44 THE STUDY OF INSECTS. 



is a dry season, it infests fruit-trees in the open air. As it 

 thrives only in a dry atmosphere, it can be subdued upon 

 house-plants by a liberal use of water. When it occurs 

 upon plants in the open air it can be combated with any of 

 the washes found useful in destroying scale insects. 



Some of the mites that infest plants produce galls. 

 These galis are of various forms, but differ from those pro. 

 duced by gall-flies (Family Cynipidce of the Order Hymen, 

 optera] in having open mouths, from which the young mites 

 escape. 



A common disease of the pear, known as the pear-leaf 

 blister, is produced by a four-legged mite, PJiytoptus pyri 

 (Phy-top'tus py'ri) (Fig. 52). The blisters characteristic of 



FIG. 52. Phytoptus pyri, greatly enlarged. 



the disease are swellings of the leaf, within which there is a 

 cavity affording a residence for the mites. Figure 53 repre- 



g 





FIG. 53. Diagram of gali of P/iytofitus pyri : 

 f, gall; w, , normal structure of leaf ; o, open- 

 ing of gall ; e, eggs. (After Soraur). 



sents a section of a leaf through one of these galis. Here 

 the leaf is seen to be greatly thickened at the diseased part. 

 On the lower side there is an opening through which the 

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



