For animals, the ability to escape from life-threatening situations is essential. Whether fleeing from a predator's grasp, or manoeuvring their way out of a fight, animals benefit from traits that allow them to escape more easily. One such trait found throughout the animal kingdom is autotomy: the ability to voluntarily shed an appendage, most commonly a leg or tail. This may seem like an extreme solution, but in instances where the choice is to lose a limb or lose your life, it's a no-brainer.
Autotomy is particularly common amongst invertebrates, with various insects, arachnids and crustaceans all reported to have the ability to self-amputate. Depending on the species, and often the animal's age, these lost legs may either grow back, or the individual may have to cope without it for the remainder of their life. However, considering that autotomy is such an extreme behaviour with potentially severe consequences, we still don't really know which species resort to autotomy, particularly within insects.
To try to bridge this gap, a team of researchers from the University of Florida focused their efforts on the Coreidae family of insects, better known as the leaf-footed bugs. Led by Zachary Emberts, they first looked at the frequency of leg loss within natural populations of nine species of this insect. By collecting adult leaf-footed bugs and noting if legs were lost, the team observed that depending on the species, between 7.9% and 21.5% of individuals had legs missing. In addition, they looked at which legs were absent and saw that there appeared to be a larger proportion of hind legs missing. Interestingly, the hind legs of many of these species of leaf-footed bug are highly decorated, have enlarged femurs and are often used as weapons when fighting over mates.
Finding individuals with missing legs does not, however, prove that the species can autotomise, so to test this, the researchers attempted to induce autotomy in the lab by grasping a hind leg with forceps to simulate attack from a predator. Supporting the field results, all nine species successfully autotomised the leg. The authors noted that all of the breakages occurred at the same leg joint and were accompanied by a stereotypical ‘raise-and-drop’ movement of the abdomen. Intriguingly, in three species, they observed that only females autotomised the limb, despite the presence of male individuals with missing legs in the wild.
This work has helped to broaden our knowledge of the range of this extraordinary trait in insects, but has also raised some questions. In some species of leaf-footed bug, the hind leg is used as a decoy during attacks, distracting predators with its decoration, and here the ability to shed it quickly makes great sense. However, these insects also often use their hind limbs as weapons during sexual competition, so it is curious that they are prepared to shed them so readily, particularly as adults cannot regrow the limb once it has been lost. As sacrificing a leg in order to survive also reduces the animals’ reproductive fitness, this appears to be one of nature's more extreme examples of cutting off your nose to spite your face.