(.KXKKAL CLASSIFICATION AND S.TKUCTrKK. 21 



Thus Fig. 8 represents the eyes of an Epei'ra, and Fig. 9 the eyes of the 

 RetiU-larian genus Theridium. Dr. Bertkau distinguishes these two tribes 

 by the presence in the Epeiroids of what he calls a basal spot (Basalfleck) 

 upon the mandibles. 



III. 



For the convenience of readers not acquainted with the anatomy of 

 spiders, a general description of the animal is here given. It is not in 

 ;MR accord with the purpose of this work to enter into the 



T / 



details of structure : for these the studies of anatomists 

 and histologists must be consulted. But some knowledge 

 ^ ^ ie P rmc il ul l organs, especially in their relations to 

 the spinning industry, is necessary to the understanding 

 o f much of what follows. 



The principal parts are the cephalothorax and abdo- 

 men. The cephalothorax consists of the cephalic part 

 * (cp, Fig. 11) and the thoracic part, tp; the two parts 



FIG. s. Face of Epeira. are un ited directly, and not by a neck, the caput being 

 set immediately upon the thorax, whence the name cephalothorax. The 

 point of juncture is marked by a suture, more or less distinct, extending 

 along the lower margin of the caput backward, on each side, and con- 

 verging in a depression more or less profound at the summit of the 

 thoracic part. The cephalothorax is externally a hard, chitinous case 

 composed of two principal plates, resembling more nearly than any other 

 part of the body the tough shell of true insects. 



The front and upper portion of the cephalic part is the caput, which 

 in Orbweavers is sometimes depressed, more frequently elevated. On the 



fore part of the caput are situated eyes, which in 

 _ e this group are eight, but in other groups sometimes 



number six and even two. The arrangement of the | 

 eyes upon the caput forms good generic and specific characters. 

 The eyes in the Orbweavers are disposed across the caput in 

 two rows of four each, known as front and rear rows (Fig. 8) ; 

 they are again divided into three groups, of which 

 _ the middle group contains four eyes, known as the FlG - 9 - Face of 



-CjyeS. . . " . -i -ii Lineweaver, 



middle eyes, those in front being mid front or middle Theridium te- 

 frout (M.F.) and those behind, the mid rear or middle rear piaarioram- 

 eyes (M.R.). The remaining four eyes, known as the side eyes (or lateral 

 eyes), are placed in two's on either side of and equidistant from the middle 

 group. They are known as the side front (S.F.) and side rear (S.R.) ; they 

 are generally quite near to each other, frequently touch, but occasionally 

 are well separated ; for the most part they are smaller than the middle 

 eyes. They, as well as the middle group, are often placed upon tubercles 

 more or less prominent. 



