OESTRUS, (Encycl.) See Plate CCCXXII.—This is one of the most curious genera of insects. They are distinguished into several species, by reason of the different places wherein they deposit their eggs. Some, instructed by nature that their eggs cannot be hatched but under the skins of living creatures, such as bulls, cows, rein-deer, stags and camels, fix upon them at the instant of laying their eggs. From the hinder-part of their body issues a whimble of wonderful structure. It is a scaly cylinder, composed of four tubes, which draw out like the pieces of a spying-glass; the last is armed with three hooks, and is the gimblet with which the œstri bore through the tough hides of horned cattle. The animal seems to experience no pain from the puncture, unless the insect, plunging too deep, attacks some nervous fibre; in which case, the beast runs about, and becomes furious. The egg being hatched, the grub feeds on the matter of the wound. The place of its abode forms upon the body of the quadrupeds a bunch sometimes above an inch high. When full-grown, the larva breaks through the tumor, and slides down to the ground; for doing which it takes the cool of the morning, that it may neither be overpowered by the heat of the day, nor chilled by the cold of the night: it then digs itself a burrow, into which it retires. Its skin grows hard, and turns to a very solid shell. There it is transformed to a chrysalis, and afterwards to a winged insect. Nature has provided for every exigence: the shell wherein the œstrus is inclosed, is of so strong a texture that it could not make its way out, if at one of the ends there were not a small valve, fastened only by a very slight filament. The first push the œstrus makes, the door gives way, the prison opens. The insect wings its way to woods and places frequented by cattle. See OESTRUS, and BOTTUS, (Encycl.)