KELLOGG 



GALLS AND GALL-FLIES i i i 



With the birth of the larva begins the development of the gall, 

 which is an abnormal or hypertrophied growth of tissue about the 

 point at which the larva lies. The excitation or stimulus for the 

 growth undoubtedly comes from the larva and probably consists of 

 irritation by special salivary excretions and perhaps also of physical 

 irritation caused by the presence of a wriggling body. In some spec- 

 ies the gall grows around and includes but a single larva, in others 

 around several to many. The larva reaches its full development 

 about coincidently with the full growth or end of the vitality of the 

 gall, this period varying much with different galls. In the galls on de- 

 ciduous leaves the vitality is shortest, ending in autumn ; in twig-galls 

 it may not end until winter or even until the following or indeed the 

 second winter. When " dead " the gall dries and hardens, thus form- 

 ing a firm protecting chamber in which the larva or larvae pupate. 

 The pupa undergoes its non-food-taking life securely housed in the 

 dry gall, which may fall with the autumn leaves or cling to the bare 

 twigs. From the galls the fully developed flies gnaw their way out when 

 new leaves and tender shoots are appearing, ready to prick in new 

 eggs for another life cycle. 



But strange to say, with some species the new eggs may be deposi- 

 ted on plants of another kind and the hatching larvae stimulate the 

 growth of entirely different-shaped galls, and they themselves develop 

 into gall-flies of marked different appearance from their mothers. 

 These new gall flies in their turn lay eggs on the first host-plant ; 

 the forming galls are like those of the grandparent generation and 

 the fully developed flies are of the grandparent kind. This alterna- 

 tion of generations — a condition in which a single species appears in 

 two forms and produces two kinds of galls, usually on different host- 

 plants — has been long known, but still remains a problem which 

 interferes sadly with a number of popular biological generalizations. 

 One of these generations appears exclusively in only one sex, the 

 female, so that the other generation, composed of both males 

 and females, is produced uniformly from unfertilized eggs. The 

 adults and galls of the two generations were formerly described as 

 belonging to different Cynipid species. Not all gall-flies, however, 

 show this dimorphic condition ; some appear habitually in but one 

 form and produce but one kind of gall ; in most if not all of these 

 cases the species is represented only by female individuals. 



The great variety of the galls, the extraordinary instinct which 

 leads the adult flies to the right selection of plant and position on 



