24 AMERICAN SPIDERS 



among different spiders, some of them being long, fine stilts on 

 which the spider hangs, and others stubby props. 



With so many walking appendages, the means of synchronizing 

 all of them is of some interest. In order to take a step, the spider 

 moves the first and third leg of one side in conjunction with the 

 second and fourth legs of the other side of the body. The remain- 

 ing legs of both sides go into action while the other series is at rest, 

 and thus the creature advances step by step. 



The appendages and other parts of the body are usually covered 

 with hairs and spines of different kinds. Some of these lie flat against 

 the integument and serve as a covering blanket. Others are heavier 

 or longer or more erect, and are used in many ways by the spider 

 to perform important functions during the spinning of silk, for the 

 preening of the body, preceding and during the mating, and as 

 aids in capturing and holding the prey. Many of these setae are 

 extremely sensitive to touch and vibration, and some may be re- 

 ceptors for various chemical stimuli. By means of its sensory hairs 

 the spider has a keen knowledge of its surroundings. 



Abdomen. The juncture between the cephalothorax and the 

 abdomen is made by a narrow waist or pedicel, which represents 

 the first true abdominal segment. In the antlike spiders the pedicel 

 is visible from above as a small tubular connection armed above and 

 below by hard plates, but in most other spiders it is not evident, 

 its presence being largely masked by the overhanging abdomen. 

 Through the tiny channel of the pedicel must pass the several struc- 

 tures essential to maintenance of life in both body parts: the ventral 

 nerve cord, a large artery, part of the midgut, and, frequently, 

 numerous tiny tracheal tubes. 



Ordinarily the abdomen is a saclike structure without visible 

 segmentation and, though covered by a sclerotized cuticle, is usu- 

 ally much softer than the cephalothorax. In the primitive liphistiids 

 and their relatives, the dorsum of the abdomen is armed with a 

 series of hard transverse plates, or tergites, each set with erect black 

 spines. In a few of the primitive true spiders there are evidences 

 of dorsal segmentation, especially in the spiderlings, but in some 

 well-known cases this segmentation may have been acquired sec- 

 ondarily. 



The abdomen frequently exhibits on its upper surface a series 

 of small, rounded depressions that mark the internal attachments 

 of muscles. Often brightly painted, and variegated with contrasting 



