Most people are familiar with beetles, but it is often only adult beetles that people recognise. Juvenile beetles are grub-like and are known as larvae, and are very different in appearance to their adult counterparts. They often spend this early part of their lives away from our view. This is their growth stage, and like butterflies, juvenile beetles need to go through a dramatic metamorphosis to become the adult forms that we know so well.
To become adults, beetle larvae must pupate in much the same way as caterpillars do when they enter the chrysalis. This usually takes place out of sight in a protected place, often within a chamber which the larva constructs for the occasion. The metamorphosis transforms the animal from the grub into the adult with wings, longer legs, antennae and all the other attributes that beetles have. Once beetles have metamorphosed they do not grow any more; all the growing is achieved as larvae.
There are many beetles which begin their lives underground feeding on the roots of plants and decaying matter in compost. People often unearth them while gardening and they are colloquially known as curl-grubs or erroneously as witchetty grubs. Their soft, pale bodies are curled in a ‘c’ shape, and they have large, hard heads and powerful jaws for cutting and crushing their food. Many of these are scarab beetles, a group which includes the well known Christmas and Rhinoceros beetles.
Some beetle larvae are borers – not in the sense that they are totally dull and uninspiring, but in that they chew and bore their way through wood. Longicorn beetles are one such group. The larvae are soft-bodied with hard heads and very tough mandibles for cutting into the wood.
Some of the most well-known beetles in the world, the ladybirds, have unusual looking spiny larvae which are quite mobile. They wander around on plants just like the adults, catching and eating smaller insects such as aphids.
One large group of beetles belonging to the family Tenebrionidae have larvae which are hard, shiny and a little worm-like in appearance. They are usually found underground where they wriggle though the soil to feed. Some of these are bred commercially throughout the world and the larvae (known as Mealworms) are used to feed pet birds and many zoo animals.
The juvenile stage of many beetles is often the longest period of their lives. They may spend a year or more underground, and only a month or so as the mature beetle that we get to see.