214 



ENTOMOLOGY 



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. 253. Cockscomb gall of 

 Colopha ulmicola, on elm. Slightly 

 reduced. 



FIG. 252. Galls of Holcaspis duricoria, on oak. Natural size. 



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

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



(Hymenoptera) and Trypetidae (Diptera), 

 and one or two among Coleoptera and 

 Lepidoptera. 



Cynipidae affect the oaks (Figs. 251, 

 252) far more often than any other plants, 

 though not a few species select the wild 

 rose. Itonidid galls occur on a great 

 variety of plants, and those of aphids on 

 elm (Fig. 253), poplar, and many other 

 plants; while psyllid galls are most fre- 

 quent 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 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, 



