Foraging behaviour of a neglected pit-building predator: the wormlion

1. What they did

Wormlions are the larvae of small flies. Like the better-known antlions, they dig cone-shaped pits in loose soil and wait at the bottom to ambush small arthropods, mainly ants. Wormlions and antlions belong to different insect orders, so their shared hunting method is a classic example of convergent evolution. While pit-building antlions and web-building spiders have been studied a great deal, almost nothing was known about how wormlions forage. The authors set out to see how hunger, body size, crowding and spacing affect wormlion pit size and the larvae’s response to prey.

They worked with an undescribed wormlion species (Vermileo sp.) found in shaded, sheltered spots of very fine dusty sand beside walls and buildings in Tel Aviv and Ramat Hasharon, Israel. They ran four sets of observations and experiments:

• Spacing in the field. They photographed 13 patches of wormlion pits, measured the area and position of every pit, and used a computer null model (random placement repeated 1000 times) to test whether the pits were spaced randomly, regularly or in clumps.
• Crowding in the laboratory. They placed 6, 12, 18 or 24 larvae in sand-filled trays (23.5 by 17 cm), four trays per density, and photographed the pits three days later.
• Field pits versus laboratory pits. They measured the diameter and depth of 51 pits in the field, then weighed the same larvae and let them build new pits in cups of sand from their own habitat.
• Hunger. They fed 103 larvae, then over three weeks kept them either starved, fed once, or fed every week. They measured pit diameter repeatedly, and at the end offered each larva a flour beetle larva and noted whether it reacted within one minute.

They predicted that pits would become more regularly spaced as crowding increased, that pits further from neighbours would be bigger, that bigger larvae would build bigger pits, and that hungry larvae would build bigger pits and react faster to prey, as seen in antlions and spiders.

2. What they found

Spacing in the field

• Average density was about 348 wormlions per square metre, higher than the densities reported for antlions (18.3 to 200 per square metre).
• Pits were regularly spaced in 5 of the 13 patches, tended towards regular in 2, and were random in 6. Spacing was not linked to density.
• Large pits tended to sit next to large pits, and small pits next to small ones.
• Overall, the distance to the nearest neighbour was not linked to pit size. Patch by patch, the link was positive in three patches, negative in three and absent in seven.

Crowding in the laboratory

• As density rose, the pits became more regularly spaced and smaller on average.
• Pits closer to a neighbour were slightly larger, and pits further from the tray edge were larger.
• Unlike in the field, the sizes of neighbouring pits were not linked.

Body size and pit size

• Surprisingly, body mass was only weakly linked to pit size: not significantly in the field until the largest larva was removed, and only weakly in the laboratory. Body mass and length explained just 2.6 to 6.4% of the variation in pit size, compared with up to 87% reported for other trap-building predators.
• The same larva’s pit size in the field and in the laboratory were more strongly linked than either was to body mass.
• Pit diameter and depth were closely linked; pits were on average about 1.25 times as wide as they were deep.
• Larger larvae were less likely to build a pit on the first day.

Hunger

• Hunger had a weak effect, and in the opposite direction to the prediction: after the feeding treatments, fed larvae built larger pits than starved ones, especially among the smaller larvae.
• Only 23 of 92 larvae reacted to prey within 60 seconds (11 starved, 6 fed once, 6 fed weekly), with no significant difference between treatments. Larvae that did not react were larger.

3. What they concluded

• Wormlions only partly followed the predictions of foraging theory.
• In the laboratory, more regular spacing and smaller pits at higher densities suggest interference competition, probably from neighbours throwing sand while building and maintaining their pits. This was not seen in the field, perhaps because the laboratory densities covered a wider range, and because disturbance from leaves, stones and sticks probably shapes spacing in the field more than competition does.
• In the field, large pits clustered together. The authors suggest that stronger larvae may gather in the best spots and push weaker ones out.
• Hungry wormlions did not build bigger pits or react faster to prey. Starved larvae may switch to an energy-saving mode, some may have been preparing to moult, or the starvation may not have been long enough.
• Because pit size was consistent for the same larva in the field and laboratory but only weakly tied to body size, pit size probably depends on other internal factors, such as physical condition or past experience, which were not studied.
• Wormlions may be less sensitive than antlions to competition from their own kind but more sensitive to the physical features of their microhabitat. This remains to be tested.

6. Other technologies, skills or innovations

• Photographs of pit patches measured with ImageJ image-analysis software.
• A null model written in MatLab, which randomly placed the same number of pits in each patch 1000 times to show what random spacing would look like.
• Pit diameter measured twice at right angles with a calliper accurate to 0.1 mm; larvae weighed on an analytical balance and photographed under a stereomicroscope to measure body length.