16

Beetle anatomy

male genitalia of many families of beetles are also

important for taxonomy, to distinguish between

similar species. When the external morphology is

uniform, the structure of the genitalia can still be

different, often as a deliberate evolutionary

strategy to prevent male beetles from accidentally

mating with females of similar but distinct species

that occur in the same environment. Mating will

not proceed if the genitalia are incompatible,

because mating with an incorrect species may

result in no offspring or offspring that are infertile.

This is known as the “lock and key” hypothesis.

Scientists often routinely dissect the genitalia when

preparing specimens of beetles of difficult groups.

Some species are called physogastric (swollen

bellied), as the abdomen in the female swells with

eggs, making flight impossible. This is particularly

common in some leaf beetles (Chrysomelidae),

such as the common European Green Dock Beetle

Gastrophysa viridula.

Beyond defecation and reproduction, the

abdomen of beetles serves a variety of other

functions. It is where the bioluminescence is

produced in Lampyridae, for example, in a light

ABDOMEN

The abdomen (see diagram on page 11) consists

of segments called tergites, five to nine of which

are generally visible (others being withdrawn into

the apex, and able to extend during mating or

egg laying). Each tergite has a pair of spiracles,

external openings of the respiratory system that

allow air into the trachea from which oxygen is

diffused into the tissues.

In a typical beetle at rest, the abdomen is

covered by the folded flight wings and the closed

elytra, though the apical few segments, called a

pygidium, may extend beyond the elytra. In some

families, especially the rove beetles (Staphylinidae),

but also some Nitidulidae, Cantharidae, and other

families, the elytra are short, leaving several

abdominal segments exposed. This makes them

more flexible and able to move through narrow

tunnels or dense substrate.

At the apex, that is the tip, of the abdomen are

the anus and the genitalia. The flexibility of the

apical segments of the abdomen is important in

mating in many beetles, since many parts of the

exoskeleton are rigid and not freely movable. The