506 The Outline of Science 



General Characters of Insects 



Insects, Peripatus, Centipedes, and Millipedes have in com- 

 mon a respiratory system consisting of tubular trache;e, which 

 marks them off from the gill-breathing Arthropods (Crusta- 

 ceans), and sensitive feelers, which distinguish them from the 

 Spider and Scorpion group (Arachnids) . In the class of Insects 

 the body in the adult state is divided into three main regions: 

 (1) the head; (2) the thorax or fore-body; (3) the abdomen 

 or hind-body. 



The outer covering of most insects is hard and firm, com- 

 posed of a non-living cuticle made of chitin, a somewhat horn-like 

 substance secreted by the underlying living skin. The chiti- 

 nous plates, which make a protective armour, are firmly fused 

 in the head region, but in the thorax and in the abdominal part 

 the different rings are joined by flexible areas, permitting more 

 freedom of movement. Thus the segmented architecture of the 

 body is more clearly seen in the thorax and abdomen than in 

 the head region, where fusion has obliterated the boundaries of 

 the successive segments of the body. In rapidly flying insects 

 there is often a fusion of thorax rings to form a firm basis 

 for the action of the wings. 



It must be clearly understood that in the insect's body the 

 muscles are inside the skeleton, whereas in ourselves the skeleton 

 is covered by the muscles. The two plans of architecture are 

 utterly different. 



The Insect's Head 



The insect's head, which bears one pair of feelers or antennae 

 and usually three pairs of jaws, is relatively small, firm, and 

 compact, separated from the thorax by a narrow membranous 

 neck allowing freedom of movement. One sees this very well 

 on a common house-fly. All adult insects (except some prim- 

 itive and some degenerate species) have a pair of compound 

 eyes, though simple eyes may be present also. The compound 



