Wasps, Bees, and Ants 
469 
of the wriggling body. In some species 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 deciduous 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 deposited 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 markedly 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 grand¬ 
parent kind. This alternation 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 belong¬ 
ing to two different Cynipid species. Not all gall-flies, however, show this 
dimorphic condition; some appear habitually in but one form and pro¬ 
duce 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 twig or leaf for ovi- 
Fig. 657.—Galls made by a 
Cynipid gall-fly. (Natural 
size.) 
