32 



REPORT 108, U. S. DEPARTMENT OF AGRICULTURE. 



The mandibles have a peculiar structure; they consist of a stout rod with a short, 

 acute, stout spur near the tip. They are supposed to suck blood from the lizards; 



nothing is known of their earlier stages. 



Family TETRAXYCHID^. 



The members of this family, commonl}' known 

 as ''red spiders," have for many years attracted 

 attention, owing to their ravages to cultivated 

 plants. Since many of the species can spin a 

 silken thread they have also been called "spin- 

 ning mites." Their structure presents few re- 

 markable characters. The body is oval or ellipti- 

 cal, provided with a few, mostly long simple hairs, 

 arranged in four rows. The cephalothorax (fig. 

 40) is separated from the abdomen by a furrow, 

 and bears on each side one or two simple eyes. 

 The palpi are short, the penultimate joint ending 

 in a claw. The last is "thumblike" and bears 

 one or more appendages called "fingers." In 

 some species the palpus of the male has a short 

 curved spine at the tip on the upper side of the 

 third joint. The mandibles (fig. 39) have their 

 basal joints united in a plate; the apical joint, 

 being very long and flexible, is fitted for piercing 

 plant tissues, and is known as the stylet. The legs 

 (fig. 41) are moderately slender, the first pair the 

 longest, with scattered hairs, and ending in one 

 or two claws. In many species of Tctraiu/chus 

 the claw (fig. 43) is split into four pieces, whence the name. On the under side of 

 the abdomen are two simple openings, the basal the genital (fig. 42), the other the 



Fig. 38.— Tetranychus bimaculatui. 

 (Author's illustration.) 



Fig. 39. 



Tetranychus, mandibular plate, dorsal and lateral views: 

 (Author's illustration.) 



Plate; b, stylet; c, spina; d, palpus. 



anal. The former in the female is usually transverse. In the male it is longi- 

 tudinal and often shows the slender curved penis. Ewing has used the shape of 

 the penis in the distinction of s])ecies. 



