694 



ARTHROPODA 



the tick fe^ver of Africa, Tsutsugamushi disease, the tick fever of 

 the Rocky Mountains, etc. They are parasitic on mammals, birds, 

 and reptiles, but are not strictly confined to one host. The Arga- 

 sidse, one of the two families included in the superfartiily, are now 

 nocturnal in habits, and do not become greatly distended with blood ; 

 while the other family, Ixodidae, or true ticks, are very common 

 in tropical countries. 



Morphology. — A tick is divisible into a head, rostrum, or capitulum, and a 

 body. The capitulum is the small anterior structure with the mouth parts, 

 and is easily visible from above in the Ixodidae {vide Fig. 313). but must be 

 viewed from below in the Argasidae (Fig. 321). It joins the rest of the body 

 at the camerostome. The capitulum consists of the following parts: — 



1 . Neck. — This is the junction of the rostrum with the rest of the body. 



2. Base. — This is the hard basal portion, and is usually quadrangular, but 

 its outline varies. 



In females of the Ixodidae there are two porose areas on the upper surface 

 {vide Fig. 311). Attached behind the base is the neck; in front, in the middle 



line, is the haustellum, composed of the 

 hypostome and the mandibles and their 

 sheaths, on each side of which lie the 

 palpi. 



3. The hypostome is an elongated 

 structure composed of two symmetrical 

 halves, with numerous minute teeth, 

 called denticles, on its ventral surface. 



4. The chelicerae, or mandibles, are 

 two in number — one on each side of the 

 median line, lying dorsal to the hypo- 

 stome. The posterior portion enclosed 

 in the body is swollen for the attachment 

 of muscles, while the anterior is flat and 

 narrow, and terminated by a hook-like 

 digit, which has two or three processes 

 — the apophyses — an internal, an ex- 

 ternal, and a middle, with hook-like 

 teeth. 



5. The mandibular sheath lies dorsal 

 to the mandibles. 



6. The palpi are composed of four 

 segments or articles — basal, antepen- 



They are of importance in classification 

 into genera, as they possess hairs, bristles, edges, angles, and spines, varying 

 in difierent species. There is sometimes a group of long hairs on the internal 

 aspect of the palpi, which are probably sensory. 



The body varies greatly in form, colour, outline, and structure. It is 

 divided into: — (i) Dorsal surface; (2) ventral surface; {3) to {5) anterior, 

 posterior, and lateral margins. 



I. Dorsal Surface. — On the dorsal surf ace may be noted (Figs. 313 and3i8): — 



{a) The scutum, found in the Ixodides, and well marked in the adult 

 male, but much smaller in the adult female. It is a hard, chitin- 

 ous plate, with two longitudinal grooves (cervical grooves). 



(6) The eyes, only sometimes present on each lateral margin of the 

 scutum in Ixodidae, or near the first coxa in Argasidae. 



{c) The dorso-submedian porose plates are small, circular, or oval 

 chitinous structures on each side of the median line, near the 

 third and fourth legs. 



(d) The poster o-marginal festoons, when present, are eleven areas 



Fig. 310. — Margaropus annulaius 

 VAR. australis Say: Dorsal View 

 OF THE Male. 



ultimate, penultimate, and apical. 



