Why Do Insects Cross Their Legs When They Die?

  @rpalmerscience on

Even though science allows us to peer into galaxies hundreds of millions of miles away and tear atoms apart to look for clues about the origin of the universe, there are still lots of unanswered questions closer to home. Weighty questions such as: why do insects cross their legs when they die? It's actually still kind of a mystery!

While there doesn't appear to be a definitive answer, some entomologists offered theories.

I suspect this is rigor mortis -- that post-death stiffening that occurs due to a chemical transformation of muscle tissue -- contracting legs as much as possible in the directions of least resistance, says Brian Farrell, an entomologist at Harvard University.

In other words, in death it's easier for insect limbs to bend than to break - or straighten.

Cole Gilbert, who studies insect physiology at Cornell University, elaborates on how the muscles of insects contribute to this cross-legged position post-mortem:

The joints of each segment of an insect's leg are relatively simple. The leg can either flex or extend around the joint, much like our knee.

In insects, the flexor muscles that bend the limb are almost always bigger and stronger than the extensor muscles that straighten it - though there are some exceptions to this rule, such as the big jumping muscles in the hind legs of grasshoppers, Gilbert says.

Nevertheless, in most insects the flexor muscles are stronger at the main joint of the leg, he says.

So, when a dead bug goes into rigor mortis, it could be that the stronger flexor contracts and pulls the leg into a crossed position. Then, since insects dry out much quicker than humans or other mammals, which rot into a gooey mush after rigor mortis, you get a mummified insect corpse with crossed legs.

However, post-muscle stiffness might not be the only part of the answer.

I hypothesize that a lot of insects cross their legs when they dry, not when they die, says Jeffrey Shultz, an entomologist at the University of Maryland.

The key, Shultz says, may lie in a structure called the arthrodial membranes. These pliable membranes are present in all arthropods - insects, lobsters and crabs. Having a flexible membrane between the bony plates of an arthropod's exoskeleton allows the creature's joints to move.

Arthrodial membranes, also known as the soft cuticle, are pliable because of their water content. When an insect dies, this cuticle would dry and contract, flexing the bug's joints into the cross-legged position, Shultz hypothesizes.

A good way for someone to test this theory would be to observe insects dying in water; if the drying out of the arthrodial membrane really is responsible for the leg-crossing phenomenon, a hydrated bug corpse would not have crossed legs, Shultz speculates.  

So pay attention the next time you find a dead cockroach floating in your sink - you might be able to solve a scientific riddle!

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