CHAPTER VII 
GALLS CAUSED BY MITES (ACARI) 
M ITES and ticks are included in the order Acari. The 
systematic position of this order is in the class 
Arachnida, which also includes scorpions, spiders, and 
harvest spiders. Mites and ticks are creatures of such 
peculiar organization that they seem far removed from the 
other members of the Arachnida, but they appear to bear 
some affinities with the harvest spiders. 
The order contains two groups, the typical mites and 
ticks ( Acarina) and the worm-like group ( Vermiformia ). Gall- 
causers occur only in the latter group. In the Acarina the 
larva has at first only three pairs of legs, it acquires 
later the fourth pair; eyes are usually present. In the 
Vevmiformia there are no eyes and no tracheae. In some 
species the adult has four pairs of three-jointed legs, 
but in the family which contains the gall-mites the 
third and fourth pair of legs are missing; the first and 
second are placed on the forepart of the body, which is 
long and furnished with bristles arranged more or less 
symmetrically. 
The Vermiform mites are very minute, and are often over¬ 
looked in the absence of microscopic examination for them. 
The species known as Eriophyes fvaxini, which is responsible 
for the curious fasciations of the flowers of the Common Ash, 
is one of the pigmies of this pigmy race; it is quite invisible 
to the unaided eye, and may be best seen by washing a gall 
in a little water and examining a drop of the fluid under a 
J-inch objective. 
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