External Factors that Influence Growth 269 



particularly those on the oak, willow, and rose. The most com- 

 mon of these contain the larvae 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 swelHng 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 

 Cecidomyidae, 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 



