When the louse feeds, it everts the anterior part of the mouth cavity, with its circle of hooks. The latter serve for anchoring the bug, and the piercing apparatus is then pushed out. [Illustration: 65. Pediculus humanus, ventral aspect of male. (×10)] Most writers have classed the sucking lice as a sub-order of the Hemiptera, but the more recent anatomical and developmental studies render this grouping untenable. An important fact, bearing on the question, is that, as shown by Gross, (1905), the structure of the ovaries is radically different from that of the Hemiptera. Lice infestation and its effects are known medically as _pediculosis_. Though their continued presence is the result of the grossest neglect and filthiness, the original infestation may be innocently obtained and by people of the most careful habits. Three species commonly attack man. Strangely enough, there are very few accurate data regarding their life history. _Pediculus humanus_ (fig. 65), the head louse, is the most widely distributed. It is usually referred to in medical literature as _Pediculus capitis_, but the Linnean specific name has priority. In color it is of a pale gray, blackish on the margins. It is claimed by some authors that the color varies according to the color of the skin of the host. The abdomen is composed of seven distinct segments, bearing spiracles laterally. There is considerable variation in size. The males average 1.8 mm. and the females 2.7 mm. in length. The eggs, fifty to sixty in number, stick firmly to the hairs of the host and are known as nits. They are large and conspicuous, especially on dark hair and are provided with an operculum, or cap, at the free end, where the nymphs emerge. They hatch in about six days and about the eighteenth day the young lice are sexually mature. [Illustration: 66. Pediculosis of the head. The illustration shows the characteristic indications of the presence of lice, viz: the occipital eczema gluing the hairs together, the swollen cervical glands, and the porrigo, or eruption of contagious pustules upon the neck. After Fox.] The head lice live by preference on the scalp of their host but occasionally they are found on the eyelashes and beard, or in the pubic region. They may also occur elsewhere on the body. The penetration of the rostrum into the skin and the discharge of an irritating saliva produce a severe itching, accompanied by the formation of an eczema-like eruption (fig. 66). When the infestation is severe, the discharge from the pustules mats down the hair, and scabs are formed, under which the insects swarm. "If allowed to run, a regular carapace may form, called _trichoma_, and the head exudes a fœtid odor. Various low plants may grow in the trichoma, the whole being known as _plica palonica_."--Stiles.