Injurious insects occur among the following orders: Coleoptera, Hymenoptera, Lepidoptera, Diptera, Hemiptera (both heteroptera and homoptera), Orthoptera, Neuroptera and Thysanoptera.
By far the most destructive insects in warm climates belong to the Hemiptera, especially to the Coccidae or scale insects.
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.
In aquatic insects various devices for obtaining or entangling air are found; these modifications are described in the special articles on the various orders of insects (Coleoptera, Hemiptera, &c.).
In Hemiptera only eleven and in Collembola only yolk, and that the mesenteric epithelium becomes reinforced by six abdominal segments have been detected.