252 ELEMENTARY ZOOLOGY. 



bug, the scale insects, the destructive chinch bug. Related to these 

 are the Cicadas (the 1 7-year locust, Fig. 97) and various water bugs 

 (as the water-boatman and back-swimmers). For further details 

 of these groups the student should refer to some special work on 

 entomology. 



266. The Spiders and their Relatives (Arachnida). 

 These animals have two pairs of head appendages and 

 four pairs of legs. The head and thorax are more com- 



FIG. 97. Periodical cicada. Natural size. Photo by Folsom. 



Questions on the Figure. To what order of insect does the cicada 

 belong? Which of its habits are most familiar to you? What are 

 its nearest relatives among the insects? How long does it take to 

 mature? 



pletely fused than in the insects (Fig. 99). They do not 

 have antennae. Most of them are air-breathers, although 

 some of them live on or under the water. They do not 

 have the compound eyes that are so common among 

 the arthropods. Most of them prey upon insects and 

 are thus of advantage to mankind. They do not pass 

 through a metamorphosis. 



With the spiders proper are classed the scorpions 

 (Fig. 98), the daddy-long-legs or harvest-men, the ticks, 

 and the mites. The ticks and mites are external parasites 

 on mammals and birds. Some mites are parasitic on 



