Why do insects avoid being on their back?
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Insects don't "like" being on their back because they can't get back on their legs and they therefore get stuck somewhere where they can gently wait for death to come.

Not all insects are unable to switch back on their legs but some can't do that (or can hardly do that). The reasons for which many insects are not able to get back on their legs (while mammals for example usually can do that) is that they have a exoskeleton. Their exoskeleton limits drastically their movements.