Minute Aptera, in which the mouth is formed for suction ; and there are two simple eyes.
This order comprises insects which are commonly parasitic upon man and other animals, and are known as Lice (Pedi-culi). The common Louse (fig. 180, a) is furnished with a simple eye, or ocellus, on each side of a distinctly differentiated head, the under surface of which bears a suctorial mouth. There is little distinction between the thorax and abdomen, but the segments of the former carry three pairs of legs. The legs are short, with short claws or with two opposing hooks, affording a very firm hold. The body is flattened and nearly transparent, distinctly segmented, and showing the stigmata very plainly. The young pass through no metamorphosis, and their multiplication is extremely rapid. Many Mammals are infested by Lice, the same animal often being subject to the attacks of more than one species of Louse. Three species commonly attack man - viz., Pediculus humanus corporis, P. capitis, and P. pubis; and a fourth species (P. tabescentium) is of rare occurrence, and gives rise to the loathsome disease known as Phthiriasis.
Fig. 180. - Morphology of Aptera. A, Pediculus humanus capitis,' B, Docophorus hamattts, one of the Bird-lice ; C, Campodea ; D, Degeeria, one of the Podnridae ; E, Scale of a Podurid, as seen under the microscope ; F, Degeeria purpurascens. All the figures are greatly enlarged. (After Packard and Gervais.)
The Lice are now very commonly associated with the Hemiptera, of which they are regarded as constituting a degraded and aberrant group.