66 
this country. So it happens that throughout our economic literature 
there occur accounts of Zetranychus telar‘us without any consider- 
ation as to whether the form mentioned was identical with the Euro- 
pean species of that name. The first break from this custom was 
made by Dr. Riley, who, in 1889, recognized that the form occurring 
on oranges was different from 7! telarius. In 1893 Professor Harvey 
described a form found in greenhouses in Maine and New York as 7: 
bimaculatus. With these two exceptions Tetranychus telarius is still 
considered responsible for all ‘‘red spider” damage. In 1877 Prof. 
A. Targioni Tozzetti, ina paper on the mouth and feet of Zetrany- 
chus, showed that the form known as 7! socdus differed from 7° telarius 
in the nature of the tarsal claws. In the latter, as in many other 
forms, the claw is suddenly bent before the middle and the apical 
part four-cleft; in 7. soczus the claw is less curved and simple.  Simi- 
lar differences obtain in certain of our species, as will be noticed 
below. In Prof. A. Berlese’s recent work, Acari Italiani, he draws 
attention to a hitherto unused character, namely, the small projections 
on the tip of the ‘‘ thumb” of the palpus. These vary in number, size, 
and shape, and, I believe, form good characters for the discrimination 
of species. 
STRUCTURE. 
The mites are an order in the class Arachnida. In this order there 
have been various divisions, yet Zetranychus has always been kept in 
association with certain other well-known forms, as Zrombidiwm, Bdella, 
Eupodes, Erythreus, etc. This association or group of forms has 
received various names, such as 7rombidide and Prostigmata. With 
Tetranychus has been associated certain other genera to form a fam- 
ily —Tetranychide—the limits of which are uncertain, and according 
to Canestrini (Pros. Acarofauna Ital.) can not be defined. But the 
genus Tetranychus can be separated on fairly definite characters. They 
are spinning mites; the body is ovate, roughly oblong, or pyriform; 
clothed above with about 24 to 36 bristles, usually long, and arranged 
somewhat roughly in four rows. The legs are never much longer 
than the body, and the joints are never enlarged. The anterior third 
of the body shows a slight constriction, which marks the division 
of the body into cephalothorax and abdomen. On the underside 
there is no division, but if the natural division is vertical (which 
I think improbable) the third and fourth pairs of legs arise from the 
abdomen. The body is broadest at the base of the abdomen. The 
cephalothorax is somewhat semicircular in outline. On each side about 
half way and near the margin is an eye-spot, containing usually, pos- 
sibly always, two ocelli. Some authors have figured but one ocellus 
each side. The fore part of the cephalothorax incloses the mouth- 
parts. Above, there is a plate about twice as long as broad, and 
