The wings are attached to the meso- and metanotum; they consist of two membranes traversed by numerous horny ribs (Fig. I. 1W and 2W).
The abdomen is made up of nine segments (C 1 to 9), some of which are not infrequently wanting. It contains the organs of nutrition, circulation, and generation.
The digestive system, the structure of which is apparent from Fig. II., consists of the following divisions: the throat, or œsophagus (8); the crop (9); the gizzard, or proventriculus (10); the pancreas (11 11); the stomach, or ventriculus (12); the biliary vessels (13 13 13); the ilium, or little gut (14 14); and the colon (15); ending in the anus (16). In the suctorial tribes, the crop is modified into a very peculiar organ, termed the sucking stomach, which presents itself as a small bag, attached to the throat by a thin tube. This bag exhausts the air from the throat, when the insect is sucking, thus producing a vacuum therein, and causing a rapid ascent of fluid into the stomach.
The heart of insects consists of an elongated tube lying along the back, and termed the dorsal vessel. It is composed of a variable number of chambers, the blood being driven forward towards the head by its contractions. These motions may be easily seen in transparent species.
The breathing organs are distributed throughout the body in the form of numerous minute air-tubes, which are supplied with air from a variable number of apertures, situated on the sides of the insect, and termed spiracles.
The nervous system consists of a chain of ganglia, running down the ventral surface of the insect, and analogous to the spinal cord of higher animals. The number of ganglia varies greatly among the different tribes.
The metamorphosis of insects, which I have previously mentioned as one of their most essential attributes, consists of four distinct stages, viz., the Egg, Larva, Pupa, and Imago.
The eggs of these animals exhibit a great diversity in shape among the different species. They are deposited by the parent with unerring instinct on substances suitable for the food of the larvæ, which, in the majority of cases, is quite different from that on which she herself subsists.
The larva state immediately succeeds the egg, and is spent almost exclusively in feeding, the insect growing at a great rate, and being frequently compelled to change its skin.
The pupa is usually completely quiescent, the insect being at this time quite incapable of any motion, except, perhaps, a slight twirling of its abdomen. Exceptions to this rule occur, however, in two of the orders, in which the pupa state does not differ materially from that preceding it.