r. V 



1 he External Anatomy of Spiders 



organs, which have been termed the lyriform organs (Fig. 126). 



These are believed to be sense organs, and are discussed under 



that head in the next chapter. 



Leg formula:. — The relative lengths of the different pairs 



of legs often afford distinctive char- 

 acteristics, which are much used in 

 descriptive works. The four pairs of 

 legs are numbered from before back- 

 ward; and in descriptions their relative 

 lengths are indicated by a formula 

 consisting of the numerals 1, 2, 3, and 

 4 arranged in the order of the relative 

 lengths of the legs of the spider de- 

 scribed beginning with the numeral 

 indicating the longest pair of legs. 

 Thus the formula 1432 indicates 

 that the first pair of legs is the longest 

 and the second pair the shortest. 

 Where two or more pairs of legs are 

 of equal length the fact is indicated 

 bv placing a bar or bracket above the 



Fig. 126. LYRIFORM ORGANS " , . , , , 



of argyrodes numerals, thus the formula 2341 



indicates that the third and fourth 

 pairs of legs are of the same length. 



Spine formula:. — The number and arrangement of the spines 

 on the segments of the legs are often indicated by formulae. Thus 

 if there are two rows of spines of three each under the tibia of the 

 first legs and two rows of two each under the metatarsus of these 

 legs, the fact is indicated as follows: spines, tib.I., 3-3, met. I, 2-2. 



THE ABDOMEN 



The abdomen of spiders is more or less sac-like and almost 

 always without traces of segmentation except in a very small 

 area at its caudal end. In most spiders the abdomen is of an 

 elongated, rounded form; but an exceedingly great variation 

 in the form of this region exists. 



The Parts of the Abdomen. — Owing to the almost total 

 lack of segmentation comparatively few distinct parts can be 

 recognized in the abdomen; spiders differ greatly in this re- 

 spect from insects, in which the abdomen is composed of a 



126 



