122 CLASSIFICATION OF THE ANIMAL KINGDOM. 



With regard to the digestive apparatus of insects, it consists of a delicate 

 membranous tube, containing a crop, gizzard, stomach, and small and large 

 intestines. The crop is only met with in Hymenoptera, Lepidoptera, and 

 Diptera, which have no gizzard. The gizzard is found in mandibulate 

 insects that live on solid, animal, or vegetable substances. In some instances, 

 also, the small intestine ' is wanting, so that the stomach in such cases 

 passes at once to the terminal large intestine or colon. Besides these, the 

 digestive apparatus possesses occasionally various secreting organs, such as 

 salivatory glands, bile vessels, and others. The respiratory organs of perfect 

 insects consist of spiracles, from which a great number of delicate air-tubes 

 or trachese spread out through the body, and by this process, the blood, 

 which is all arterial, is being continually oxygenized, as it is brought in 

 contact with the tracheal tubes. 



In the nervous system of insects, we find a gradual concentration of 

 parts. The principal ganglion, or brain, becomes considerably developed, 

 and a chain of smaller ones runs on the base of the body, to guide the 

 muscles of the legs and wings. Interesting experiments have been made 

 with reference to this ganglionic nervous chain, and there seems good 

 ground for presuming that, analogous to the distinct columns that exist 

 in the spinal axis of vertebrate animals, there are at least two distinct 

 tracts also in the central axis of insects; for, as Professor Jones observes, 

 "It has been well ascertained that the nerves given off to the muscular 

 system of the Homogangliata, are not derived from the ganglionic masses 

 themselves, but from the cords which connect them together; while the 

 nerves, distributed to the integument and external parts of the body, 

 communicate immediately with the ganglia." 



Various are the senses which insects possess. That of touch is common 

 to all. It is evident also, that some at least, as for instance the Flesh-fly, 

 have the power of smelling; while there are abundant proofs of their being 

 able to hear sounds. Their eyes are of two kinds, simple and compound; 

 and many insects possess both. As regards their means of reproduction, 

 the sexes in all of this class are distinct, and the generative organs vary 

 considerably in different tribes, and are of a complex description. Those 

 of the female terminate in a common oviduct, attached to which are 

 certain appendages called gluten-secretors and spermatheca; and in many 

 insects there is a long external intestinal tube of elaborate construction at 

 the end of the oviduct, called the ovipositor. The number of eggs laid 

 by different species varies considerably; some, as the Flea, lay about twelve 

 only, while the Queen Bee averages some forty or fifty thousand; but 

 they are all beaten by the Termite, which, if it were to continue the 

 process throughout a whole year, would, says Professor Jones, produce the 

 astonishing number of two hundred and eleven millions four hundred and 



