THE RED SPIDERS OF AMERICA AND A FEW EUROPEAN 

 SPECIES LIKELY TO BE INTRODUCED. 



E. A. McGregor/ 



Of the Bureau of Entomology , United States Department of Agriculture. 



The so-called red spiders or s{)innmo- mites (which were all included 

 in the genus Tetranychus until within the last few years) are without 

 doubt the most important economically of the mites, and cause 

 gi-eat damage annually to many of the most valuable cultivated 

 plants. The writer has previously estimated that at normal prices 

 the cotton crop of the United States may suffer through the ravages 

 of the common red spider (Tetranychus himacvlatus Harvey) to the 

 extent of $2,000,000. Leading crops in various parts of the world 

 are Imown to suffer similarly. 



As Tragardh and others have pohited out, the systematic knowledge 

 of the red spiders is by no means on a plane with the comprehension 

 of their economic status. Moreovei-, not until the completion, re- 

 cently, of the investigation of the red spider on cotton (23 )2 by the 

 Bureau of Entomology, was much known as to accurate details of 

 their biology. Banlcs in the United States and Berlese in Europe 

 were the pioneers who undertook the taxonomic separation of the 

 species of this group. As Zacher (13) and the present writer (20) 

 have already pointed out, the ordinary microscopic equipment with 

 which these mites previously have been studied was far too crude, 

 and only on such grounds could the finer taxonomic details have 

 escaped such excellent observers as the two above mentioned pioneer 

 acaridologists. 



In Banks's Red Spiders of the United States (9) he separates the 

 species chiefly through the palpal and tarsal characters. These are 

 still very useful in the taxonomy of the group, but to them have been 

 added the so-called collar tracheae and the penis. In order properly 

 to examine these structures it is absolutely necessary to employ an 



' Since this paper was written the author has resigned from the Bureau of Entomology. 

 " The numbers in parentheses refer to " References " at the end of this paper. 



Proceedings U. S. National Museum, Vol. 56— No. 2303. 



1 15690— 19— Pioc.N.M.vol.56 41 641 



