24 INVERTEBRATE 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 
