_Small Insects, wingless, with large head; thorax usually of two, rarely of one or three segments; prothorax always distinct; hind body consisting of eight to ten segments, in addition to the posterior two thoracic segments which usually are but little or not at all separated from it. The metamorphosis is very slight. The creatures live on the skins of birds or mammals, finding nourishment in the epidermal products._ The whole of the Insects of this family live a parasitic, or rather epizoic, life. They all creep about those parts that are near to the skin, the feathers of birds or the hair of mammals; they rarely come quite to the surface, so that they are not detected on a superficial examination. It is curious that under these circumstances they should exhibit so great a variety of form and of anatomical characters as they do. [Illustration: FIG. 214.—_Trinoton luridum_. Lives on the common duck and various species of _Anas_. (After Giebel.)] They are very depressed, that is, flat, Insects, with a large head, which exhibits a great variety of shape; frequently it is provided in front of the antennae with some peculiar tubercles called trabeculae, which in some cases are mobile. The antennae are never large, frequently very small; they consist of from three to five joints, and are sometimes concealed in a cavity on the under side of the head. {346}[Illustration: FIG. 215.—Under-surface of head of _Lipeurus heterographus_. (After Grosse.) _ol_, Labium; _md_, mandible; _mx_, maxilla; _ul_, labium.] [Illustration: FIG. 216.—Under lip of _Nirmus_, A; and of _Tetrophthalmus chilensis_, B. (After Grosse.) _m_, Mentum; _g_, ligula; _pl_, palp; _pg_, paraglossa; _hy_, lingua.]