ii8 



ARACHNOIDEA. 



CASE 

 VIII. 



Nos. 

 «3. 24. 



Bryobia speciosa {Koch)^ Ubers., 61. 



Genus Petrobia {Murr., n.g.). 

 Characters the same as those of Bry- 

 obia, but with three eyes in the posterior 

 angles of each side of the cephalothorax, 

 and abdomen without triangular margi- 

 nal papilla. Duges hesitated whether 

 to place the species from which we have 

 named this genus among the Tetranychi 

 or Raphignathi. It appears to us to 

 come so near Bryobia that it would not 

 be well to place it apart from it, and 

 yet too different to be put in the same 



genus. We have therefore thought it better to make a separate 



genus for it. 



Petrobia lapidum {Koch). Trombidium lapidum {Rcrm., Apt. pi. vii.); 

 Tetranychus cristatus {Diiges, Ann. Sc. Nat. 1834).— 23. Magnified 

 figure of larva. 24. Magnified figure of perfect insect. 



yobia speciosa. Copied from figure 

 in Kocli's Ubersicht. 



Petrobia lapidum. younj. Ditto, full grown. 



This is easily recognised by a sort of crest or ridge which crosses 

 the back like a girdle, as shown in the woodcut It, however, 

 disappears when the insect is very well filled. The presence of 



