Preference of antlion and wormlion larvae (Neuroptera: Myrmeleontidae; Diptera: Vermileonidae) for substrates according to substrate particle sizes

1. What they did

Antlion larvae and wormlion larvae (the larvae of a fly family) live in sand and loose soil and hunt by waiting for prey. Many build cone-shaped pitfall traps and wait at the bottom; other antlions build no pit and lie buried, ambushing prey that walk across the surface. The authors asked how much the size of the sand grains matters to where these larvae live.

• Natural habitats. They took 12 sand or soil samples from each of 17 sites in 11 countries (Tunisia, Cyprus, Yemen, Albania, Austria, Croatia, Greece, Hungary, Slovenia, Spain and the United States), where 13 antlion species and one wormlion species lived. With a spoon they took sand from the pit with the larva, or from where a buried larva lay. They dried the samples, sieved them into nine size fractions (from under 60 µm up to over 2.2 mm) and weighed each fraction. They then grouped the sites into fine, fine to medium, medium and coarse substrates.
• Choice experiment. They tested larvae of four antlion species: Myrmeleon hyalinus (from Cyprus, 27 larvae), Euroleon nostras (Slovenia, 25), Myrmeleon immaculatus (United States, 26) and Neuroleon microstenus (Slovenia, 26). Each larva was fed one ant a day for five days, then went without food for five days so all were equally hungry. It was then dropped into the centre of a round container divided into eight wedges, each filled with sand of a different grain size. After 24 hours they recorded which wedge the larva was in: where a pit had been built, or, for the non-pit-builder, where it reacted to an ant released on the surface.

2. What they found

Natural habitats

• The finest substrate was a mix of powder and fine sand in rocky pockets at Parc Grüel, Barcelona, where the wormlion Lampromyia iberica lived. The next finest was Saharan sand at Douz, Tunisia, where the pit-builder M. hyalinus was very common.
• Fine to medium substrates came from Djerba and El-Jem (Tunisia), Berat (Albania), Leibnitz (Austria) and Bugacpuszta (Hungary).
• Medium substrates came from Salamis (Cyprus), both Yemen sites, Ilovik (Croatia), Kos (Greece), Boč and Pohorje (Slovenia) and Connecticut (United States).
• Coarse substrates, at Rovinj (Croatia) and Črni Kal (Slovenia), held two non-pit-builders, Neuroleon microstenus and Distoleon tetragrammicus.
• Antlions lived in a wide range of substrates whatever their way of catching prey. The wormlion was found only in fine substrate and the two coarse-sand species only in coarse substrate. M. hyalinus lived in a wide range, from fine sand to medium substrates, and E. nostras in a wide range of medium substrates.

Choice experiment

• Larvae wandered around at first (a behaviour called “doodling”), and some crossed more than one fraction before settling. They never built a pit across the line between two fractions.
• M. hyalinus chose the finest fraction (under 60 µm) more than any other: 37% of larvae.
• M. immaculatus and E. nostras built most pits in a medium fraction (230 to 540 µm): 54% and 52%.
• N. microstenus was most often in the 540 to 1000 µm fraction (42%), with some in the next two coarser fractions, 1000 to 1540 µm and 1540 to 1750 µm (15% and 4%).
• Each species’ choices differed clearly from chance. The species differed from each other, except M. immaculatus and E. nostras, whose choices were alike.

3. What they concluded

• Grain size matters to antlions and wormlions in choosing where to live and hunt, together with other factors such as shade, sand depth, prey and temperature.
• The larvae fall into four groups by the sand they live in: fine (the wormlion, and M. hyalinus from desert sand), fine to medium and medium (species of Cueta, Euroleon, Myrmeleon, Nophis and Synclisis) and coarse (Distoleon and Neuroleon).
• In the experiment the four species mostly chose medium grains, except M. hyalinus, which preferred fine grains like those of its natural habitat. Antlions were found in nature in coarser substrates than they chose in the laboratory.
• The authors suggest M. hyalinus may prefer the finest sand because of its slender head. Since larvae rarely move far, where they end up may depend mostly on where females lay their eggs, sometimes in the only suitable sand available.
• Medium grains are thought best for sensing prey vibrations: fine sand dampens vibrations, so prey are detected at shorter distances, and very coarse particles are hard for small insects to dig through.

6. Other technologies, skills or innovations

• Sieve analysis of sand into nine particle-size fractions, weighed on a scale accurate to 0.001 g.
• An eight-way choice arena: a round container (20 cm across) split into equal wedges with a removable plexiglass divider.
• Releasing an ant on the surface to reveal where a buried, non-pit-building larva was lying.