distill into the wound a liquid venom which occasions the irritation that follows the bite. Fig, 11 shows the form of the larva and nymph of this insect. The former will be recognized as the little "wriggler" that may be seen in such numbers in stagnant water in summer. Fortunately, these insects are harmless until they acquire wings, and after that their life is a short one; but, unfortunately, they breed at an enormous rate, and thus maintain the supply, to the infinite annoyance of man and other tender-skinned animals.
Another blood-sucking parasite of both man and beast, whose staying tendencies are proverbial, is the louse. Fig. 12 represents the species that inhabits the head of man. The mouth of this insect consists of a sucker contained in a sheath, without articulations. It is armed at the point with retractile hooks, within which are four bristles
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| Fig. 12.—Louse of the Head. | Fig. 13.—Louse of the Head. 2, 3, Sucker. | Fig. 14.—Louse of the Head, Claw. | ||
that aid in breaking through the skin. They have climbing feet terminated by pincers, with which they maintain their hold on the hairs. The sucker and claw are illustrated in Figs. 13 and 14. The nits, or eggs, hatch in five or six days after they are laid, and in eighteen days more the creature is able to reproduce its kind. Leeuwenhoek calculated that two females might become the grandmothers of 10,000 lice in eight weeks.
A not less annoying parasite that lives on the blood of man and the higher animals is the flea. Both male and female get their living in this way, and even the larvæ are supplied from the same sources by the mother, who sucks for herself first, and then divides with her young ones. The ordinary flea (Pulex irritans, Fig. 15) is common in both Europe and North America. It may be called a fly without wings, and, together with others of its kind, forms a distinct family under the name Pulicidœ. The four principal species are Pulex irritans of man, Pulex canis of the dog, Pulex musculus of the mouse, and Pulex vespertilionis of the bat. Great numbers of human fleas, half as large as the common fly, are found in summer on the sandy
