246 THE INSECTS 



form), is known as the spinneret. This is a set of glands which secrete 

 in a Hquid state the silk which the spider spins. On exposure to air, 

 this fluid hardens and forms a very tough building material which com- 

 bines lightness with strength. 



Uses and Form of the Web. — The web-making instinct of spiders 

 forms an interesting study. Our common spiders may be grouped ac- 

 cording to the kind of web they spin. The web in some cases is used as a 

 home ; in others it forms a snare or trap. In some cases the web is used 

 for ballooning, spiders having been noticed clinging to their webs miles 

 out at sea. The webs seen most frequently are the so-called cobwebs. 

 These usually serve as a snare rather than a home, some species remain- 

 ing away from the web. Other webs are funnel-shaped, still others are of 

 geometrical exactness, while one form of spider makes its home under- 

 ground, lines the hole with silk, and makes a trapdoor which can be closed 

 after the spider has retreated to its lair. 



A poisonous centipede from Texas. Half natural size. From photograph by Davison, 



Myriapods. — We are all familiar with the harmless and common 

 thousand legs found under stones and logs. It is a representative of the 

 group of animals known as the millepedes. These animals have the body 

 divided into two regions, head and trunk, and have two pairs of legs for 

 each body segment. The centipedes, on the other hand, have only one 

 pair of legs to each segment. Both are representatives of the class 

 Myriapoda. None of the forms in the eastern part of the United States 

 are poisonous. 



Insects and Crustaceans Compared. — Both crustaceans and 

 insects belong to a great group of animals which agree in that they 

 have jointed appendages and bodies, and that they possess an 

 exoskeleton. This group or phylum is known as the Arthropoda. 

 Spiders and myriapods are also included in this group. 



Insects differ structurally from crustaceans in having three 

 regions in the body instead of two. The number of legs (three 



