220 INSECT TRANSFORMATION 



aspect of most of the segments, but the mouth-armature 

 has become extremely feeble, the hooks being far smaller 

 than those in the first or second stage. The larva, relatively 

 stouter than in the previous stages, has now no further paths 

 to travel through the tissues, but it makes a hole through 

 the skin and lies with its tail-spiracles just beneath this hole, 

 and therefore in touch with the atmosphere, while its body, 

 directed obliquely downwards, lies in a cavity the hollow 

 of the small tumour or " warble " due to its presence. This 

 swelling increases with the growth of the maggot, which, after 

 another moult, enters its fourth and final stage (Fig. 109 b). 

 Its spiny armature is now again strong ; as in the first stage 

 there are dorsal as well as ventral rows of spines on most of 

 the body segments, and the tail-spiracles are large and con- 

 spicuous, but the mouth-hooks are minute. During the third 

 and fourth stages the maggots feed by imbibing the fluid 

 which exudes from the inflamed tissues affected by their 

 presence. Ripe warbles, forming prominent swellings along 

 the host-animal's back on either side of the spine, are evident 

 from February on till May or June, and the fully fed larvae 

 become barrel-shaped and dark in colour. Ultimately each 

 maggot works its way through the breathing-hole in its host's 

 skin and falls to the ground, where it pupates, the hardened 

 larval cuticle forming a firm, black puparium (Fig. 109 d) 

 from the front end of which a round lid splits off to allow the 

 escape of the fly. The pupal period lasts about six weeks. 



While many insect -larvae live thus as parasites in the bodies 

 of large animals, a far greater number are parasitic in other 

 insects, the female adult laying her eggs on the body of an 

 appropriate host, or if, as is usually the case, she possesses a 

 piercing ovipositor, inserting this through the cuticle and skin 

 so that the eggs are laid among the internal tissues on which 

 the larvae feed. The vast majority of these insect-larvae 

 parasitic in other insects, feed on the larva of their host, and 

 a single species of insect may serve as host to a large company- 

 of parasites. For example, caterpillars of the Gipsy moth 

 (Porthetria dispar), a common European species, often destruc- 

 tive in woodlands, and inadvertently introduced some years 

 ago into the eastern United States, are known to serve as 

 hosts for more than forty hymenopterous and twenty-six 



