286 THE ARACHNOIDS 



As the greatest confusion exists as to the classification of ticks, Dr. Charles W. 

 Stiles has now in hand a system of classifying ticks according to the appearance 

 of these plates as seen under the high power of a microscope. There is great varia- 

 tion in the outline and general picture of these stigmal plates in the different species. 

 The stigmal orifice, the opening of the tracheal system, is in the center. The 

 Ixodinse have a scutum or shield-like chitinous structure on the dorsal surface. It 

 covers almost the entire back of the tick in the male and only a small portion anteri- 

 orly in the female. The genital opening is toward the anterior part of the ventral 

 surface. The anus, with anterior or posterior anal grooves, is near the posterior 

 third of the venter. The legs have six segments, the coxa being flattened out on the 

 surface of the body and the terminal tarsus ending with a pair of hooks and at times 

 with a pulvillus. The nymph has stigmal plates but has no genital opening while 

 the larva has neither genital apertures nor stigmal orifice. 



Life History of Ticks. -This varies greatly according to the sub- 

 family, genus, and species. The female Ornithodoros savignyi lays 

 about 140 eggs. The larva does not leave the egg, but moults inside, 

 and finally emerges as an eight-legged nymph. It lives in the dust in 

 the cracks of the native huts and comes out at night to feed on the sleep- 

 ing natives. As the possibilities for destruction are not so great as with 

 many Ixodinae the necessity for thousands of eggs is not imperative for 

 the continuation of the species as with the Ixodinae. With some of 

 the Ixodinae the females lay from 5000 to 20,000 eggs during several 

 days or weeks and then die. The eggs are preferably deposited near 

 grass. The egg stage lasts from two to six months, when the six-legged 

 larva (" seed tick ") emerges. It crawls up a blade of grass and gets on a 

 passing animal. After feeding, or at times without taking nourishment, 

 the larva drops to the ground, and changes to the pupal stage which has 

 four pairs of legs. The pupa crawls up a blade of grass and gets on a 

 passing animal (the second host). Feeding, it falls to the ground where 

 it remains eight to ten weeks. It moults and develops into an adult 

 tick. These males and females gain access to a third animal host 

 the males fecundate the females, after which the female gorges herself 

 with blood; afterward dropping off the animal and laying eggs. With 

 some ticks fewer hosts suffice. 



Cleland has noted reports of serious symptoms, chiefly cardiac and visual, from 

 the bite of ticks in Australia (Ixodes holocyclus). This is exceptional, however, as 

 the symptoms following the bites of such ticks are only those of skin irritation. 



Classification of Ixodidae. 



Subfamily Argasinae. Head concealed by body when viewed 

 dorsally. No scutum. Stigmal plates between third and fourth legs. 



