External Factors that Influence Growth 269 
particularly those on the oak, willow, and rose. The most com- 
mon of these contain the larve of gallflies. Adler and Beyer- 
inck have studied experimentally the process of gall formation 
by gallflies, and their results have thrown much light on the 
processes involved. Previously it had been generally held that 
the growth leading to the formation of the gall is caused by a 
poison injected by the insect at the time of deposition of the egg. 
The swelling caused in animals by the sting of bees, for instance, 
may have led to this idea; but it has been shown that, in most 
cases examined, the secretion poured over the egg at the time of 
deposition only serves to fix the egg in place. It has also been 
shown that the poison of the bee does not produce a swelling or 
a gall when injected into the young tissue of a plant. Most galls 
do not begin to develop until the larva hatches and fastens its 
jaws in the surrounding cells. In only two forms has it been 
shown that a secretion may be responsible for the gall formation, 
which begins at once and is far along before the egg hatches. 
It has also been supposed that the wounding of the tissue caused 
by the puncture of the ovipositor is responsible for the growth, 
but this has been entirely disproven, because some forms, the 
Cecidomyide, do not pierce the tissue, but push the ovipositor 
into the bud without wounding it; also because the wounded 
part does not, as a rule, produce the gall, but only the region 
around the larva; the egg itself may be placed on a free surface 
not pierced by the sting. Furthermore, in some cases, the plant 
is pierced a long time before the gall develops, the latter occur- 
ring only when the larva emerges; thus Trigonaspes crustalis 
pierces the young leaf in May, but the larva does not hatch until 
September, and then the galls begin to develop. 
The galls become the abode of other species of gallflies and 
of other insects, parasites, and inquilines. When the larva that 
makes the gall is parasitized by the invading insects, the growth 
of the gall stops, as a rule, when the larva that made it is 
killed; but there are a few instances known in which the 
presence of the parasite seems to suffice to cause the continued 
growth of the gall, although it does not appear that the parasite 
