Phthirius pubis (= P. inguinalis), the pubic louse, or so-called "crab louse," differs greatly from the preceding in appearance. It is characterized by its relatively short head which fits into a broad depression in the thorax. The latter is broad and flat and merges into the abdomen. The first pair of legs is slender and terminated by a straight claw. The second and third pairs of legs are thicker and are provided with powerful claws fitted for clinging to hairs. The females ([fig. 69]) measure 1.5 to 2 mm. in length by 1.5 mm. in breadth. The male averages a little over half as large. The eggs, or nits, are fixed at the base of the hairs. Only a few, ten to fifteen are deposited by a single female, and they hatch in about a week's time. The young lice mature in two weeks.
The pubic louse usually infests the hairs of the pubis and the perineal region. It may pass to the arm pits or even to the beard or moustache. Rarely, it occurs on the eyelids, and it has even been found, in a very few instances, occurring in all stages, on the scalp. Infestation may be contracted from beds or even from badly infested persons in a crowd. We have seen several cases which undoubtedly were due to the use of public water closets. It produces papular eruption and an intense pruritis. When abundant, there occurs a grayish discoloration of the skin which Duguet has shown is due to a poisonous saliva injected by the louse, as is the melanoderma caused by the body louse.
The pubic louse may be exterminated by the measures recommended for the head louse, or by the use of officinal mercurial ointment.
Hemiptera
Several species of Hemiptera-Heteroptera are habitual parasites of man, and others occur as occasional or accidental parasites. Of all these, the most important and widespread are the bed-bugs, belonging to the genus Cimex (= Acanthia).
The Bed-bugs—The bed-bugs are characterized by a much flattened oval body, with the short, broad head unconstricted behind, and fitting into the strongly excavated anterior margin of the thorax. The compound eyes are prominent, simple eyes lacking. Antennæ four-jointed, the first segment short, the second long and thick, and the third and fourth slender. The tarsi are short and three segmented.
It is often assumed in the literature of the subject that there is but a single species of Cimex attacking man, but several such species are to be recognized. These are distinguishable by the characters given in Chapter XII. We shall consider especially Cimex lectularius, the most common and widespread species.