402 THE BIOLOGY OF INSECTS 



Hypodenna, as recently pointed out by E. W. Laake (192 1), 

 though it resembles the preceding instar in general form, 

 mouth-hooks and tail-spines, differs from it in the complete 

 absence of spines on the body-segments, the surface of the 

 cuticle being quite smooth. Nevertheless it wanders by 

 devious ways, to the final position beneath the skin of the 

 beast's back where the warble-maggots may be found in 

 the third, fourth, and fifth stages from January until June, 

 by which time most have become " ripe," although some 

 belated individuals remain in the cattle until July or even 

 August. The maggots pursue various paths in their journey, 

 some taking a direct route backwards by way of the dorsal 

 muscles and the vertebral canal, others travelling in the 

 tissues of the diaphragm and thence working to the region 

 of the back. Hadwen (19 16) traced the course of one from 

 the gullet to the margin of the diaphragm and thence 

 through the intercostal muscles between two ribs to the 

 neighbourhood of the backbone. The travels of these 

 maggots through the body of the bullock or heifer, by 

 various paths to a definite goal are surprising, and illustrate 

 vividly the adaptation of parasite to host. It is certain, 

 however, that a considerable proportion of the young larvae 

 which, after hatching, bore into the skin, may lose their way 

 in the tissues of their host and perish long before the 

 attainment of their full growth. In the course of experi- 

 mental work it was found that out of 187 eggs laid by a 

 female Hypoderma hovis on the heel of a calf only 41 ripe 

 maggots were developed in the succeeding spring. 



Soon after arrival beneath the skin of the back the 

 warble-maggots bore through the hide, thus establishing 

 contact with the outer air for breathing and later emergence, 

 undergo another moult (Plate XVI, A), and assume the 

 fourth larval form. This is relatively stout with rows of 

 spines on the ventral aspect of the body-segments and with 

 conspicuous circular tail-spiracles directed to the air-hole 

 through the host's skin (Plate XVI, B). The most remark- 

 able change in this instar as compared with the earlier stages, 

 is seen in the mouth armature ; as described by G. Phibbs 



