268 LECTURES ON BIOLOGY 



the blade becomes black, for chemical union has taken place 

 between the iron and the tannic acid of the gall, the result being 

 ferrous gallate or, in other words, ink. Apart from gall-flies, 

 several beetles, leaf-wasps, plant-lice, and gall-midges also 

 produce galls. 



Galls are nothing else but the brood-chambers in which the 

 ova pass their development. Thus 'two birds are killed with 

 one stone ' : the growing larvae enjoy a perfect protection against 

 most animals, and have at the same time in the juices of their 

 host-plant a rich source of food. Sometimes each gall contains 

 only one larva, sometimes several. 



One of the most interesting kinds is the little oak gall-fly, 

 JBiorhiza renalis, whose strange life-cycle was first discovered by 

 Adler. Before the beginning of spring, the first generation of 

 small wingless creatures leave their brood-chambers. They differ 

 so much in appearance from the gall-flies that it is hard to believe 

 they belong to the species. All individuals are females. As 

 soon as they enter the outer world they set about their most 

 important and indeed only task, egg-laying. The oak-trees, like 

 all nature, are still in their winter sleep ; young and tender 

 leaves are, therefore, not available for the brood of these wasps, 

 and they must undertake the laborious task of sinking their eggs 

 into the firmly closed winter buds. For this purpose the females 

 are splendidly equipped, their strong, dagger-like ovipositor ap- 

 pearing expressly created for perforating the firm cover-leaves. 

 Nevertheless it requires many hours before the wasp has pene- 

 trated to the interior of the bud, bored numerous fine tubes 

 and deposited into each one egg. In the course of the spring 

 these buds develop into large galls containing numerous larvae. 

 In July the second generation emerges, this time males and 

 females, typical gall-flies, distinguished, apart from their wings, 

 by their larger size, longer legs, and different formation of their 

 ovipositor. After fertilization the females lay their eggs singly 

 into the parenchyma of the under-side of the young oak-leaves, 

 and around each leaf is formed a small, longish gall from which 

 the parthenogenetic generation of females is hatched before, the 

 coming of the next spring. 



How ingeniously has Nature cared here again for the pre- 

 servation of the species? In the inclement season a maleless 



