INSECTS IN RELATION TO PLANTS 



197 



by mites (Acarina) and a few plant excrescences are due to nematode 

 worms and to fungi. 



Among insects, Cynipidae (Hymenoptera) are pre-eminent as gall- 



FIG. 249. Galls of Holcaspis ditricoria, on oak. Natural size. 



makers and next to these, Cecidomyiidae (Diptera), Aphididae and 

 Psyllidae (Hemiptera); a few gall-insects occur among Tenthredinidae 

 (Hymenoptera) and Trypetidae (Diptera), 

 and one or two among Coleoptera and Lepi- 

 doptera. 



Cynipidae affect the oaks (Figs. 248, 249) 

 far more often than any other plants, though 

 not a few species select the wild rose. Ceci- 

 domyiid galls occur on a great variety of 

 plants, and those of aphids on elm (Fig. 250), 

 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 char- 

 acteristic, so that it is often possible to classify 

 galls as species even before the gall-maker is 

 known. 



Gall-Making. The female simply lays the egg on the epidermis, or 

 else punctures the plant and deposits an egg in or near the cambium, or any 

 other tissue capable of growth; the egg hatches and the surrounding plant 

 tissue is stimulated to grow rapidly and abnormally into a gall, which 



FIG. 250. Cockscomb gall 

 of Colo pita itlniicola, on elm. 

 Slightly reduced. 



