All insects, from the largest to the most minute, contain internally a well-formed heart and a digestive, reproductive, respiratory, and nervous system (Fig. 3), while the spaces surrounding these organs are, for the most part, packed with a complex system of muscles. The heart is a delicate tube lying along the middle of the back or dorsal surface of the body, immediately under the skin, and extends almost from one end of the insect to the other; in an almost similar position, close to the lower or ventral surface of the body, the nervous system is situated, and consists of a chain of nerve centres, or ganglia, connected by a double nerve cord, the most anterior of these ganglia being in the head and forming the brain, the following three lying in the thorax, one to each segment, while the remainder are confined to the abdomen, one ganglion to each segment, as in the thorax. In many insects the number of nerve centres is reduced, owing to the fusion of two or more. The reproductive organs are located in the abdomen. The digestive system consists of a tube (Fig. 3), with its appendages, opening at the mouth and at the posterior end of the body; this alimentary canal may be straight and simple, or convoluted and complex, according to the insect and the nature of its food. Respiration in insects is carried on by means of a system of air tubes (Fig. 3), which branch and re-branch to form an intricate system of delicate tubular airways, carrying the atmosphere to all tissues of the body; the main air tubes open at the surface by a series of breathing pores normally arranged along each side of the body, except on the head; these pores are best seen on a caterpillar or on the abdomen of adult insects. CHAPTER IV. Life Histories of Insects. No doubt owing to the endless assortment of sizes, from mere specks to giants of a few inches, a widespread idea has arisen, particularly in regard to such insects as have a general resemblance to one another, that the smaller individuals are the younger stages of the larger. Though gradation in size may be a sign of successive ages in certain insects, the presence of functioning wings denotes that growth has ceased; in the case of wingless insects, the characters of maturity may be less conspicuous. Although there may be at times a fairly wide range in size among fully-grown individuals of the one species, such variation is not due to age, but to certain factors influencing the insect during growth, such as the abundance or scarcity of food supply, and favourable or unfavourable climatic conditions. On the other hand, the sex to which an individual belongs is often responsible for difference in size, males very frequently being smaller than females. Size, therefore, is by no means a sign of age, and the smaller winged insects must not be regarded as the young of the larger ones, no matter how close is the resemblance.