Although Phthirus pubis occurs in Europe, Asia, Africa, North America and Australia, and is found on negroes as well as whites, as herein recorded, further data are required relating to the geographical distribution of the species and the races of man it infests.

The crab-louse occurs less frequently on man than does Pediculus and appears to be parasitic chiefly on persons leading an active sexual life. It has, so far, only been twice recorded on another host than man, i.e. the dog.

Whilst the crab-louse is usually found confined to the pubic and perianal region, it frequently spreads upward upon the abdomen and breast, and may infest the axillae severely, or it may spread downward along the thighs. Generalized infestation is rare, in such cases only the hands and feet are spared, although the head and neck are as a rule also free from parasites. The crab-louse has occasionally been found localized upon the head in infants, being either confined entirely to the eyelids, the nits being encountered on the eyelashes, or occurring also upon the eyebrows and hairy portions of the scalp. In adults, the parasite occurs much more rarely in these situations; a case is recorded of an Arab in whom, in addition, the beard and moustache were infested. Such cases are rare in children, very rare indeed in adults.

Apparently the head is so rarely infested because it is less suitable as a habitat. As Waldeyer points out, the head is probably not a suitable habitat because the scalp-hairs are crowded close together and finer than on the pubis and in the axillae. The majority of the active stages are found clinging to two hairs on the regions of the body they most infest, and where the hairs are sparser than on the head. In these parts, the hairs are 2 mm. or more apart. The reach between the extended two hind leg-pairs of the adult insect is about 2 mm., these legs being the ones that are used for grasping the hair.

Whilst infestation usually takes place through coitus, there are many exceptions to the rule as proved by the occurrence of crab-lice on infants. The latter may become infested by their parents or other adults. The insect may pass from one infant to another through their sharing a cradle, or it may pass from soldier to soldier when crowding occurs in barracks or billets. Scratching of their persons by infested individuals no doubt aids in the spread of the parasite upon them and the dissemination of the insect further afield. Both the act of coitus and scratching promote the shedding of hairs, and these, when bearing nits or young larvae, may be of considerable importance in disseminating the parasite. Such hairs, and no doubt occasionally active stages of the louse, are shed on clothing, bedding, the seat of the privy, etc., and readily become entangled with the pubic or other hair of clean persons who may come in contact therewith. A detached louse promptly clings to any hair with which it comes in contact. Therefore, whilst Pthirus is commonly conveyed directly, it may also be acquired indirectly. It is a helpless creature when removed from the hair to which it clings continuously upon the body, where it moves about by shifting from hair to hair; it is therefore much more likely to be conveyed passively from host to host than is Pediculus.

Unless disturbed, the parasite remains confined throughout its life to a limited area upon the host's skin as I have demonstrated experimentally.

The females appear to preponderate over the males in number when upon the host, their relative proportion being about 3:2. In copulation, the male does not seize the female as in Pediculus; he seizes the hairs to which the female clings, using these as a support. Oviposition occurs as in Pediculus. A female that was raised experimentally, laid up to three eggs per day, laying a total of 26 eggs. The hatching period (on the leg) lasted 7–8 days. Like Pediculus, the crab-louse passes through three moults (there being as many larval stages) before it attains sexual maturity. When the adults are about 10 days old they exhibit to a well-marked degree the greenish coloration of the fat-body which is seen by transparency at the sides of the abdomen in the living insect. The life-cycle, from egg to egg, is completed in 22–27 days (on the leg).

Young unfed larvae usually die within ten hours of emergence. When removed from man, they survive longer at 16–20° C. than at 30° C. and die much more rapidly in a dry than in a moist atmosphere; none of the numerous lice of all stages that were tested were found to survive up to 42½ hours, when maintained under different conditions. Living on man a male survived 22 days, a female 17 days, but the insects can doubtless live longer.

The feeding habits of Pthirus recall those of Ixodidae in being practically continuous, and, like Boophilus, they stop feeding to moult upon the host. After moulting they promptly proceed to feed again, shifting but slightly to a fresh feeding ground. On emerging, the young larva clasps a single hair and feeds at its base, and after a day or more it may clasp two hairs as the later stages do usually. The habit of continuous feeding explains why the insects die so quickly when removed from the host.

Whilst a brief exposure to light renders the insects restless but does not cause them to wander away whilst feeding, a warm body brought within their sphere promptly produces great activity in Pthirus when it is removed from man.