Why Do Bugs Die On Their Backs?

The Reasons Insects Die Belly Up

A cockroach on its back
PAN XUNBIN/Science Photo Collection/Getty Images

You've probably noticed a variety of dead—or nearly dead—crawly critters, from beetles, cockroaches, flies, crickets—and even spiders—in the same position: flat on their backs with their legs curled in the air. Lots of bugs die in this particular pose but have you ever wondered why?

This phenomenon, common as it is, has sparked plenty of debate among amateur insect enthusiasts and professional entomologists alike. In some respects, it's almost a "chicken or the egg" scenario. Did the insect die because it was stranded on its back and was unable to right itself, or did the insect wind up on its back because it was dying? Both scenarios have merit, and either might actually be correct, depending on the circumstances of a particular bug's demise.

Dead Insects' Limbs Curl When They Relax

The most common explanation for why bugs die on their backs is something called the "position of flexion." When a bug is dead or dying, it cannot maintain tension in its leg muscles and naturally falls into a state of relaxation. (If you rest your arm on a table with your palm up and relax your hand completely, you'll notice that your fingers curl slightly when at rest. The same is true of a bug's legs.) The argument goes that in this relaxed state, the bug's legs curl or fold up, causing the insect (or spider) to topple over and land on its back before it expires.

But why would the bug simply fall over rather than face-plant? The explanation has to do with gravity. The heavier mass of the dorsal side (back) of the bug's body falls hits the pavement, leaving the lighter side where the legs are to push up the daisies.

Blood Flow to the Legs is Restricted or Stops

Another possible explanation involves the flow of blood—or lack thereof—in a dying insect's body. As the bug dies, blood flow to its legs ceases, causing them to contract. Again, as the critter's legs fold up beneath its considerably heavier body and the laws of physics take over.

''I've Fallen and I Can't Get Up!"

Although most healthy insects and spiders are quite capable of righting themselves should they inadvertently wind up on their backs—very much like turtles and tortoises—but they do sometimes find themselves irreversibly stuck. A diseased or weakened bug might be unable to flip itself over and subsequently, it would succumb to dehydration, malnutrition, or predation—although in the latter case, you wouldn't find the bug corpse since it would have been eaten.

Insects or spiders with compromised nervous systems likely have the most difficulty righting themselves. Many of the most popular commercial pesticides act on the nervous system, often causing targeted insects to go into convulsions. As the bugs uncontrollably kick their legs, they get stuck on their backs, unable to muster the motor skills or strength to turn over, again, leaving them with their legs pointing toward heaven as they make their final curtain call.