24 LN VERTEBRATE ZOOLOGY 



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 embryonic segmentation indicate as to the ultimate relation- 

 ships 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 in regard 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 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 only strike an insect 

 which is beneath it. 



The second pair of appendages are the pedipalps. These are 

 leg-like and contain one less segment than the legs, the missing 



