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68 which it is derived, is exactly synonymous with the Greek one, from which we obtain the principal component part of the term Entomology. Both words therefore are sufficiently descriptive of the whole articulated races, and, in fact, when originally applied, were designed to embrace them all. In the more limited and precise sense in which it is now used, the word insect is applied to such animals only as present the following characters:—no internal skeleton; a nervous system composed of ganglions; an imperfect circulating system; respiration by means of tracheæ communicating with the air by stigmata; oviparous, the sexes distinct; body covered by a coriaceous or membranous integument, and divided into three distinct sections, viz. the head, provided with two antennæ; the thorax, with six articulated legs; and the abdomen, usually having the sexual organs at the extremity; and, finally, not presenting these parts in full developement till after having passed through (with very few exceptions,) several successive changes called metamorphoses. These negative and positive characters, derived both from external and internal parts, will be found distinctive, and completely exclusive of all the other