512 The Animal Kingdom 



six characteristic pairs of appendages. Each chelicera has a basal seg- 

 ment and a distal pincerlike segment which bears the opening of the 

 poison duct near its tip. This duct is connected with the gland in the 

 cephalothorax. The six- jointed palpi have the basal segment modified 

 for breaking up food. In the males, the palpus is further modified for 

 the transference of sperm. Each of the four pairs of walking legs con- 

 sists of seven segments : the coxa, the trochanter, the femur, the patella, 

 the tibia, the metatarsus, and the tarsus; each of the tarsi terminates in 

 claws and variously modified hair pads. Ventrally the mouth is located 

 between the bases of the coxae of the palpi. 



The distal part of the abdomen contains the anal opening and the 

 spinnerets. These latter consist of several pairs of blunt projections 

 which contain small tubes through which the silk passes from the ab- 

 dominal silk glands. Ventrally the reproductive duct opens at the an- 

 terior portion. While in the male there is no special structure around 

 the opening, the female has a complicated apparatus, the epigynum. On 

 either side of this opening are the openings of the book lungs, while just 

 anterior to the spinnerets is often another opening, the tracheal spiracle. 



The Biology of the Spiders. — The spiders are an extremely varied 

 and successful group of animals. It has been estimated that there are 

 over 20,000 different species. On the whole they are of little economic 

 importance, with their destruction of harmful insects probably being 

 somewhat helpful. As mentioned, they all have poison glands which are 

 used to kill their prey. With rare exceptions, this poison has no effect 

 on human beings with the black widow spider (Latrodectus mactans) 

 possibly being an exception. It is probable that the danger of this 

 spider is highly exaggerated, but there is some evidence that it may be 

 dangerous or even fatal to small children or persons in a poor state of 

 health. 



The multitude of form and appearance of spiders defies generaliza- 

 tion. There are forms that resemble bird droppings and thorns. Some 

 are elongate and slender ; others are short and stout. Many are bril- 

 liantly colored and range in size from less than a millimeter to the large 

 bird-eating spiders of South America. The most spectacular develop- 

 ment among the spiders is associated with their production of silk and 

 web building. Some build large symmetrical webs for the capture of 

 their prey, while others are content with an irregular loose pile of silk. 

 One group of spiders even uses the web for transportation. The 

 young spiders spin a short thread and then allow the currents of wind 



