PAE KEDPSPIDERS OF THE UNITED STATES. 
(Tetranychus and Stigmeus. ) 
By NatHan BANKs. 
HISTORY. 
The genus Zetranychus was established in 1832 by Dufour for a small 
mite found living gregariously on clumps of gorse. He noticed that 
there was a milky whitish irregular web or mesh of threads upon 
which crawled tiny red objects, which, by the aid of a glass, he discoy- 
ered were mites. He kept specimens in 
a bottle, and saw them produce a similar 
web. Since then these creatures have 
been known as ‘‘spinning mites.” Simi- 
lar mites were known from the time of 
Linneeus, and one described by him as 
Acarus telarius is considered identical 
with Dufour’s form. These spinning 
mites were found on a great variety of 
plants, and showed differences in size and 
color, which, coupled with their different 
habitat, led the naturalists of that day to 
consider these forms as separate species. 
Gradually, it was discovered that certain 
species were extremely variable*in color, 
and occurred on more than one plant. 
This overthrew the old classification, and 
since then it has never been determined 
what was a species in this genus. The 
best two modern authorities working on 
the same (Italian) fauna reach quite differ- 
ent results. Certain forms of these mites 
have caused considerable injury to plants, 
especially in greenhouses, and from the red color of most of them, 
have been termed ‘‘red spiders.” As some of the early accounts con- 
sidered the common red species as 7) felar/us Linn., this name has 
been applied indiscriminately to all red spiders, both in Europe and 
21490—No. 8 4) 65 
N fh 
Fic. 1.—Tetranychus bimaculatus—en- 
larged (original). 
