INSECTS IN RELATION TO PLANTS 
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than any other plants, though not a few species select the wild 
rose. Cecidomyiid galls occur on a great variety of plants, and 
those of aphids on elm (Fig. 249), poplar, and many other 
plants; while psyllid galls are most frequent on hackberry. 
The galls may occur anywhere on a plant, from the roots to the 
flowers or seeds, though each gall-maker always works on the 
same part of its plant,—root, stem, 
bud, leaf, leaf-vein, flower, seed, etc. 
Galls present innumerable forms, 
but the form and situation of a 
gall are usually characteristic, so 
that it is often possible to classify 
galls as species even before the 
iG: 3210: 
gall-maker is known. 
Gall-Making.—The female cy- 
nipid punctures the plant and lays 
an egg in the wound; the egg 
hatches and the surrounding plant 
tissue is stimulated to grow rapidly 
and abnormally into a gall, which 
serves as food for the larva; this 
transforms within the gall and es- 
capes as a winged insect. The 
physiology of gall-formation is far Tonnies gee ae ene 
from being understood. It has been 
found that the mechanical irritation from the ovipositor is not 
the initial stimulus to the development of a gall; neither is 
the fluid which is injected by the female during oviposition, this 
fluid being probably a lubricant ; if the egg 1s removed, the gall 
does not appear. Ordinarily the gall does not begin to grow 
until the egg has hatched, and then the gall grows along with 
the larva; exceptions to this are found in some Hymenoptera 
in which the egg itself increases in volume, when the gall may 
grow with the egg. It appears that the larva exudes some 
fluid which acts upon the protoplasm of certain plant cells (the 
cambium and other cells capable of further growth and multi- 
plication) in such a way as to stimulate their increase in size 
