If there’s anything that everyone knows about insects, it’s that they have six legs. So what’s going on with caterpillars, which have between ten and sixteen? Why are caterpillars insects?
Up at the front end, there are three pairs of thin, jointed legs that look typically insect-like. These are called “true legs” and are used to hold and manipulate leaves during feeding. Behind these are two to five pairs of stouter, fleshier limbs called prolegs, which are used for walking.
But where did they come from? Are they entirely novel structures found only in caterpillars?
Only the true legs are retained by the adult butterfly or moth; the prolegs are lost at the pupal stage. But where did they come from? Are they entirely novel structures found only in caterpillars? Are they extra true legs that have become modified over eons?
It turns out that they are neither. They are the product of genetic machinery used hundreds of millions of years ago by insects’ crustacean ancestors to build appendages on the abdomen that functioned as swimming and breathing aids. These were lost in insects, but the machinery for building them was not – it is just de-activated in most insects (including sawfly larvae, which are remarkably caterpillar-like, but lack prolegs). Only caterpillars have turned it back on again.
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Our experts have complied guides to caterpillars and insects, including how to identify 12 British caterpillars, how does a caterpillar turn into a butterfly and what are insects?