Trap-Jaw Hunters and Jumping Springtails

Trap-jaw ants are awesome – few dispute this fact. Yet despite the remarkable nature of the trap-jaw mandibular structure, quantitative assessments of predator-prey interactions and ecology in this group are fairly rare. This is particularly surprising given that trap-jaw ants are an ideal system for understanding how morphological structures vary within species across a wide geographic range. Recognizing this utility, Dr. Kyohsuke Ohkawara and colleagues recently conducted an interesting study in this group, which was published last month. They investigated the impact of variations in prey size on the shape of mandibles in Japanese trap-jaw ants.

Head view of the trap-jaw ant Strumigenys lewisi. Photo: AntWeb

The species in the study, Strumigenys lewisi, are unrelated to the group of trap-jaw ants featured in The Daily Ant last month, but they sport similar, spring-loaded mandibles that can rapidly capture prey. These ants, like other species in the genus Strumigenys, prey on an order of insects called Collembola. As a side note, many collembolans are actually pretty cute:

Cute collembolan. Photo: Edward Nurcombe

Some of these little soil-dwelling insects, which are commonly called “springtails”, use long tails to quickly spring away from any threat they may face. But S. lewisi workers can move their mandibles even faster than the escape mechanism of their prey. Using this predator-prey system, Dr. Ohkawara and colleagues asked: Does the mandibular shape of S. lewisi vary across different populations according to the body size of their collembolan prey?

Indeed, the researchers found that mandible length was positively correlated with body size in particularly one group of collembolans, the Entomobryidae family. Trap-jaw ant colonies with longer mandibles were found in habitats that on average hosted larger Entomobryid collembolans. This is perhaps not too surprising, but such quantifications of trait function in real ecological communities help us to better understand what factors impact ant species distributions, and to what degree.

A different Strumigenys species, next to a large springtail. Photo: Alex Wild

Furthermore, the researchers found that in S. lewisi, the queen’s mandibular length did not differ significantly from worker mandibular length, despite the queen having a significantly larger overall body size compared to the workers. This is quite interesting, as it suggests that a specific ecological factor – prey size – has evolutionarily constrained the mandibles of these ants not just in the workers but also in the queens, breaking the typical developmental pattern of size difference between the reproductive and workers castes.

Trap-jaw ants are amazing even just considering the remarkable speed of their mandibles. But this work by Dr. Ohkawara and colleagues shows that trap-jaw ants are an excellent system for understanding the interaction between morphological traits, ecological selection pressures, and evolution.