GENERAL CLASSIFICATION AND STRUCTURE. 21 
Thus Fig. 8 represents the eyes of an Epeira, and Fig. 9 the eyes of the 
Retitelarian 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. 
II. 
For the conyenience of readers not acquainted with the anatomy of 
spiders, a general description of the animal is here given. It is not in 
accord with the purpose of this work toa enter into the 
details of structure; for these the studies of anatomists 
and histologists must be consulted. But some knowledge 
of the principal organs, especially in their relations to 
the spinning industry, is necessary to the understanding 
of much of what follows. 
The principal parts are the cephalothorax and abdo- 
men. The cephalothorax consists of the cephalic part 
(ep, Fig. 11) and the thoracic part, tp; the two parts 
Fig. 8. Face of Epeira. are united 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 
this group are eight, but in other groups sometimes 
number six and eyen 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 2S 
the middle group contains four eyes, known as the Fis. 9. Face of 
middle eyes, those in front being mid front or middle Eee taoaen 
front (M.F.) and those behind, the mid rear or middle rear Pidariorum, 
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 (8.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. 
The 
Caput. 
The 
Hyes. 
