Skeletons need to be strong. If you're a terrestrial animal, they need to be strong enough to support the pull of muscles and the impact of steps, leaps and other gravity-defying feats. But being strong often has its drawbacks. Skeletal tissue that is too thick can burden an animal with unnecessary weight and hinder motion, so a fine balance has to be struck. Eoin Parle and his collaborators at Trinity College, Dublin, Ireland, wanted to understand how insect exoskeletons are designed to be strong enough to deal with daily forces they encounter, while being light enough for efficient mobility.
The authors explored this question with three insects: the desert locust (Schistocerca gregaria), American cockroach (Periplaneta americana) and death's head cockroach (Blaberus discoidalis – which is more than twice the size of the American cockroach). To understand the structural aspects of each insect exoskeleton, they measured tibia geometry, stiffness and the force required to bend the tibia to the point of breaking (strength). Then they estimated the ecologically relevant stresses encountered by each species by using previously published forces incurred by the tibia during common activities such as walking, running and jumping, as well as those of more strenuous survival activities – usually performed during an emergency – such as pushing through small holes and crevices (wedging) and righting when overturned (both performed by the death's head cockroach), and anti-predator jumping by the locust.
Parle and his collaborators found that tibia strength did not differ across insects, even though they varied in material properties (composition, orientation and amount of each component). One reason for this, the authors state, is that exoskeletons bend before they break, unlike materials such as wood. In a previous study, the authors suggested that the thin-walled tubular legs of insects are similar to many engineered structures in that they are light and strong, but will buckle when their strength is tested.
Not surprisingly, running generated more stress than walking for all three insects, and all emergency maneuvers generated more stress than routine activities. The authors found that of all the emergency maneuvers, jumping in the locust generated the most stress. In addition, wedging through a tight space generated more stress than righting in the cockroach. And although these emergency maneuvers appear to place limbs at perilous risk, Parle and his colleagues state that the insects only take these extreme chances on a small number of occasions.
Interestingly, although the death's head cockroach is larger and heavier than the American cockroach, and had larger and thicker tibia, there was no difference in tibia strength or stiffness between the two. The authors propose that this is because the American cockroach runs more and generates proportionally greater daily forces than its ghoulishly named cousin. So, the similarity in strength and stiffness of the smaller cockroach to the larger compensates for the greater forces it has to endure.
The authors also found that safety factors – essentially the margin of extra protection built into a design or material – were lowest for the emergency maneuvers, particularly for jumping in the locust. This, the authors say, suggests that insects have evolved to have light limbs that allow economic movement, which they trade off by avoiding riskier behaviors that may cause their limbs to buckle.
