ARACHNIDS, CENTIPEDES, AND MILLIPEDES— ARTHROPODS 199 



taken in, and fifteen or twenty folds within, which resemble the leaves 

 of a book. Gaseous exchange takes place as the air circulates over these 

 leaves. 



The web plays a vital part in the life of the spider and its many 

 uses make an interesting study. Inside the abdomen there are several 

 silk glands that secrete a viscid fluid from which the web is formed. 

 This liquid is forced out of the spinnerets under rather high pressure and 

 hardens almost instantly on contact with the air to form the web. This 

 is the same principle which we use to produce rayon and nylon thread, 

 forcing a viscid liquid through tiny holes to harden in the air, but the 

 spiders had been using the method long before man thought of it. One 

 of the best known uses of the web is as a trap for insects which provide 

 most of the food for spiders. To the inquisitive mind of a child, one of 

 the great mysteries of nature is how the spider keeps from getting caught 

 in its own web. The spider can walk around over the web, yet an insect 

 becomes entangled immediately on contact. At least a partial answer 

 can be found in the types of silk glands ; there are some that spin a web 

 that is sticky and others that spin a web that is not. The spider seems 

 to mix the strands in forming the trap and knows which to walk on, but 

 the hapless insect does not. The funnel-shaped and the orb-shaped webs 

 are the most popular types of traps used by the spiders. There is no 

 sight more beautiful than the geometrical pattern of the orb weaver's 

 web sparkling with dew drops in the early morning sun. 



When an insect contacts the web the spider goes into action. It 

 rushes out and begins squirting sticky web at its struggling prey. The 

 more the insect struggles the faster it is entangled in this stream of web 

 and is soon helplessly awaiting its doom. The spider may hang it by a 

 thread and spin it around while emitting web on it so that it is soon 

 trussed up like a mummy and can be hung up for future use, just as we 

 would hang a smoked ham in the pantry. On the other hand, if the 

 spider is hungry it will eat the insect on the spot. Since spiders do not 

 have jaws, the insect cannot be chewed up and swallowed ; but the spider 

 will bite a hole in it and inject digestive enzymes, and suck up the liqui- 

 fied digesting food. The empty exoskeleton is then cast aside and an 

 accumulation of these skeletons can usually be found below a web as a 

 testimony of the effectiveness of the trap. 



Some spiders, such as the wolf spider, do not use the web as a trap, 

 but stalk and pounce on their prey like a tiger. However, these may 

 spin web to line their nests, which are usually holes in the ground, so 

 that they are soft, dry, and cozy. 



The web may be used as a convenient means of transportation for 

 the spiders. A spider on the ceiling can lower itself to the floor by stick- 



