If, in addition to these characteristics, one considers that these animals are not provided with interior skeletons--that their nervous system is formed of a double cord, swelling at intervals, and placed along the under-side of the body, with the exception of the first swellings or ganglions which are under the head--that they are not provided with a complete circulating system--that they breathe by particular organs, termed tracheæ, extending parallel to each other along each side of the body, and communicating with the exterior air by lateral openings termed spiracles--that their sexes are distinct--that they are reproduced from eggs--and, in conclusion, that the different parts we have mentioned are not complete until the creature has passed through several successive changes, called metamorphoses, a general idea may be formed of what is meant in zoology by the word "insect." Insects, whose general organisation we have briefly traced, have been classed by naturalists as follows:-- 1. APTERA (Fleas and Lice). 2. DIPTERA (Gnats, Flies, &c.) 3. HEMIPTERA (Bugs, &c.) 4. LEPIDOPTERA (Butterflies and Moths). 5. ORTHOPTERA (Grasshoppers, Crickets, Cockroaches, &c.) 6. HYMENOPTERA (Bees, Wasps, &c.) 7. THYSANOPTERA (_Thrips cerealium_). 8. NEUROPTERA (_Libellula_, or Dragon-fly; _Ephemera_, or May-fly; _Phryganea_, or Alder-fly). 9. COLEOPTERA (Beetles). We shall commence the history of the various orders by examining the Aptera. I. APTERA. Insects of this order are without wings, and the name is derived from two Greek words, [Greek: a], privative, and [Greek: pteron], wing, indicating the negative character which constitutes this order.[9] It consists of Fleas and Lice. The Flea (_Pulex_), of which De Geer formed a separate group, and called _Suctoria_, includes several species. [9] It is probable that one day the order Aptera will be superseded. The absence of wings is not really a character of great value. De Blainville, Mollard, Pouchet, Van Beneden, and Gervais, have made several attempts in that direction. The fleas have been placed among the Diptera, and the lice among Hemiptera in the "Traité de Zoologie Médicale" of these two last authors. The common flea (_Pulex irritans_, Fig. 17) has a body of oval form, somewhat flattened, covered with a rather hard horny skin of a brilliant chestnut brown colour. It is the breaking of this hard skin which produces the little crack which is heard when, after a successful hunt, one has the happiness to crush one of these parasites between one's nails. [Illustration: Fig. 17.--Flea (_Pulex irritans_).]