THE INTEGUMENT 5 



C. in the (dead) Rhotlnius) the crystalline layer of wax, which is only 

 about 0-25 micron thick, begins to loosen, and the rate of water loss 

 increases rapidly. The wax is secreted by way of the pore canals and 

 crystallizes on the surface, so that the insect is rendered waterproof 

 before moulting. The pore canals end below the epicuticle; but they 

 contain bundles of exceedingly fine filaments, the 'wax canal fila- 

 ments', about 80 A thick and visible only with the electron micro- 

 scope. These tubular filaments probably transmit wax or wax pre- 

 cursors from the cells to the surface of the cuticle. At the termination 

 of the pore canal they spread out fanwise and pass right through the 

 epicuticle. Other waxes may be secreted in association with shellac 

 or with protein and then gradually crystallize out - but the precise 

 mechanisms are not yet fully understood. 



Surface forces and the insect cuticle 



Since the surface of a body varies with the square of its radius, 

 whereas its volume or mass varies with the cube of its radius, it 

 follows that the ratio of surface to mass become progressively greater 

 as the size of the body diminishes. Consequently, the forces resident 

 in the surface become relatively stronger. In the case of insects, the 

 surface forces are, indeed, frequently great enough to move the 

 whole mass of the body ; and they introduce a factor into the life of 

 insects, particularly when they come into contact with water, which 

 is unnoticed by larger animals. It is necessary, therefore, to consider 

 the properties of the cuticle from this point of view. 



As a general rule, the cuticle is not readily wetted by water; there 

 is, in fact, a large 'angle of contact' between the water surface and 

 the cuticle (Fig. 2, A). Consequently, when an insect stands on water, 

 the surface tension acts in the opposite direction to gravity, and the 

 insect is held up as though by an elastic membrane. This 'hydro- 

 phobe' property may belong to the epicuticle; but in some cases at 

 least it is accentuated by the presence of glands producing a fatty 

 secretion. That is the case, for example, around the spiracles of mos- 

 quito larvae, and around the respiratory tube of the larva of Eristalis, 

 by means of which this insect, like many other aquatic larvae, is 

 enabled to hang suspended from the surface film with the remainder 

 of the body submerged (Fig. 2, B). 



