THE ACARINA OR MITES— BANKS. 



103 



Nail-gai,j,s dk an erio- 



PHYES. 



hairs. These hairs are usually thickened and twisted, and the whole 

 mass is of an even height. The mites live amonu- these deformed 

 hairs, sucking the juices of the leaf. As the juice becomes exhausted 

 the erineum becomes reddish or rusty brown 

 in color, and is a very prominent object. 

 At this stage, when the erineum is most 

 easilv noticed, one is apt to tind few if anv 

 mites, as they have left for fresh pastures. 

 The galls may l)e on either surface of the 

 leaf, though conmionly abo^'e. The form 

 is quite characteristic of the species, though 

 there is usually some variation. These 

 galls alwavs have an opening through which 

 the mites can pass. This character will dis- 

 tinguish these galls from those of Diptera 

 and Hymenoptera, but not from Homop- 

 terous galls. The opening is often ver^- 

 small and concealed by tufts of hairs. 

 Within, the gall is often parth' tilled up 

 with folds and projections, and sometimes 

 with hairs. In color the gall is at first like the leaf, but gradually 

 turns yellow or reddish, and then brown or black. Sometimes the 

 gall covers a great deal of space, l)ut does not sw^ell up much, in ap- 

 pearance nuich like a blister. 



Galls are formed while the leaf is growing rapidlv. It is supposed 

 that the puncture of the plant-cells by the mite 

 induces an increased flow of sap in that direction, 

 which causes the spot to grow faster than the 

 surface around it, so that this spot must sw-ell up 

 in the form of a gall. This, however, does not 

 account for the diversity of form of the galls, 

 and wh}' each gall is characteristic of the mite 

 that made it. Some species of Eriophyidje live 

 in plant-buds, and their feeding prevents the 

 opening of the bud, which after a time dries up 

 and dies. With other species the buds swell to 

 a great size, but never open. Other mites pro- 

 duce a curling or rolling of the edge of the leaf, 

 •or a slight folding of the surface. Some live on 

 the surface of fruits, as the orange-rust mite. 

 A few species produce galls or excrescences on 

 twigs, especially near the base of terminal buds. 

 The diseased condition produced l)y these mites 

 has been termed phytoptose or erinose. 



The eggs of the Eriophyidiv are laid u})on the surface of the leaf. 



Fig. ]y6. — Rib-gai,i,s of ax 



ERIOPHYES. 



