50 



The Living Things of the Earth unit 



Fui. 68 The black widow 

 spider 7J?agnified. ]Vith legs 

 stretched otit it is ctbout one 

 and one half inches in size. 

 The lower side of the abdo- 

 men with its distinct hour- 

 glass is shown at the nppcr 

 right. At the lower right cor- 

 ner is the body of the n/ale. 

 How does it compare in 

 size with the female';' (v. s. 



DEPARTMENT OF AGRICULTURE ) 



w (jndered what was meant by the "quiet" 

 of the countryside. 



The sound-producing apparatus of the 

 cricket is peculiar. The front pair of 

 wings is thickened. The edges of the wing 

 covers have a set of "teeth." As one roug^h 

 surface rubs over the other the stiff winos 

 vibrate. It is the vibration that is heard as 

 a shrill chirping. In katydids and cicadas 

 the apparatus is slightly different. In these 

 insects it is only the males that are so 

 equipped. Other insects, such as bees and 

 flies, make noise by the rapid beating of 

 their w inos. 



The insects. So numerous and so varied 

 are the insects that many books have 

 been filled with accounts of their extraor- 

 dinary structure and fascinating lives. 

 This brief account has onlv scratched 

 the surface. Tlie biologists who study 

 insects (called entomologists, en-toh- 

 niol'o-jists) can tell man\' excitinu^ tales 

 of the doings in the highh' populated 



insect world. For review do Exer- 

 cise 3. 



Other arthropods — the spiders. If you 

 turn again to the chart on page 41 vou 

 will see that besides the very laroe and 

 varied class of insects there are three 

 other classes in the arthropod phylum. 

 One of them is the spiders and their close 

 relativ^es. Does it astonish you to learn 

 that spiders arc not insects? It should 

 not. Being arthropods, of course, they 

 have a firm outer covering and jointed 

 legs; but ^'ou \x\\\ count four pairs of 

 legs (not three), and only two body 

 parts. The head and thorax are joined 

 together. And x\\q\ lack three structures 

 found in insects: ^ings, antennae, and 

 compound eyes. Now draw the diagram 

 suggested in Exercise 4. 



Most spiders can give off a special 

 ]i(]uid from the abdomen that hardens 

 in riic air into a silk thread. The webs 

 nv,i\ be used as homes or as a means of 



