254 



SYSTEMATIC ZOOLOGY. 



a sting a weapon of offence and defence, the efficiency of 

 which is increased by an associated poison-gland. 



The lowest forms are the sawflies, the larvae of which are 

 vegetable-feeders, some eating the leaves of plants, others 

 boring in the solid wood. A little higher in the scale come 

 the gall-flies, those forms which lay their eggs in various 



FIG. 76. Ichneumon-fly, enlarged. From Riley. 



plants and in some way so stimulate the vegetable tissue 

 that strange growths galls are formed. Allied to these 

 last are the ichneumon-flies (fig. 76), which lay their eggs 

 in other insects. Here the larvae hatch out, feed upon the 

 host, at last destroying it. Then pupation comes, and 



