The parasitic insects occur entirely externally. Some of them are not parasitic at all times.
The head louse (P. capitis) lives among the hairs. It forms a chitinous sheath for its ova, which it cements to the hairs. The young, when they emerge from the egg, are like the adult in form, there being no further metamorphosis. The body louse (P. vestimen-torum) is like the former but considerably larger. The ova are deposited in the clothing, especially the seams, where also the adults congregate. The crab louse (P. pubis) has its popular name from the fact that it has long curved claws with which it attaches itself to the hairs. It occurs in the parts of the body furnished with stiff hairs, chiefly the pubes, but also the axillae, eyebrows, beard, eyelashes, etc. It is smaller and less elongated than the other two forms.
This animal is only partly parasitic. Its larvae, which are about an eighth of an inch in length, occur in quantities in the neighbourhood of mouldering organic matter, in dusty corners of rooms, etc.
Cimex lectularius (Common bug) is still less of a parasite. It lives chiefly about beds, and comes out of retired parts on to the skin to extract blood.
This is common in the West Indies, Central and South America, and southern parts of North America. The female, which resembles an ordinary flea, penetrates the skin, usually of the toes, where it swells up into a sac about the size of a pea, the abdomen being distended with ova. It produces painful inflammation.
The Larvae of insects or Maggots are occasionally found in the tissues of man. There are a few cases in which such larvae have, by migrating under the skin, produced considerable inflammation. There are also cases in which, deposited in, neglected wounds, or even in the mouth and nostrils of excessively debilitated persons, they have actually produced considerable destruction by feeding on the tissues. In neglected military hospitals wounds are often abundantly tenanted by maggots.
Fig. 199. - Larva of anthomyia canicularis. Many such larvie were passed by the intestine. x 8.
The Larvae of insects are also sometimes passed by the bowel. The form which has been chiefly observed is the Anthomyia canicularis. In some cases enormous numbers of the larvae of this fly (see Fig. 199) have been passed, stated as quarts in one report. (See Finlayson, in Glas. Med. Jour., xxxi., 225, 1889).