The class Insecta includes such an enormous number of species, genera, and families, that it would be impossible to treat of these satisfactorily otherwise than in a treatise especially devoted to entomology. Here it will be sufficient to give simply the differential characters of the different orders, drawing attention occasionally to any of the more important points in connection with any given family.

As already said, the Insecta are divided into three divisions, termed Ametabola, Hemimetabola, and Holometabola, according as they attain the adult condition without passing through a metamorphosis, or have an incomplete or complete metamorphosis. The Insects which come under the first head (viz., Ametabola) are not furnished with wings in the adult condition, and the four orders which compose this section are commonly grouped together under the name Aptera. By some, however, this division is entirely rejected, and the orders in question are placed amongst the Hemimetabola, or even grouped with the Myriapoda. Indeed, it is certain that the orders of the so-called Apterous Insects are not, strictly speaking, scientific divisions. It is, however, a matter of convenience to retain them in a separate form, as it is by no means absolutely certain how they may most naturally be distributed amongst the higher orders.

Sub-class I. Ametabola

Young not passing through a metamorphosis, and differing from the adult in size only. Imago destitute of wings; eyes simple, sometimes wanting.