INSECTS IN RELATION TO PLANTS 



255 



FIG. 249. 



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 

 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 

 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 is 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 



Cockscomb gall of Colopha ulmicola, 

 on elm. Slightly reduced. 



