STEUCTIJRE. 9 



or tiplpdcle. Each plate is a corneous piece, the surface of which is 

 marked by granules of smaller or larger size. The shape and sculp- 

 ture of the stigmal plate is quite constant in each species, but differs 

 in the sexes. 



In some genera there are on the dorsum, near the middle, a pair 

 of small circular or oval plates, called by Doctor Stiles the dorso- 

 submedian forose plates; no one has yet discovered their nse. 



Ticks are usually dull-colored, but some forms are brightly mottled 

 with brown, wdiite, yellow, or red. However, each species has a 

 characteristic shade of color, which, when once known, helps in field 

 identification; the colors accredited to the species in this paper are 

 those shown by alcoholic materials. In life most of the colors are 

 lighter or more vivid than after immersion in spirits. 



LIFE HISTORY. 



Ticks are parasitic during the greater period of their life; most 

 of them, how^ever, leave the host to moult, and all, to deposit eggs. 

 Mammals are their ordinary hosts, bnt birds and reptiles are also 

 infested, and tw^o species of ticks have been taken from insects. 

 Many of them show a decided preference for a certain animal, but 

 a number of our common species have been found on a great variety 

 of animals. Some ticks have apparently changed their host; for 

 example, the Texas fever tick, now chiefly found on cattle, originally 

 infested deer, and possibly bison. 



In sucking the blood of their hosts the ticks, unless extremely 

 numerous, do but slight harm, but several of them have been shown 

 to transmit the germs of some disease, so that they become, in several 

 cases, economic pests of prime importance. 



The life history of ticks has been investigated by several authors, 

 notably Curtice, Morgan, Lounsbury, and Hunter and Hooker. The 

 female tick, as the result of her bloodthirsty nature, becomes enor- 

 mously distended, and is then mature. The male mates with the 

 female for some days, after which the latter drops to the ground to 

 deposit eggs. These issue as elongate masses in front of the tick, and 

 may be as many as 1,000 to 10,000 in number. During the operation 

 the head is wnthdrawn into the body so that the surface of the 

 capitulum is close to the genital pore. As the eggs issue they are 

 coated with a viscous substance secreted from glands opening in the 

 membrane between the shield and the head. These glands are partly 

 eversible and enwrap each egg as it issues from the ovipositor. The 

 eggs are usually placed upon the surface of the soil or just beneath 

 it, and the larva? hatch in a few days. The young ticks, known as 

 "seed-ticks," ascend the nearest support of grass or herb and patiently 

 await the coming of some animal. Delay and disappointment must 

 often end in starvation and death. The seed ticks are, however, able 



