INSECTS IN GENERAL 9 



also the muscles are attached. In adult insects the 

 chitin is often deposited as thick, hard plates, like 

 armour ; but in many young insects, such as cater- 

 pillars, the layer is very thin, flexible, and trans- 

 parent. 



Another point that calls for explanation is the 

 manner in which insects breathe. They have no 

 lungs, nor is their blood pressed into service to carry 

 oxygen to the tissues. Atmospheric air passes into 

 the body through small openings called " spiracles," 

 some of which may be seen along the abdomen of the 

 poplar hawk-moth, whose photograph is reproduced 

 on Plate IX. After entering the spiracles, the air 

 circulates through a system of minute tubes, or 

 tracheae, which ramify among the insect's living 

 tissues, and supply to them the oxygen requisite for 

 the various vital processes. The air-tubes, or tracheae, 

 are formed by an in-pushing of the outer skin, or 

 epidermis, and their inner wall is supported by a 

 spirally coiled thread of chitin. Thus, when an 

 insect moults (we shall see shortly that this is a 

 frequent occurrence while growth is proceeding) it 

 discards not only its crust, or exoskeleton, but the 

 lining of its tracheae as well. 



With the internal organs of the insect we have not 

 space to deal. Since, however, we shall find much 

 food for thought in the behaviour of insects their 

 individual habits, parental activities, and apparent 

 cuteness in surmounting the difficulties of their every- 

 day life, it seems well to state that these creatures 

 are endowed with a highly developed nervous system, 

 which differs in important respects from that of a 

 vertebrate animal. In its more simple, ancestral 

 form, the insect's central nerve-cord was composed of 



