52 THE BIOLOGY OF THE SEASONS 



of the cells of the silk-glands, and outside the silk there is 

 a delicate fatty sheath which can be removed by washing. 

 The caterpillars use the silk to attach themselves to twigs, 

 to save themselves when they are about to fall, to lower 

 themselves from a branch to the ground, and to make a 

 cocoon for the period of metamorphosis. But there are 

 many caterpillars that are not silk-spinners. 



Returning to the structure of the caterpillar, we see that 

 the first three rings behind the head bear five-jointed legs 

 ending in a point. This region corresponds to the thorax 

 of the full-grown butterfly or moth, and though the legs of 

 the larva do not in the strict sense become those of the adult, 

 they correspond to them. Behind the thorax, and quite 

 continuous with it, is the abdomen, which consists of ten 

 segments. In most cases, however, the ninth is telescoped 

 and difficult to see. Short unjointed legs, ending in peculiar 

 gripping structures with a double crown of hooks, occur on 

 the third, fourth, fifth, and sixth rings of the abdomen, 

 and a large pair (sometimes transformed into protrusible 

 whips) is borne on the tenth and last. There are, of course, 

 occasional departures from the rule that there are five pairs 

 of pro-legs. Thus in the " loopers," which move in a 

 characteristic way, familiar in the common magpie moth 

 (Abraxas grossulariata), there are only two pairs. 



There are paired breathing apertures, leading into air- 

 tubes or tracheae, on the first thoracic ring, and on the eight 

 foremost abdominal rings. Not infrequently the body is 

 thickly covered with hairs which have irritant properties ; 

 or instead of hairs there may be bristles, spines, or warts. 

 Some caterpillars have offensive thoracic glands ; thus those 

 of the puss-moth larva secrete formic acid. Inside the body 

 there are, of course, organs corresponding to those that we 

 possess brain and nerve-cord, food-canal and associated 

 glands, complex muscles, air-tubes, a heart, excretory tubes, 



