8 BRITISH INSECTS 



the hindmost section of the body the abdomen. 

 The latter is made up of a varying number of rings or 

 segments, some of which are often fused together and 

 otherwise modified, so that their identification is 

 difficult. For the most part, however, the segments 

 form a more or less flexible series, thus permitting 

 the tip of the abdomen to be moved from one point to 

 another. Since the six legs of an adult insect are all 

 attached to the thorax, the abdomen must be de- 

 scribed as legless ; but it frequently carries appen- 

 dages, which probably represent the paired " limbs " 

 that were present on the posterior segments of the 

 wormlike ancestral stock. Indeed, among certain of 

 the lowliest existing insects we shall see that the 

 abdominal appendages still assist locomotion. In 

 many insects (e.g., cockroaches and May-flies) the 

 extremity of the abdomen bears two, or three, 

 antennae-like processes, termed " cerci," which appear 

 to be sense-organs perhaps tactile or olfactory. 

 Finally, we find that the tip of the abdomen is 

 usually adapted to meet the requirements of the 

 reproductive processes. In the female insect there is 

 often an elaborate egg-laying apparatus, or ovipositor. 

 We have now constructed a kind of mental key to 

 the outward form of an insect. It remains to be said, 

 however, that the visible crust, or cuticle, of the 

 creature is not its true " skin " in the strict sense of 

 the word. It consists of a peculiar non-living sub- 

 stance, horn-like in appearance, but chemically quite 

 distinct from horn, termed chitin. This is excreted by 

 the underlying layer of living cells which form the 

 true outer skin, or epidermis. The insect has no 

 bony skeleton. Its soft tissues are supported and held 

 together by this outer layer, or exoskeleton, to which 



