THE INTEGUMENT 9 



new cuticle is ready) the old skin needs but the slightest force for it 

 to split along the line of weakness (Fig. 3, D). 



fig. 3. — A, B, moulting of a cuticle in which an exocuticle is wanting 

 (Modified after Wigglesworth.) In A the new epicuticle is formed; 

 the digestion of old endocuticle has scarcely begun. In B the digestion 

 and absorption of the old endocuticle is almost complete. C, D, trans- 

 verse sections through the prothorax of an insect at stages corres- 

 ponding with A and B, showing the absence of exocuticle at the line 

 of weakness. (Semischematic) 



end, endocuticle; ep, epicuticle; ex, exocuticle; gl, dermal gland; mf, moulting fluid; 

 nc, new cuticle; oc, old cuticle; w, line of weakness in cuticle 



The rupturing force is the pressure of the blood. The insect con- 

 tracts the abdominal muscles, often repeatedly in a peristaltic fashion, 

 and so increases the pressure of blood in the thorax, which bulges 

 outwards until the old skin splits. Many insects add to the efficiency 

 of this process by first swallowing air (as vertebrates fill the lungs 

 with air during defaecation or parturition) or, if they are aquatic, by 

 swallowing water. Once the old skin is split, the insect escapes from 

 it by gentle peristaltic movements, often aided by gravity - for many 

 insects hang head downwards during moulting. 



It is commonly stated that the moulting fluid serves as a lubricant 



