30 MEDICAL ENTOMOLOGY 



wood tick (Dermacentor variabilis) or the Texas fever tick 

 (Mar gar opus annulatus), before you, study and draw the follow- 

 ing characters, making one drawing of the whole animal and 

 separate drawings necessary to show the details. 



a. Rostrum, consisting of the head and mouthparts. 



b. Labium or hypostome, a continuation forward of the head 

 (snout-like) possessing longitudinal rows of teeth. 



c. Mandibles or cheliceres, one pair, terminating in well de- 

 fined teeth. The cheliceres work, each in a separate sheath. 



d. Palpi or pedipalpi, segmented structures varying consider- 

 ably in relative length in the various genera. 



e. The eight legs are composed of the following parts: coxa, 

 trochanter, femur, tibia, protarsus, tarsus, ungues (claws) and 

 pulvillus (absent in Argasinae). 



f . Scutum or shield, a chitinous plate covering a portion of the 

 dorsal part of the body back of the head. This shield is often 

 ornamented with spots, furrows and perforations. 



g. Eyes, present in Dermacentor and a few other genera, but 

 absent in a number of other genera, including Margaropus. 



h. Anus, ventrally located, removed from the posterior border 

 and guarded by anal plates. 



i. Anal groove, a furrow either posterior or anterior to the 

 anus, depending on the genus, and useful in classification. 



j. Spiracles with guard plates, situated laterally and ven- 

 trally, in some species in front of the fourth coxae, or in some 

 behind the fourth coxae. 



fc. Marginal festoons; the marginal border of the abdomen is 

 often crenellated. 



1. Sexual differences; the shield or scutum of the male covers 

 the greater part of the dorsum, while in the female it extends 

 over only a small portion posterior to the head. The male is 

 usually considerably the smaller and more slender, and is louse- 

 like in appearance. Though this is true for the Ixodinae, to 

 which group the specimens studied belong, in the Argasinae the 

 sexes are more difficult to distinguish. 



