It seems that caterpillars got their prolegs from ancient crustaceans. (Image credit: Javier Millán Photography/Getty Images)
Scientists have finally figured out where caterpillars get their extra sets of legs. It turns out that these plump little limbs descended from their crustacean ancestors more than 400 million years ago.
Insects have six legs, except when they don’t. Caterpillars – the larvae of butterflies and moths – have extra sets of limbs known as prolegs. So do the larvae and even the adults of a handful of other insects. These prolegs are an evolutionary mystery, and scientists have long struggled with how and why they got them.
A new study published on October 12 in
Scientific progress suggests that these prolegs originated from the primitive crustaceans from which insects evolved during the Ordovician Period (485.4 million to 443.8 million years ago).
Prolegs are unconnected and feature a set of grappling hooks that function as spiked suction cups. Some species have up to 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 driven primarily by hydraulic pressure—the movement of fluid in each limb.
Connected: The fuzzy caterpillar has a sting “like being hit with a baseball bat,” and now we know why
“Caterpillars just feed on tubes. They maximize their potential for nutrition and growth. So they have developed a gut-based body plan with several legs to support the gut,” co-author
Antonia Monteiroevolutionary biologist at the National University of Singapore (NUS), told Live Science.
Caterpillar front legs are powered by hydraulic pressure. (Image credit: Ridho Arifuddin/Shutterstock)
“Their legs help either grab hold of substrates, while the other legs help them feed or move them along the substrate,” she said. After the metamorphosis of the caterpillar, the forelegs disappear. “When you become an adult insect, you don’t need them. You have a beautiful body plan with massive wings and you just don’t need those little stomach supports. You have a different way of life.”
Scientists have already suggested this
prolegs refer to the pectoral legs — saying they are extra sets of legs that disappeared during insect evolution and were reactivated when they became useful again. Others believe that they are entirely new adaptations.
A third hypothesis is that they are
modified endites — inward-facing leg structures that were evident in ancestral crustaceans.
In the new study, the scientists tested how genes direct the growth of these appendages by altering the embryonic development of squint brown butterflies (
Bicyclus anynana). In doing so, they hoped to determine which of these hypotheses—if any—was valid.
By disrupting a gene that determines the placement of limbs and other structures while the larva is still in the embryonic stage, the researchers were able to elucidate the pathways by which the prolegs develop. When the gene was partially inactivated, progenitors of typical legs as well as prolegs developed on the abdominal segments of the caterpillar. When it was completely disabled, only the progenitors of typical legs were present.
Because both types of limbs were present when the gene was partially disabled, the researchers demonstrated that the prolegs did not develop from the same cell types as the pectoral legs.
Rather, they look like modified endites. As crustaceans evolved into insects, the endites were largely lost. But in butterflies and moths, the gene for them is reactivated, giving caterpillars their stride.
The only other place where endites seem to persist in insects is in the mouthparts—the mandibles, maxillae, and labium, which are actually modified legs themselves. The cutting edge of the lower jaw, for example, appears to be a highly modified endite.
“Prolegs have a lot of affinities with head appendages in terms of the cocktail of genes they express,” Monteiro said.
So structures that trace their origins back to the crustacean ancestors of insects have been evolutionarily repurposed multiple times and for multiple functions—helping very hungry caterpillars move their heavy bodies and satiate their voracious appetites.