A brilliant toddler has told me that a butterfly can’t be an insect because it has more than six legs when it’s a caterpillar. He’s wrong, but I’m not sure why. What should I tell him?
• If your young friend had turned a caterpillar upside down, he would have seen that there are in fact six fully jointed legs at the front end, just behind the head. These will become the legs of the butterfly or moth. The remaining “legs” have no joints and are really just fleshy outgrowths from the skin called prolegs. Caterpillars have up to four pairs of these, plus an extra pair known as claspers at the tail end.
The caterpillars of geometrid moths have no prolegs, just the true legs and claspers. To move along, they have to bend themselves into a loop to bring the claspers just behind their legs, and then throw themselves forward as if measuring their own length on the ground. That explains their family name – they “measure the Earth” with this inching gait – and their popular names: “loopers” in the UK and “inchworms” in the US. Watching a looper walking illustrates very well how useful a caterpillar’s prolegs are.
“Watching a caterpillar walking illustrates very well how useful its prolegs are”
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Hazel Russman, Harrow, Middlesex, UK
This article appeared in print under the headline “Insect imposter”
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