Antlion pits take advantage of ant slips

The feeling of sand between your toes on a sunny day at the beach. For some, this feeling is tied to happy memories. For others, a source of dread when thinking of all the places that sand will get stuck. Ants also have a love-hate relationship with this particular substrate. While ants depend on sand to build their homes, sand is also used by ant predators to trap ants in pits of doom! Ants are not the only insects plagued by substrate struggles. Spiders spin sticky webs that make escape attempts futile. Pitcher plants use slopes that their prey can climb in but not out of. In their article, Humeau and colleagues test if the sandy substrate or the slope of antlion pits are to blame for the ant’s downfall.

Larval antlions, Euroleon nostras, are clever predators. While their pits are not very deep, pits are dug with a slope close to the critical angle, the steepest slope possible without leading to an avalanche. The authors of this study predicted that ants would slide on the particles, decreasing the ant’s stability, and making it difficult to climb out of the antlion pits. To test this hypothesis, the group first tested how woodland ant workers, Aphaenogaster subterranean (common antlion prey), walked under different experimental treatments. The treatments included different substrates (solid vs granular), slopes (flat vs incline), and roughness (smooth vs rough) and then compared these treatments to how ants moved in an antlion pit. The scientists then analyzed how the ant’s limb movements differed among the different treatment groups.

Figure 1A from Humeau et al., 2019; Showing the definition of the five leg variables on the factorial design experiments. Stride length (black continuous line on image 3) is the distance traveled between two consecutive contact events with the surface. Slip length (red dotted line on image 3) is the distance traveled between the beginning of a stance phase and the beginning of the next swing phase. Swing length (blue dashed line on image 3) is the distance traveled between the beginning of a swing phase and the
beginning of the next stance phase.

This study also tested the effect of sand grain size on ant climbing and avalanche angle. To test this the group had ants walk over glass beads at 3 different diameters (500, 250, and 180 µm) and calculated an abnormality index, which measured the climbing difficulty for an ant given the orientation, slope, and particle size.

*Figure 1D from Humeau et al., 2019; Showing* Set-up used for measuring the ant’s trajectories on an inclined
plane at angle. With d representing the diameter of the glass beads.

Surprisingly, the team found that neither sand nor slope alone inhibited ant escape. Instead it was the interaction of both sand and slope together that make the antlion pits so treacherous. On granular inclined slopes, ants moved 3-4 times slower and were less stable compared to any other treatments. To combat this, the ants took shorter strides and used a more stable gait, keeping more legs in contact with the ground than the typical gait. For the second part of the study, the authors were able to confirm that their abnormality index accurately defined the difficulty of locomotion for the ants. Near the avalanche threshold, ants display a strong walk abnormality index and do slide considerably but do not create avalanches. Antlion pits are therefore specific not just in the substrate used, but also in the slope at which the pit is dug. Both the slope and the sand grain size must match the prey size in order to trap ant prey. As the authors state, insects have succeeded at conquering “the whole Earth.” Which makes the results of this study somewhat satisfying knowing how clever ant predators must be to catch them.

Kelly Diamond is a PhD candidate at Clemson University finishing up her dissertation on the environmental impacts on escape and climbing performance in a group of goby fishes.