Larvae are not ancestral throwbacks; many evolved later than the adults
It is tempting to see a caterpillar or tadpole as a glimpse of its group's distant past. Often that is wrong. In insects the larval stage evolved secondarily, so a larva can differ more from the common ancestor than the adult does. Larvae are specialists in their own right, built for a different life.
A larva is a distinct juvenile form that many animals pass through before metamorphosis. It is typical of species with indirect development, among them insects, some arachnids, amphibians and cnidarians. Larvae usually look nothing like their parents, with organs the adult lacks and often a different diet. Primitive arachnid larvae even have one pair of legs fewer, three instead of four.
Living differently has advantages. Tadpoles stay in the water while adult frogs can venture onto land, and separating the generations shelters young from some predators and stops them competing with adults for food. In barnacles, sea squirts and polychaete worms the roles are reversed: adults are fixed in place, so the mobile larva does the travelling, either feeding as it drifts or living off stored reserves. Some larvae depend on others entirely. In many eusocial Hymenoptera, female workers feed them; in one paper wasp, males also help but do it far less efficiently.
Occasionally the larval stage becomes permanent. Some salamanders, including the axolotl, reach sexual maturity while still in larval form and never transform, a phenomenon known as neoteny.
Among insects, only the group with complete metamorphosis has a true larval stage. A widely used classification dating to Antonio Berlese in 1913 sorts these larvae by body plan. Legless apodous larvae, such as those of the Apocrita, are poorly hardened. Polypod larvae, like caterpillars, carry extra stumpy legs along the abdomen and stay close to their food. Oligopod larvae have six legs and adult-like mouthparts but no compound eyes; some are flattened, long-legged hunters such as lacewing larvae, while others are plump, short-legged burrowers like beetle grubs. Protopod larvae, hatching from eggs with very little yolk, can look barely insect-like at all.
Source: Larva
