10 INSECT PHYSIOLOGY 



to facilitate this process - indeed, that is often considered to be its 

 chief function. But in many insects the moulting fluid has practically 

 disappeared by this stage; the insect is quite dry, the inner surface 

 of the old skin only slightly moist ; and in many aquatic and terres- 

 trial pupae a layer of air appears before eclosion, between the pupal 

 skin and the developed adult - due, no doubt, to the energetic ab- 

 sorption of the moulting fluid. 



When the insect has withdrawn its limbs, and freed itself com- 

 pletely from the old skin or from the pupa, it must enlarge the new 

 cuticle rapidly to the required size, and bring about the expansion of 

 its wings, before the epicuticle has hardened. This it does in the 

 same manner, by swallowing more air or water, and forcing the 

 body fluid under pressure into the wings. In the Ephemeroptera and 

 in many Lepidoptera the sole function of the gut in the adult insect 

 is to receive air for this purpose. If the crop of the cockroach is 

 pricked with a needle, the distension of the new cuticle cannot take 

 place, and the insect collapses like a pricked balloon ; or if the tips 

 of the wings in the dragon-fly are cut off, blood drips from them and 

 they cannot expand. 



We may note here that the process of hatching from the egg is 

 similar in many particulars to that of moulting or emergence from 

 the pupa. In the mature embryo of grasshoppers there are glandular 

 organs ('pleuropodia') on the first abdominal segment which secrete 

 enzymes that dissolve the inner layers of the shell. The amniotic 

 fluid in which the embryo is bathed may be in part absorbed through 

 the cuticle of the young insect ; but in many cases the fluid is swal- 

 lowed by the mouth shortly before hatching (like the moulting fluid 

 of the silkworm). Many insects then tear or bite their way out of the 

 egg. Others swallow air, and by muscular contraction of the abdo- 

 men, drive the head or thorax against the shell and cause it to burst 

 open. Certain groups have special structures, spines, saws, hard plates 

 or distensible bladders, which enable them to increase the efficiency 

 of this process by concentrating the impact at one point of the egg, 

 and so piercing the shell or forcing off the egg cap. (This last arrange- 

 ment finds a parallel in the ptilinum of emergent Muscid flies, which 

 they distend with blood in order to force off the cap of the 'pupa- 

 rium'.) When the insect has left the egg, it commonly swallows air 



