126 . 
badly affected by the little blister-makers presents a pitiful appear¬ 
ance, with its speckled or spotted leaves and its stunted growth. 
The fruit, if any, is poor in size and quality, ^and there is evidence 
every way of the slow mischief accomplished.” 
It has been supposed bv some that plants must be diseased before 
insects will attack them, but Boisduval has shown that such is not 
the case. Plauts in perfect health may be infected and injured by 
placing them with those already attacked, or by bringing infested plants 
among them. That strong and actively growing plants sometimes do 
not appear to be affected by mite attacks, is what we should expect, 
and is not necessarily evidence that mites have not inflicted injuries 
upon them. In the case of the healthy plant, the growth may 
overbalance the injuries, and the latter not be noticeable, while in 
the unhealthy plant, the injury is more liable to overbalance the 
gain to the plant by growth, and so attracts our attention. Moreover, 
as in the case of the attacks of the chinch-bug upon corn, seasons 
most favorable to the development of plant-feeding mites are, as a 
rule, least favorable to vegetation, and their attacks are, in conse¬ 
quence, all the more dangerous. 
The plant-feeding mites may be roughly divided into two groups: 
those which live exposed upon the plant, and those which cause 
abnormal growths on the leaves or stems, wdiich afford them shelter. 
To the first group belong the spinning mites, Tetranychi, and those 
of this group best known for their injuries belong to the genus 
Tetranychus. These mites are commonly known as red spiders, 
from their prevailing color and their habit of spinning a fine web 
on the surface of the leaves they infest; but they are true mites, 
differing from spiders in their minute size and in the character of 
their mouth parts. They work on the under side of the leaves, and 
may be there found in great numbers on badly injured plants. To 
the second group belong what are known as gall-mites (Phytopti) 
from the galls and growths of hair which their attacks cause to 
appear. They are best known from their galls, and the injuries 
they inflict, since the mites themselves are so small that even 
when abundant they escape detection. By opening one of the 
galls .and washing it out in a little water, the mites will appear as 
small whitish particles floating on the surface. Under favorable 
conditions they become so numerous that they leave their galls and 
collect upon plants in such quantities as to resemble a powdery 
coating on the leaves and twigs. 
Mites injure plants partly by puncturing them with their needle- 
shaped maxillae, (of which each mite has a pair), and sucking the 
juices of the plants, but quite as seriously also by interfering with 
the respiratory and assimilative processes in which the leaves are 
engaged. From this double injury the healthy green color of the 
leaves is exchanged for a sickly yellow hue, or brown spots appear 
at the points attacked, and by spreading and fusing, give a prevail- 
ng brown color to the leaves. The disease, so-called, is known as 
acariasis ; if the injury has been done by the gall-mites, it may be 
called phytoptosis ; the former term comprehends the latter, and 
answers all the requirements of convenience. 
