PHYLUM ARTHROPODA. SPIDERS AND THEIR ALLIES 



275 



Chigger mite 

 (Eotrombicula) 



Spotted fever tick 

 (Dermacentor) 



Chicken mite 

 (Dermanyssus) 



Vector of relapsing fever 

 (Ornithodoros) 



Figure 167. Some parasitic ticks and mites. 



Human itch mite 

 (Sarcoptes) 



Texas cattle tick 



(Boophilus) 



gorged with blood 



RELATIONS OF 

 ARACHNOIDEA TO MAN 



Most of the Arachnoidea are not only 

 harmless, but they feed largely on injurious 

 insects and are to a certain degree beneficial. 

 A few species damage food plants; others 

 attack man and domestic animals directly; 

 and a few transmit disease germs. King 

 crabs are trapped in great numbers and 

 ground up for fertilizer. 



The red spider, Tetranychus, is a green- 

 house pest that is destructive to many 

 species of plants; it may become a pest also 

 in cotton fields. The clover mite injures 

 clover and fruit trees, and the pear-leaf 

 blister mite damages pears and apples. 



Scorpions are noted for their poisonous 

 sting; but they are mostly confined to the 

 tropics and some are not very poisonous. 

 The black widow spider, Latrodectus (Fig. 

 168), appears to be the only spider in the 

 United States whose bite needs to be feared. 

 The adult can be recognized by its glossy 

 black color, globose abdomen, and the red- 

 dish spot on the under surface of the ab- 

 domen usually shaped like an hour glass, 

 but sometimes like a triangle or two tri- 

 angles. 



Although not dangerous, the chigger mites 

 or red bugs (Fig. 167) produce a very dis- 

 tressing itch which may continue for several 

 days; an immunity may be acquired to their 

 invasions of the skin. One of the itch mites, 



