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 galls are of various forms, but differ from those pro. 
duced by gall-flies (Family Cynipidce of the Order Hymen\ 
opterci) 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, Phytoptus 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- 
Fig. 53.—Diagram of gall of Phytoptus pyri : 
g* gall; n, m, normal structure of leaf ; o , open¬ 
ing of gall ; e , eggs. (After Soraur). 
sents a section of a leaf through one of these galls. 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 
) 
