CLASS ARACHNIDA 299 



struggling prey, and sometimes in making a broad, wavy band across 

 the center of the web. Larger threads are used in the construction of the 

 web itself. 



The spiders make use of two kinds of silk, one of which is dry and 

 inelastic, the other viscid and elastic. In the orb web, which may be 

 taken as a type, the framework is composed of threads made up of the 

 first kind of silk, while the spiral threads which pass around the web 

 from one radiating thread to the next are made of silk of the second kind. 

 If examined with a lens this viscid and elastic silk will be found to have 

 numerous beadlike masses of sticky material which help to hold the prey 

 when it touches the web. It is supposed that these two kinds of silk 

 are spun from dilTerent spinnerets. 



Silk is used not only to line a nest or form a web but also to fashion 

 the cocoon. Some spiders spin an anchor line by means of which they 

 may return to a certain point from which they have leaped or fallen. 

 Others make bridges of silk, spinning threads off into the air until they 

 become attached to some object on the farther side of a space; the lines 

 are then drawn taut by the spider. Still other spiders make use of silk 

 in the construction of balloons, spinning a loose mass of threads which 

 are sufficient to buoy the animal up and enable it to be carried along by 

 the wind. These aeronautic or flying spiders have been known to travel 

 hundreds of miles in this fashion. 



326. Behavior. — Spiders can see but a very short distance, apparently 

 distinctly only within a radius of four or five inches. They do not seem 

 to use other senses, except the one of touch, but that sense is exceedingly 

 delicate, especially on the pedipalpi and on the terminal joints of the 

 legs. Spiders act largely from instinct but they form some habits and 

 have many activities which are as justly considered intelligent as are 

 those of any insects. 



327. Economic Importance. — Spiders are of little economic impor- 

 tance other than the service which they render in the destruction of 

 injurious insects, but this is more or less offset by their destruction of 

 beneficial ones. Many spiders, particularly those of the tropics, are 

 feared as being poisonous. The larger ones might be able to inject by 

 their bite a sufficient amount of poison to cause marked effects, though 

 rarely, if ever, are they fatal to man. In this country the only spider 

 the bite of which is serious is a greasy-looking black species, Latrodediis 

 mactans (Fabricius), with a very large globular abdomen on the lower side 

 of which are some yellow or reddish spots. It is usually found under 

 objects lying upon the ground but may also inhabit dark outbuildings. 



328. Scorpions. — Another type of arachnid is the scorpion (Fig. 196), 

 the body of which is clearly metameric. It is divided into a prosoma or 

 cephalothorax ; a mesosoma, which is made up of the broadened anterior 

 abdominal metameres; and a metasoma, which includes the slender 



