Page:Popular Science Monthly Volume 14.djvu/526

510 to have poisoning powers similar to those of the spider. They seize their victim by the leg or any other part, who, after the lapse of fifteen or twenty seconds, becomes paralyzed, makes no resistance to the devourer, and remains passive till nothing is left but an empty and shrunken shell. The poison has no effect on its own species: they frequently feed on each other; but in that case the prey struggles so long as any fluid seems to be left in the body. It is found in the dust of hay, old grain, meal, and flour, on collections, and also on books, from which latter it derives its name; but, in the opinion of Mr. Murray, it is so found because other gregarious acari congregate there, and "we should no more think that it was there for the purpose of feeding on those stuff's in which it is found than we should admit that a cat eats hay because it is found in a rat-infested hay-stack."

The occurrence of the phenomena of parthenogenesis among acarids has been shown by this species. Mr. Beck succeeded in rearing three successive generations from a female without any intervention of the male. The female is very careful of her eggs: she lays them in a heap, and rests brooding over them, guarding them from attack.

The itch-mites, Sarcoptes, infest the larger mammals. The commonest species, S. scabiei, which preys on man, when seen with the naked eye, looks like a. white, shining globe, or a little bladder of



water, of an inch long and  of an inch wide. The microscope brings to view (Fig. 9) a round, flat body, with a head not unlike that of a tortoise, provided with powerful jaws and nippers, the structure of which is well adapted to mining the skin. The body is set with