68 
ing these organs in a large series of specimens as well as in the various 
species precludes the present possibility of using them for specific 
distinction. Iam inclined to believe that there is some variation in 
the form of the vulva, due to the condition of the specimen. The lines 
and ridges are in a soft tissue, and so are lable to modification and 
distortion. 
There are four pairs of legs in the adult; in the young, as in the 
young of most other mites: but three pairs are present. Each leg 
consists of at least six joints, a coxa, more or less anchylosed to the 
body; a small trochanter; a femur, the longest joint; a patella, about 
one-half the length of the femur; a tibia, slightly longer than the 
patella; a tarsus, longer than the tibia and tapering to a fine point. 
The tarsus, near its tip, has a division more or less distinct, the small 
terminal piece being called the onychium; from the tip of this arises 
the claw, which is usually cleft into four slender parts, whence the 
name of the genus— 7etranychus—four-claw. The first pair of legs is 
the longest, though often scarcely longer than the fourth pair; the 
second and third pairs are plainly shorter than the other two and sub- 
Fig. 4.— Tetranychus : mouth parts, lateral and ventral views—highly magnified (original). 
equal in length. On the ventral surface the basal limits of the coxve 
are not well defined, but the cox are rather widely separate. 
The surface of the body is very finely and often regularly striate, 
mostly in a transverse manner. The hairs or bristles are minutely 
serrate, those on the anterior part of body curve forward, those on 
the posterior portion of body bend backward; the long ones on the 
legs appear to stand nearly erect in life, and doubtless have scme 
sensory function. 
There appears to be much uncertainty regarding the spinning appa- 
ratus, and an examination of a large number of specimens does not 
throw much light on the subject. Dujardin, Pagenstecher, and Don- 
nadieu believed that the thread issued from the mouth or in its close 
vicinity. Dufour and Dugés assert that it comes from the anal end of 
the body. Claparéde found some glands opening on the tips of the 
palpi which he considered as silk glands. The thread is not produced 
during the ordinary wanderings of the mite, but at certain times. 
When mites first attack a leaf there are only a few scattered threads 
lying close to the surface of the leaf, which are attached here and there 
