14

Beetle anatomy

THORAX

All six legs, as well as the elytra and flight wings,

are attached to the thorax (see diagram on

page 10), and this holds the muscles to operate

these limbs. The intestine passes through the

center, but most of the thorax consists of muscle.

The exoskeleton of the thorax, to which these

powerful muscles are attached from the inside,

needs to be strong, and the upper surface of the

pronotum is usually the hardest part of the beetle.

It may have horns, for example in the Hercules

Beetle Dynastes hercules (Scarabaeidae: Dynastinae)

the forward-pointing thoracic horn is much

longer than the head (or cephalic) horn, and

protects the head from above. A few weevils in the

subfamily Baridinae have forward-pointing horns,

or tusks, on the underside of the thorax, which the

males use in fighting for females; each male places

its horns into special sockets in the thorax of the

other male, and then they have a trial of strength

where they try to push one another off a twig.

left | Cyclommatus asahinai

(Lucanidae) This Oriental

Stag Beetle is taking flight,

showing the rear parts of

the thorax that are usually

covered by the elytra.

opposite above | Theodosia

rodorigezi (Scarabaeidae)

A male of this Philippine

flower chafer shows cephalic

and thoracic horns.

opposite below | Cossyphus

rugosulus (Tenebrionidae)

This Mediterranean darkling

beetle has the thorax

covering the head; pale

spots let some light

through to the eyes.

Many other beetles that are not horned,

such as some Tenebrionidae (Cossyphus rugosulus,

illustrated opposite below) and many species

of Lampyridae, Cantharidae, Chrysomelidae,

and Bostrichidae, have part of the thorax

shielding the head from above, so that the

“front” of the beetle is the front margin of the

thorax, with the head below and behind. Some

have pale spots in the thorax corresponding to

where the eyes are, so that the beetle can still

see changes in light.

The extended thorax protects the head, eyes,

and antennae, but it may also have other uses.

For example, in male fireflies (Lampyridae),

which fly in search of the light produced by

a female, the projection of the thorax shields

the eyes from above, like the blinkers worn by

a horse. This prevents the beetle from being

distracted by the stars, which might otherwise

appear brighter than the glowing female they

are looking for.