MITES AND TICKS 137 



she visits the surface and returns with a fresh supply of air, which 

 she at once discharges under her dome-shaped nest until it becomes 

 inflated by the air, after the fashion of a diving-bell, and looks 

 as if it were filled with quicksilver. Within the diving-bell home 

 the spider spends the greater part of her life, only quitting it 

 in search of food, or to bring down a fresh supply of air to replace 

 that which has become exhausted." l 



" Harvest-spiders " or " Harvestmen" (Opiliones), as the long- 

 legged spiders frequently found creeping over the walls in dwelling- 

 houses in the autumn are popularly called, are not true spiders. 

 The cephalothorax and the abdomen are fused together so that 

 the body of a Harvestman has the appearance of a small, round 

 button. There are several species of Opiliones, most of which 

 are rather small, but some found in South America reach ex- 

 ceedingly large dimensions, while others are very grotesque in 

 shape. 



The order Acari, Mites and Ticks, includes a vast number 

 of forms, most of which are very minute. A few species are 

 J inch or more in length, but the majority of the Acari are ex- 

 tremely small, some species being invisible unless viewed through a 

 magnifying glass. In spite of their small size, however, the Acari 

 are of considerable economic importance. They attack plants, 

 doing a vast amount of damage to crops of all kinds; many are 

 parasitic on animals ; and in tropical countries they often act as 

 transmitting agents of certain diseases to man and domesticated 

 animals. 



All Mites undergo a metamorphosis, many passing through 

 several stages before arriving at the adult state. They vary some- 

 what in form ; in some the body is sac-like, and appears to have 

 no division ; while in others the body is distinctly divided into 

 two parts the cephalothorax and the abdomen. The legs are 

 normally eight, but may be as few as two in number, and eyes 

 may be present or entirely absent. 



The Red Spiders and the Gall-Mites are only too well known 

 to fruit-growers from the damage they cause to fruit trees and 

 bushes; poultry - keepers are familiar with the species which 

 attack domestic fowls; while most people who pay visits to the 

 country are acquainted with the " Harvest-Mite," the micro- 



1 F. Martin Duncan : " Our Insect Friends and Foes." 



