The nervous system is remarkably concentrated in some beetles, the abdominal ganglia showing a tendency to become shifted forward and crowded together, and in certain chafers all the thoracic and abdominal ganglia are fused into a single nervecentre situated in the thorax, - a degree of specialization only matched in the insectan class among the Hemiptera and some muscid flies.
Most of the dominant families - such as the Carabidae (ground-beetles), Scarabaeidae (chafers), or Curculionidae (weevils) have a distribution as wide as the order.
The Passalidae are a tropical family of beetles generally considered to be intermediate between stag-beetles and chafers, the enlarged segments of the feeler being capable of close approximation.
The Scarabaeidae or chafers are an enormous famil y of about 15,000 species.
Among the vegetable-feeding chafers we usually find that while the perfect insect devours leaves, the larva lives underground and feeds on roots.
They never bear segmented limbs (palps) and only exceptionally (as in the chafers) is the skeleton composed of more than one sclerite.
Insects a remark - able concentration of the trunk-ganglia takes place, all the nerve-centres of the thorax and abdomen in the chafers and in the Hemiptera, for instance, being represented by a single mass situated in the thorax.
Out of 118 species of butterflies, belonging to four important classes, no fewer than 86 are peculiar; while among the rose-chafers or Cetoniinae the same is the case in 19 out of 30.