It is not intended to investigate the anatomy of insects in this work thoroughly; but, as we are about to treat of the habits and economy of certain created beings, it is necessary first to explain the principal parts of their structure, and the stages which every perfect insect or _imago_ has undergone before arriving at that state. We, therefore, proceed to explain, as simply as possible, the anatomy of an insect, and the functions of its organs. If we take an insect, and turn it over, and examine it carefully, the first thing that strikes us is that it is divided into three parts: the head; the thorax, or chest; and the abdomen, or stomach. The head (Fig. 1) is a kind of box, formed of a single piece, having here and there joints more or less strongly marked, sometimes scarcely visible. It is furnished in front with an opening--often very small--which is the mouth; and with some for the eyes, and with others for the insertion of the antennæ or horns. [Illustration: Fig. 1.--Head of an Insect] The integuments of the head are generally harder than the other parts of the body. It is necessary that this should be so. Insects often live and die in the midst of substances which offer some resistance. It is necessary, therefore, that the head should be strong enough to overcome such resistance. The head contains the masticatory organs, which, frequently having to attack hard substances, must be strongly supported. The exception to this rule is among insects which live by suction. It would be out of place here to mention the numerous modifications of the head which are presented in the immense class of insects. The eyes of insects are of two kinds. There are compound eyes, or eyes composed of many lenses, united by their margins and forming hexagonal facettes; and there are also simple eyes, or ocelli. The exterior of the eye is called the cornea (Fig. 2), each facette being a cornea; and the facettes, which vary in size even in the same eye, unite and form a common cornea, which is represented by the entire figure. [Illustration: Fig. 2.--A Compound Cornea.] In order to show the immense number of the facettes possessed by many insects, we give the following list:-- In the genus Mordella (a genus of beetles) the eye has 25,008 facettes. In the _Libellula_ (dragon-fly) 12,544 " In the genus _Papilio_ (a genus of butterflies) 17,355 " In _Sphinx convolvuli_ (the convolvulus hawk-moth) 1,300 " In _Bombyx mori_ (the common silkworm moth) 6,236 " In the house-fly 4,000 " In the ant 50 " In the cockchafer 8,820 " The facettes appear to be most numerous in insects of the genus _Scarabæus_ (a genus of beetles). They are so minute, that they can only be detected with a magnifying glass.