The geometry of prey capture in praying mantis forelegs

The form of an animal's limbs has to balance multiple functions: locomotion, grasping, climbing, and jumping, among others. For cryptic animals, especially those that resemble elements of their habitat like sticks or grasses, the limbs may also be modified to enhance the camouflage. The performance of a limb in one category may require a tradeoff, reducing performance in another category. Praying mantises provide a diverse group of insects who all use their forelegs for one function, capturing prey, while some species use them as part of their camouflage. Here we use a large database of images of mantis species to capture the variation in morphology across the order, and to calculate the largest prey that their forelegs can hold. We find that the length and thickness of the femur and the length of the tibia comprise most of the variability across species. The majority of species have similar foreleg morphology, with two large groups extending into areas of the morphospace with thicker or thinner forelegs. A geometric relationship between dimensions of the foreleg and the optimal prey diameter maps directly onto the variability across species determined by principal components analysis; legs with thinner femurs and shorter tibia can't hold large prey, and the distribution of the species across the morphospace follows the gradient of optimum prey size. These results suggest that some species trade ability to grasp larger prey for benefits including crypsis, and the praying mantises are an ideal system for studying morphological and functional variation in limbs.