186 



EEPOET 108, U. S. DEPARTMEXT OF AGRICULTURE. 



Fig. 283.— Nail gall 

 (Original.) 



Fig. 284.— Blister gall. (Original.) 



on the same plant and produce galls of different shape. These galls are of various 

 shapes, according to the species of mite, and in general those on leaves may be classi- 

 fied in groups. Some, a simple erineum (fig. 287) or "frost gall" ; in others the area of 

 the erineum is depressed, making a "dimple gall" (fig. 280); in 

 others tliis depression becomes deeper and the margins grow toward 

 each other, making a "round " (fig. 288) or "capsule gall " (fig. 281). 

 The roimd gall may become elongate, forming a "pouch gall" 

 (fig. 282); or become flattened, making a "blister gall" (fig. 284); 

 or may lengthen into a pointed process and is then called a "nail 

 gall" (figs. 283, 285). Swellings along a vein are termed "rib galls" 

 (fig. 286). 



The eggs (fig. 278) of the Eriophyidse are laid upon the surface of 

 the leaf, sometimes before the leaf has opened. They are attached 

 singly, are nearly spherical in shape, and pale yellowish or gray- 

 ish in color. The eggs are quite large as compared with the mite. 

 The young, at birth, are helpless and without tarsal appendages, 

 but soon molt and obtain them. The mites can move quite swiftly, considering their 

 size and sometimes they spread over a tree with wonderful rapidity. The anal sucker 

 aids them in holding onto a surface, but not in locomotion. They molt four times, it 

 is said, before becoming adult, but pass 

 through no changes in structure, except 

 the development of the genital apertures. 

 At each molt there is a resting period when 

 the mite is within its old and now loose 

 skin. With the drying up of the food plant in the fall the mites seek winter quartero 

 within the buds or beneath the bud scales. Sometimes, doubtless, they winter under 

 pieces of bark. When in a bud they begin to feed on the leaf and produce the gall 

 before the bud is fully open. 



The Eriophyidse are, perhaps, more closely related to the Tarsonemidae than to any 

 other group. Many of the Tarsonemidae feed exposed on leaves or stems of a plant; 



some of them cause swellings or deformities of the 

 plant, and some have the body more or less distinctly 

 segmented. Moreover, in several genera of Tarsone- 

 midae there is a tendency to the reduction in the num- 

 ber of the legs, and in other genera the hind legs are 

 very slender and of little use in walking. 



The few tropical species known belong to the com- 

 mon European genera, their galls are similar to those 

 of the temperate zone, and several of our species are 

 identical with European forms. There is not much 

 diversity of form in the family, and generic classifica- 

 tion is based on few and rather simple characters. 

 Quite a number of galls have been collected in the 

 United States, but the mites have not been studied 

 except by Profs. Garman and Ilodgkins, who have 

 described a few species. Several European acarolo- 

 gists have carefully studied these mites in recent 

 years, but the work of Alfred Nalepa has been pre- 

 eminent. 

 The better-known genera may be separated as follows: 



Number of abdominal rings on dorsum and venter nearly equal Eriophyes. 



Number of alxlominal rings on venter nearly twice as many as on dorsum. . . 2 



Dorsum \Wth the middle area highly arched 4 



Dorsum of an even curve 3 



Fig. 285 —Nail galls of an Eriophyes. 

 (Author's illustration.) 



