Why Do Bugs Die On Their Backs?

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

You've probably noticed dead or dying beetles, cockroaches, flies, crickets, and even spiders all wind up in the same position—upside down with their legs curled in the air. Have you ever wondered why bugs always seem to die on their backs?

This phenomenon, common as it is, has sparked plenty of debate among amateur insect enthusiasts and professional entomologists alike. In some respect, it's almost a "chicken or the egg" scenario. Did the insect die because it was stranded on its back and unable to right itself? Or, did the insect wind up on its back because it was dying?

Dead Insects' Limbs Curl When They Relax

The most common explanation given for why bugs die on their backs is something called the position of flexion. A dead (or near death) bug cannot maintain tension on its leg muscles, and they naturally fall into a state of relaxation. In this relaxed state, the legs will curl or fold up, causing the insect or spider to topple over and land on its back. If you rest your arm on a table with your palm up and relax your hand completely, you will notice that your fingers curl slightly when at rest. The same is true of a bug's legs.

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. When the bug dies, blood stops flowing to its legs, and they contract. Again, as the critter's legs fold beneath its considerably heavier body, the laws of physics come into play and the bug flips over on its back.

'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, they do sometimes find themselves stuck. A diseased or weak bug may be unable to flip itself over and subsequently succumb to dehydration, malnutrition, or predation--although in the latter case, you won't find a dead bug on its back, of course, as it will have been eaten.

Pesticides Affect the Bug's Nervous System

Insects or spiders with compromised nervous systems will have the most difficulty righting themselves. Many pesticides act on the nervous system, and their intended bug targets often spend their final moments writhing and squirming on their backs, unable to muster the motor skills or strength to turn over.