At the same time it is to be observed that the Diptera, and among others the house-fly, can sustain their flight longer than the Hymenoptera and Neuroptera, although one would not think so from their appearance. In conclusion, if an insect's power of flying is not considerable, its power of traction and propulsion are immense, compared with the vertebrate animals; and, in the same group of insects, those that are the smallest and lightest are the strongest. The proportion between the muscular strength of insects and the dimensions of their bodies, would not appear to be on account of their muscles being more numerous than those of vertebrate animals, but on account of greater intrinsic energy and muscular activity. The articulations of insects may be considered as solid cases which envelop the muscles, and the thickness of these cases appears to decrease in a singular manner according to the size of the creature. The relative bulk of the muscles being less in the smaller species than in the larger, it is necessary to explain the superior relative strength of the former by supposing them to possess a greater amount of vital energy. These astonishing phenomena will perhaps be better understood if we consider the obstacles which insects have to overcome to satisfy their wants, to seek their food, to defend themselves against their enemies, &c. To meet these requirements they are marvellously constructed for both labour and warfare, and their strength is superior to that displayed by all other animals. It is also much greater than that of the machines we construct to replace manual labour. They represent strength itself. God's workmen are infinitely more powerful than those invented by the genius of man, which we call machines. We think it necessary, in closing this chapter, to give a sort of general outline of the great class of animals which we are about to study. If we wished to characterise insects by their exterior aspect, we might consider them as articulate animals, whose bodies, covered with tough and membranous integuments, are divided into three distinct parts: the head, provided with two antennæ, and eyes and mouth of very variable form; a trunk or thorax, composed of three segments, which has underneath it always six articulated limbs, and often above it two or four wings; and an abdomen, composed of nine segments, although some may not appear to exist at first sight.