PLANT-EATING INSECTS 199 



require two seeds to enable it to complete its growth. 

 After consuming one, it drops to the earth, and (being a 

 legless maggot) drags itself along by its jaws until it 

 comes to another pod, into which its bites its way. 



Not a few insects are able to modify the growth of 

 plants in such a way that an excrescence, called a gall, 

 is formed. This power is not confined to one family. 

 Certain species of aphides, thrips, scale insects, two-winged 

 flies, beetles, as well as numerous Hymenopterous insects 

 and the caterpillars of a few small moths all produce char- 

 acteristic gall-structures ; while the bright scarlet " horse- 

 bean galls," which are often so abundant on the leaves of 

 the crack willow, are the work of a saw-fly called Nematus 

 gallicola. During April and May the parent insect lays 

 her eggs, by means of her wonderful twin-saw ovipositor, 

 within the leaf buds ; and as the leaves unroll, the galls 

 develop. For several weeks each remains a solid mass of 

 vegetable tissue, with the egg lying in a small cavity near 

 the centre. Then the larva hatches, and feeds upon the 

 inner portion of the gall, from which, when full-fed, it 

 issues and drops to the ground. Here it forms a cocoon, 

 changes to a pupa, and ultimately appears as a perfect 

 saw-fly. This happens in August or early September; 

 and each newly emerged female saw-fly oviposits at once 

 in developing leaf buds, with the result that a second 

 batch of galls shortly appears. Thus, the saw-fly achieves 

 two complete life-cycles in the course of each twelve 

 months. 



Professor Carpenter has pointed out that the pro- 

 duction of a gall seems calculated to supply the wants 

 of the insect with as little damage as possible to the 

 plant. We may appreciate this economy by comparing 

 the habits of Nematus gallicola with those of another 

 species of the same genus well known to gardeners as 



