Matsura et al. 2001). Accordingly, in RESULTS this experimental design, the inserted tip of the female abdomen would reach the paper at the bottom of the Spatial distribution of pits container and the oviposited eggs would adhere to it. The total number of pits in the research area varied The base of each container was divided into halves, and between 77 and 154 during the period from April to sand with a different particle size was put into each half.

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