Adult beetles have a distinctive appearance, with the hard elytra covering most of the body and meeting down the mid-line, which makes them easy to recognise but this apparent uniformity hides an enormous variety of life histories. There are numerous variations in larval forms and feeding habits, and beetles can be found in a wide range of habitats. In turn this makes them important in ecological research and environmental surveys. The group has always been popular with general naturalists and insect collectors, second only to the Lepidoptera, and not least because most specimens are robust and easy to keep in collections. In common with other endopterygote insects, the larvae are the main feeding stage but in many families the adults are long-lived and also feed. Sometimes larvae and adults occupy similar habitats with the same food requirements, but in other cases the adults have different habits from the larvae. The hard elytra have undoubtedly played a part in adult longevity because the well-protected insects can burrow under the ground or in wood without damage; most other adult endopterygotes have fragile wings that limit their activity. Well-known groups of beetles include ground beetles, diving beetles, woodworm, carpet beetles, longhorns, leaf beetles, ladybirds, weevils, oil beetles, soldier beetles, glow-worms, click beetles, stag beetles, dung beetles and rove beetles. Worldwide there are around 350,000 known species in 175 families; in Britain there are about 4,000 species in 112 families.