Spiders and Their Near Relatives 



which the mite that started the gall entered, and from which young 

 mites developed in the gall can escape, in order to start new 

 galls. In addition to the swelling of both surfaces of the leaf 

 its internal structure is seen to be modified. In some parts there 

 is a great multiplication of the cells, and in others a large part 



Fig. 69. GALL OF ERIOPHYES PYRI 

 g, gall n,n, uninjured part of leaf 0, opening of gall 



of the cells have been destroyed. Two eggs of mites are rep- 

 resented in this gall. As the season advances, and the galls 

 become dry and brownish or black, the thicken- 

 ing of the leaf becomes less marked. In fact, 

 in some cases there is a shrinkage of the parts 

 affected. Figure 69 represents a section through 

 a leaf collected and studied in October. 



The most conspicuous of the abnormal 

 growths on leaves caused by mites are bright 

 coloured patches, often mistaken for fungoid 

 growths; these occur on the leaves of various 

 trees, and are due to an abnormal development 

 of the epidermal plant cells or a deformation of 

 the plant hairs, caused by the mites. This type 

 of abnormal growth is termed an erineum 

 (e-rin'e-um). 



The members of the family Demodecidae 

 resemble the gall-mites in having the body 

 annulate and prolonged behind; but differ in 

 having eight legs which are three-jointed. It 

 includes a single genus, Demodex, the species of 

 which are found in the sebaceous glands and 

 hair-follicles of various mammals. The species 

 that infests man is Demodex folliculorum 

 (Dem'o-dex fol-lic-u-lo'rum) (Fig. 70). It was formerly sup- 

 posed to be the cause of "blackheads" or comedones (com-e- 

 do'nes) on the face; but it has been found that the mites occur 

 in healthy as well as diseased follicles. 



Fig. 70. DEMODEX 

 FOLLICULORUM 



(after Megnin) 



94 



