1 8 PHYLUM ARTHROPOD A 



Class: Arachnoidea. Division: Arachnida 

 A SPIDER 



As large a spider as possible should be obtained for this study. 

 If a small one is used, it is usually well to stick a slender insect pin 

 through it, in order to be able to handle it easily, and it should be 

 studied with the aid of a hand lens. Observe the form and color 

 of the animal. The body is unsegmented (although the body of 

 the embryo spider is distinctly segmented) and is made up of two 

 parts, the cephalothorax and the abdomen. What does the embry- 

 onic segmentation indicate as to the ultimate relationships of 

 spiders ? Observe the hairs which cover the body and legs. They 

 are projections of the cuticula and are important sense organs, 

 being sensitive to vibrations of the atmosphere. They thus aid in 

 giving the animal information as to what is going on about it. 



The Cephalothorax. This division of the body is equivalent to 

 the head and thorax of insects. Observe carefully the eight eyes 

 at or near its forward end, both the size and the arrangement of 

 which vary much in the various species of spiders. The ventral 

 surface bears the six pairs of appendages, the mandibles, the 

 pedipalps, and the four pairs of legs. 



The mandibles, the anterior pair, occupy a vertical position at 

 the front end of the body and consist each of a basal portion and 

 a terminal claw, near the tip of which is the pore from which 

 poison is injected into the bite. In consequence of the vertical 

 position of its mandibles the spider can strike only an insect 

 which is beneath it. 



The second pair of appendages are the pedipalps. These are 

 leglike and contain one less segment than the legs. The basal 

 segment of the pedipalp is called the maxilla. The two maxillae 

 are flattened structures situated on the underside of the cephalo- 

 thorax just back of the mandibles, their forward, medial margins, 

 which cover the mouth, being used to lacerate and squeeze the 

 food so that the animal juices can be sucked up. Spiders prey 

 exclusively upon living animals, but they can take in only liquid 

 food. The pedipalps of the female spider differ in shape from 



