4 INSECT PHYSIOLOGY 



harder than calcite, and that for their mandibles and claws which they 

 need as hard as possible the Crustacea provide a covering of sclerotin. 

 The sclerotized mandibles of some insects are so hard that they can 

 readily bite through sheets of foil of lead, copper, tin, zinc or silver. 



The epicuticle 



The endocuticle is responsible for the extensibility of the integument 

 and for combining toughness with flexibility. The exocuticle provides 

 the rigidity in the hard parts such as the head capsule, the segments 

 of the limbs, and so forth. The epicuticle is responsible for the im- 

 permeability of the cuticle, and particularly its power of preventing 

 the loss of water by evaporation. 



When the epicuticle of the fully developed integument is examined 

 microscopically it appears as a refractile amber-coloured layer, 

 usually not more than a micron in thickness. It is inextensible; but 

 where the cuticle is liable to bend or stretch it is deeply folded. The 

 epicuticle is a highly complex structure. Its nature certainly varies in 

 different insects, and the only way in which it has been possible to 

 learn something of its composition has been to observe the stages in 

 its development when a new cuticle is being formed. 



At this time the epidermal cells first lay down a thin layer of 

 lipoprotein which subsequently becomes tanned like the exocuticle 

 to form the epicuticle as visible with the microscope. When it is first 

 formed this layer is pierced by the pore-canals and from these there 

 exudes upon the surface a viscid secretion which is rich in the pheno- 

 lic substances that are used to tan the cuticle. Shortly before moulting 

 a crystalline layer of wax separates out over the surface of this secre- 

 tion and serves to waterproof the cuticle. And finally, at the time of 

 moulting or soon afterwards, little glands (dermal glands) pour out a 

 protective covering of secretion, called the 'cement layer', which 

 spreads evenly over the wax. The nature of the cement layer is not 

 fully known, but there is evidence that in the cockroach and other 

 insects it contains resinous material identical with shellac. 



The waxes of the epicuticle play a most important part in protect- 

 ing the insect from loss of water. They vary in character from soft 

 greasy materials to hard crystalline substances like beeswax. If the 

 insect is exposed to a high temperature (35° C. in the cockroach, 55° 



