GLOBAL EYE
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Scientists have finally figured out where caterpillars got their extra sets of legs from. Turns out these chubby little limbs originate from their crustacean ancestors over 400 million years ago. Insects have six legs, except when they don’t. Caterpillars, the larvae of butterflies and moths, have additional sets of limbs known as prolegs. These prolegs pose an evolutionary mystery, and scientists have long grappled over how and why they got them. A new study suggests that these prolegs have origins in the primitive crustaceans that insects evolved from during the Ordovician period.
Prolegs are unjointed and feature sets of gripping hooks that function like spiky suction cups. Some species have as many as nine pairs. Unlike the six legs that most insects have, which extend from the thorax, or midsection, prolegs emerge from the abdomen. Their movement is mostly powered by hydraulic pressure, the movement of liquid into each limb. “Caterpillars are just eating tubes. They are maximising their eating and growth potential so they have evolved a gut-based body plan with a few legs to support the gut,” said Antonia Monteiro, an evolutionary biologist at the National University of Singapore. “Prolegs help them either grab onto substrates while the other legs help them feed or move them along the substrate.” After the caterpillar metamorphoses, the prolegs disappear.
Scientists have previously proposed that prolegs relate to thoracic legs, saying they are extra sets of legs that disappeared over the span of insect evolution and were reactivated when they became useful again. Another hypothesis is that they are modified endites, internally facing leg structures that were apparent in ancestral crustaceans. Scientists tested the manner in which genes direct the growth of these appendages by altering the embryonic development of squintin
