Fig. 16.—Bed-bug.
To the same order belongs the singular hemipterous insect of our ponds, the boat-fly (Notonecta). It has some feet suited for swimming, and others for running, and it swims on its back with great rapidity. It is a dangerous neighbour for everything that has life. Always greedy of blood, it attacks great as well as little animals, and sucks the blood of its victim to the last drop, so that it must be closely watched when placed in an aquarium.
Lice, concerning which we are about to add a few words, are also free parasites, and belong to a different order of insects. Their mouth is formed of a sucker contained in a sheath, without articulations; it is armed at the point with retractile hooks, within which are four bristles. They have climbing feet, terminated by pincers, with which they seize the hair of the animals on which they live; their eggs are known by the name of nits. We have represented in Figs. [17], [18], and [19], the complete insect, the head, the sucker, and a claw more highly magnified.
Lice are hatched at the end of five or six days, and reproduce at the end of eighteen days. Leeuwenhoek calculated that two females might become the grandmothers of 10,000 lice in eight weeks. They are all
parasites of the mammalia, and three species live at the expense of man: the louse of the head, of which Swammerdam gave a detailed description in his work entitled “Biblia Naturæ”; the body-louse, which lives on the bodies of filthy people, forms a distinct species; the third species is the louse which occasions the disease called pedicularis, or Phthiriasis. These insects were formerly much more common than they are at the present day. In 1825 Dr. Sichel published a monograph concerning them; and there appeared in the “Gazette Médicale” of 1871, a long article on the history of Phthiriasis.
Fig. 17.—Louse of the Head.