Key to Families of Adult Australian Aquatic Coleoptera (Beetles)

Coleoptera (beetles) is the most speciose of all the insect orders. The majority of the approximately 120 families known from Australia are terrestrial but around 14 are wholly aquatic or strongly associated with water and a further 5 include some aquatic or semi-aquatic species. This key identifies these 19 families but may not distinguish members of this set from all other, terrestrial, beetles.

Coleoptera undergo holometabolous development, meaning there is an abrupt change of body form at the final moult. It is convenient to deal with adults and larvae in separate keys.

The chief distinguishing feature of adult beetles is that the front wings form hardened elytra. The elytra cover and protect the membranous flying wings, which are rolled and folded underneath the elytra at rest. The elytra meet at the midline of the abdomen, never overlapping as do the hemelytra of some hemipteran bugs. Functionally, the body of an adult beetle is peculiarly divided not into head, thorax and abdomen as in most other insects, but head, prothorax and hind-body. The hind-body comprises the pterothorax and abdomen, fused broadly together. Beetle mouthparts usually are of biting type. The adult head sometimes has two ocelli but never three, an often none. The abdominal sternum often displays a reduced number of visible sclerotised plates (sternites). The first few are reduced and concealed beneath the hind coxae, and the terminal segments usually are telescoped into the apex of the abdomen.

Identification to family level can be partly achieved on gross features such as size and shape, but in many cases requires a careful examination of external morphology. There is much variation of body form within many families and large-scale characters can readily be misleading. On the other hand it is never necessary to examine internal characters in order to identify a specimen to family level.

The Australian aquatic and semi-aquatic beetles fall into three suborders: Myxophaga (1 family: Microsporidae), Adephaga (6 families: Carabidae through Gyrinidae in the key), and Polyphaga (12 families: Hydrophilidae through Brentidae in the key). Myxophaga is a small suborder comprising minute, globose beetles. Adephaga includes a wide range of body sizes and two general body shapes: one in which the head, prothorax and hindbody are well separated from each other, the other in which they fit closely together to give a smooth lateral outline and overall a boat-shaped appearance. Polyphaga includes all these shapes and more.

A few families are instantly recognisable on the basis of unique characters. Most specimens, however, will fall readily into one of several not necessarily closely related families and then may require close examination for final placement.

Key Author(s): CSIRO Entomology Key Version: 1.1