98 ARACHNOIDEA 



CASE This species has been found on a great variety of plants, and 

 from its form and manners, it was supposed to be the same on 

 all of them long before this had been 

 proved by breeding them — a conclusion 

 which could not have been arrived at had 

 reliance been placed upon their colour, 

 for some are greenish, and marked only 

 with brown specks on the sides, but 

 variable, and evidently dependent on the 

 alimentary matter within them ; others 

 are rust-coloured, or reddish, or brick-red, 

 and that is the colour with which horticul- 



Tetranvchus tclnrius (female). 



Copied fron. Boisduvai. tuHsts arc most familiar. Upon the holly- 



hock Duges found at the same time individuals presenting almost 

 all shades of colour, a circumstance probably connected with some 

 peculiarity in the nutrition derived from that plant. On the vine, 

 Dr. Johnston found the colour to vary in intensity in different 

 individuals. So far as our own observation goes, the rusty colour 

 is an indication of greater maturity than the green. This, and most, 

 if not all the species of the genus, spins a web on the back of the 

 leaves of the finest and most delicate texture. The threads of its 

 web are secreted from a conical nipple situated underneath, and 

 very near the extremity of the abdomen. They are drawn out 

 -and guided by the motions of the insect, and by the action of the 

 minute claws and hairs of the legs, which seem to be only used 

 for this purpose. The threads are so slender that we fail to see 

 them, even with the assistance of a magnifier, until after they are 

 woven into a web, or network. In the construction of this web, 

 all the feet are moved with great agility ; but the movements of 

 the mite itself are not quick, and it moves with difficulty over 

 smooth and polished surfaces, as over glass. Upon leaves, espe- 

 cially on the under side of them, it finds a fitter hold, for, sup- 

 ported on the bristles that jut out beyond the claw, it spins its 

 web, affixing the threads to the prominences and hairs of the 



