Remaining immobile for an unpredictable period after being touched by a potential predator (post-contact immobility, PCI) may favor survival. However, the factors that determine variation in PCI duration are poorly understood. We tested whether PCI duration depends on the surrounding conspecific density (the selfish prey hypothesis) in the tropical antlion larvae Myrmeleon crudelis. These insects avoid predation by being immobile or burying themselves. The selfish prey hypothesis predicts a reduction in the PCI duration as conspecific density increases because this high density of conspecifics around can stimulate the redirection of the predator's interest in other nearby potential prey. In the field, we measured PCI and found that its duration was independent of the conspecific density. We also measured the PCI of a subset of larvae in a density-reduction lab experiment. We found that larvae displayed shorter PCI duration when we reduced their number of neighbors to zero. These results suggest that antlion larvae did not follow the selfish prey hypothesis. We propose two alternative explanations. First, the larvae have a limited ability to detect neighbors in the field and keep up with the changing number of surrounding active pits. Second, burying may be more important than PCI as an antipredator strategy. In our lab experiment, larvae had a more accurate idea of the conspecific density around their pit because they explored the area. We propose that knowing the absence of conspecifics triggered a shorter PCI duration. Under a high predation risk (i.e., no other prey to which the predator would redirect its attention), the larvae select the unequivocal antipredator behavior of burying. This work illustrates the relevance of accurate information in deciding how to avoid predation, especially when prey can prioritize between alternative behaviors which success varies between contexts.

Field observations

We randomly selected 103 focal antlion pits (1-5 cm in diameter) and placed a 25 x 25 cm square plot with the selected focal pit at the plot center. Next, we counted the number of traps inside the plot and measured the pit diameter of all of them with a digital Caliper to the nearest 0.1 mm. We calculated the density of conspecifics around the focal pit (i.e., # of pits/625 cm²) and the total area of the surrounding pits (i.e., the sum of the area of all pits inside the plot excluding the focal larva’s area). The area of each pit was estimated using the formula of π * r².

We measured PCI duration as the difference (in seconds) between when the antlion landed on the substrate and when its body moved for the first time. We extracted the focal larva from the ground by gently scooping the soil in the trap ~5 cm deep with a spoon. We placed the spoonful on a plastic kitchen sifter to locate the larva. Once located, we held it dorsoventrally with entomological forceps to simulate a predator attack. We dropped the larva inside the plot from which it was taken on a sector of bare soil from ~ 10 cm from the ground, simulating a predator's involuntary drop. Using a chronometer, we measured the time in seconds to the nearest 0.1s between the larva’s first contact with the soil and its first movement (i.e., PCI). When the larva began to move, it quickly burrowed into the ground. 

Laboratory experiment

In lab conditions, we tested if antlion larvae modulate their PCI in response to their number of neighbors by experimentally reducing the original neighbor density found in the field. We brought to the laboratory a subset of 30 randomly selected individuals after measuring their PCI in the field. The larvae experienced environmental conditions similar to those in the field since the laboratory does not have temperature or photoperiod control. We gently put each larva at the center of the container and let it acclimate for three days, an adequate lapse for antlions to build their pit traps. All larvae moved around the container, searching for a place to build the pit, leaving visible paths in the substrate. Larvae did not receive food during the experiment.

Three days later, we repeated the protocol performed at the field to measure the PCI. In the lab, we collected the individuals from the pits inside the container using a spoon, and we quickly took them with entomological forceps 10 m away from the lab building to a field site with the same soil substrate where we had conducted the field trials. The behavior of the larvae was similar the second time we measured the PCI (after being in the lab) in terms of their behavior in the field.