Scientists have finally figured out where caterpillars get their extra legs. As it turns out, these chubby little limbs originate from our crustacean ancestors more than 400 million years ago.
Insects have six legs, except when they don’t. Caterpillars, which are the larvae of butterflies and moths, have additional limbs known as prolegs. The same goes for the larvae and adults of a few other insects. These prolegs pose an evolutionary mystery, and scientists have long grappled with how and why they were obtained.
A new study was published on October 12th. scientific progress The origin of these prolegs suggests that they lie in the primitive crustaceans from which insects evolved during the Ordovician period (485.4 million to 443.8 million years ago).
Prolegs have no joints and feature a series of grip hooks that act like pointy suction cups. Depending on the species he may have as many as nine pairs. Unlike most insects, which have six legs that extend from the thorax or midsection, the front legs emerge from the abdomen. Their movement is primarily driven by hydraulics, the movement of fluid to each limb.
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“Caterpillars simply feed on tubes. They have maximized their eating power and growth potential. So they evolved a gut-based body plan with several legs to support the gut. ” said the co-author. Antonia Monteiroevolutionary biologist at the National University of Singapore (NUS) told Live Science.
“The proleg helps grip the substrate, and the other legs help feed or move along the substrate,” she said. When the caterpillar metamorphoses, its front legs disappear. “When the insect becomes an adult, they don’t need them. They have a beautiful body plan with large wings and don’t need the support of a small intestine. They have a different lifestyle.”
Scientists have previously proposed that Proleg relates to the thoracic leg — states that they are an extra set of legs that disappeared during insect evolution and were reactivated when they became useful again. Some believe it is an entirely new adaptation.
The third hypothesis is modified endite — an inward-facing leg structure found in ancestral crustaceans.
In a new study, scientists tested how genes direct the growth of these appendages by altering embryonic development in squint-eyed brown butterflies.bicycle aninana). In doing so, they hoped to determine which of these hypotheses, if any, were valid.
By disrupting the genes that dictate the placement of limbs and other structures while the larva is still in its embryonic stage, the researchers were able to unravel the pathway by which prolegs develop. Once the gene was partially disabled, the precursors of typical legs and front legs developed in the caterpillar’s abdomen. Once completely disabled, only typical leg progenitors were present.
The researchers demonstrated that prolegs do not arise from the same type of cells as thoracic legs, as both types of limbs were present when the gene was partially disabled.
Rather, they appear to be modified endites. Endite was largely lost as crustaceans evolved into insects. But in butterflies and moths, those genes were reactivated, providing the caterpillar with front legs.
The only other places in insects where endite appears to have persisted are in the mouthparts, namely the mandible, maxilla, and lips, which are actually modified legs themselves. For example, the cutting edge of the lower jaw appears to be highly modified endite.
“Prolegs have a lot of affinity with head appendages in terms of the cocktail of genes they express,” Monteiro said.
As such, structures dating back to the crustacean ancestors of insects have been reused many times and for multiple functions over the course of evolution. In other words, it helps the hungry caterpillar move its heavy body and satisfy its formidable appetite.
