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The Dead Bug and the Sleeping Hand

Bugs flip when they die because their legs return to what is called the "position of flexion."

Dead bug on its back

Photo: lightsoutfilms (flickr)

When many insects die, they flip onto their backs into the position known as the "position of flexion."

The dead bug and the sleeping hand, on this Moment of Science7.

Everyone has found a dead or dying roach, spider or beetle on its back with its legs up in the air. Somehow, it seems, bugs decide to flip themselves over in order to die. But why should they?

Some entomologists suggest that the reason so many dead bugs are found on their backs is in the way their legs work — or cease to work. When a bug is alive and walking, it generally is very stable, since it has a body suspended between multiple legs. When they die, however, those mobile and actively balancing legs return to what is called the “position of flexion.”

“Position of flexion” is a name for what a group of muscles do when there is no tension in them at all — that is, when they are dead. If you relax your hand, for example, you will see that the fingers do not lie flat; they naturally curl about halfway in. Although still alive, that half-curled shape approximates the position of flexion for the human hand.

For some species of bug, a similar curled-in shape is the position of flexion in death. If you imagine your hand walking along on its fingers and then curling them all inward, you can see how likely it is that the whole structure would roll over onto its back — as happens with bugs.

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