The most distinctive feature of beetles is the hardening of the forewings into elytra; it is from this that they get their formal name (koleos - sheath, pteron - wing). The elytra serve to protect the more delicate hind wings, as well as the dorsal surface of the abdomen, and may have been a key factor allowing them to exploit narrow passageways (for example, in leaf litter and under bark). During flight the forewings are opened enough to allow the hind wings to unfold and function:

Other derived characteristics of beetles are:

• hind wings folded under elytra, with reduced venation
• hind two thoracic segments (mesothorax+metathorax=pterothorax) broadly connected with abdomen, so that the primary functional units of body are head / prothorax / pterothorax + abdomen, rather than the more typical head / thorax / abdomen of many other insects.
• genitalia retracted into abdomen
• adult antenna with 11 articles

Beetles are holometabolous insects, normally with adecticous, exarate pupae. Most species have chewing mouthparts. There is a gula present on the undersurface of the head.