58 



REPORT 108, U. S. DEPARTMENT OF AGRICULTURE. 



the tip of the hody, while in other species there is a ciir\'ed groove in front of the anus. 

 and reaching back each side toward the margin of the body. In all forms there is a 

 lateral groove on each side reaching forward to the genital 

 pore. In the males of several genera there are one or two 

 corneous, triangular plates, known as the anal plates, each 

 side of the anus. Sometimes the abdomen terminates in 

 a short median process or tail. 



The legs arise on each side on the anterior part of ihe 

 venter. The cox?e are sometimes close together, some- 

 times more widely separate. The legs are usually slender, 

 subequal in length, but the foiu-th pair is rather the 

 longest, and sometimes larger than the others. Each is 

 composed of at least six joints, as follows, from base 

 outward; coxa, trochanter, femur, tibia, metatarsus, and 

 tarsus. The latter is commonly more or less definitely 

 divided into two parts. At the tip of the tarsus is a pair 

 of large claws (fig. 112) situated on a rather long pedicel, 

 and between them is a pulA-illus. On the upper surface 

 of tarsus I is a pit covered by a membrane; this is known 

 as Haller's organ, in honor of its discoverer, and is supposed 

 to be an organ of audition, but more probably to recognize the approach of a host. 

 One or more of the coxae are armed behind by spinelike processes or teeth. In the 

 males of some species the hind coxae are greatly enlarged. Above and usually 

 slightly behind the hind coxae are the stigmal plates (fig. 108), containing near their 

 centers the stigmal orifices or spiracles. Each plate is a corneous piece, the surface 

 of which is marked by granules of smaller or larger size. The shape and sculpture 

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



Fig. 107. — Hypostome and 

 mandibles of a tick. (Origi- 

 nal.) 



Fig. 108.— Stigmal plates of female ticks: a, Ixodes; b, ^fargaropus; c, Amblyomma; d, DermaceiUor: 



e, Hacmaphysalis. (Original.) 



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

 or oval plates, called the dorso-sub median porose plates; no one has yet discovered 

 their use. Ticks are often dull colored, but some forms are brightly mottled with 

 brown, white, yellow, or red. However, each species has a characteristic shade of 

 color, which, when once known, helps in field identification. In some forms the 

 contents of the intestine show through so that the a])parent markings are irregular 

 and variable. 



Ticks are parasitic during the greater part of their life; most of them, however, leave 

 the host to molt, and all to deposit eggs. Mammals are the ordinary hosts, but birds, 

 reptiles, and batrachians are also infested, and two species of ticks have been taken 

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

 number of the common species have been found on a great variety of hosts. Some tic^ks 

 have ai)parently changed their host; for exain])ie, the Texas-fever tick, now chiefly 

 found on cattle, originally infested deer, and possibly bison. In sucking the blood 

 of the'r hosts the ticks, unless extremely numerous, do but slight harm, but several ot 



