36 



Until within the year 1900 the common red spider most often occur- 

 ring in greenhouses was technically designated as Tetranychus telarhis 

 Linn., a name which has been rather indiscriminately applied to all 

 species of red spiders, both in America and abroad. 



Red spiders are not true insects, in fact not even spiders, but are, 

 more properly speaking, spinning mites. Since, however, they are 

 almost universally known as red spiders, this term is retained for 

 present purposes. 



As the word "mite" indicates, these insects are extremely minute, 

 and when they occur in ordinary numbers are not apt to be noticed 

 unless leaves are carefully scrutinized. Attention, however, is certain 

 to be drawn to them when they become excessively numerous, as fre- 

 quently happens in neglected greenhouses 

 or out of doors during droughts in sum- 

 mer. 



Red spiders spin threads, but do not, 

 like true spiders, utilize them for climb- 

 ing or descending from a height. The 

 threads spun are extremely fine and 

 scarcely perceptible to the unaided eye, 

 but a web of threads is frequently so dense 

 as to form a tissue plainty visible at a little 

 distance. Webs are usuallv constructed 



Fig. 9.— Tetranychus bimaculatus: 

 adult— enlarged (from Banks). 



Fig. 10. — Tetrany- 

 chus bimaculatus: 

 palpus— enlarged 

 (from Banks). 



Fig. 11.— Tetrany- 

 chus bimacula- 

 tus: claws— en- 

 larged (from 

 Banks). 



upon the lower sides of leaves, and attached here and there to project- 

 ing hairs, veins, or the edges of the leaves. Within the webs thus 

 formed the mites feed in their different stages, and the eggs are laid 

 from which the young develop. 



The general appearance of the red spider under consideration, which 

 is now known as Tetranychus bimaculatus Harv. as it looked under a 

 microscope, is well shown in figure 9. At figure 10 a greatly enlarged 

 palpus of the same species is illustrated, and figure 11 shows the claws 

 similarly enlarged. 



The length of full-grown individuals, including the palpus, is from 

 0.4 to 0.6 ,nm and the width 0.25 to 0.30 m,n , the thickness being 0.17 



