GALLS CAUSED BY GALL-WASPS 
25 
brood it may be greatly prolonged, the larva remaining 
unchanged for several years. The larvae of Callirhytis 
glandium, which galls acorns, may delay metamorphosis for 
three years. Mr. Fitch collected some galls in 1878 in 
which the larvae were still living in 1881. The pupa is 
white and fleshy. 
The imagines (perfect insects) are sluggish and of un¬ 
interesting habits. The sexes are much alike; there is 
no marked difference in coloration ; the males are a little 
shorter and have longer and thinner antennae. The sexual 
and agamic females show many points of difference. The 
agamic are usually larger, differently coloured, and the 
ovipositor differs both in form and size. 
In some species the imagines are so much alike that the 
only certain test of identification is the gall from which they 
have emerged. 
It is now generally agreed that cynipidous galls arise 
from the irritation of the meristematic tissue by the move¬ 
ments of the larva. The egg alone does not give rise to 
gall growth, for in some cases it is deposited weeks before 
the gall begins to form. Cameron* concluded “(1) that 
there is no evidence that the venom has anything to do 
with the origin of the gall—on the other hand, there is 
every reason to believe that its use is to close the wound; 
and (2) that as observation shows that the mechanical 
irritation produced by the birth and growth of the larva is 
the primary factor in gall genesis, we may fairly conclude 
that the theory of mechanical irritation is more in con¬ 
sonance with observed facts than the infection one.” There 
is no permanent opening in the galls of the Cynipidae ; the 
larvae are immersed in the substance of the gall, and pupate 
in it; the imagines bore their way out. 
The galls may be only slightly attached to the plant, or 
more or less embedded in the tissue. If there is only one cell 
within, the gall is styled “monothalamous ” or “ unilocular ” ; 
when many cells are present it is termed “ polythalamous ” or 
* “ British Phytophagous Hymenoptera,” vol. iv., pp. 23, 24. 
