576 MEDICAL ENTOMOLOGY 



posterior margin of the ventral surface, just anterior to the festoons ; 

 plaques are present in certain species of Amblyomma. Shields are 

 prominent raised plates of chitin ending in points, and are as a rule 

 situated on each side of the anal aperture in the males of the genera 

 Hyalomma, Margaropus and Rhipicephalus (see figs.) These shields 

 are divided into two groups, the adanal, which are always present close 

 to the anus, and the accessory, external to the adanal. In the males 

 of the genus Margaropus there is often a caudal appendage, which 

 projects from the posterior border and is best seen when the tick is 

 somewhat distended with blood ; in the male of Hyalomma aegyptium 

 (Plate LXXII, fig. 3) there are two such appendages, one on each side, 

 which end in chitinised points. 



Adult ticks always have four pairs of legs, as also have the nymphs, but 

 there are only three in the larva. Each leg is composed of six segments, 



which are united to each other by intersegmental 

 Legs. Plate LXXII . , , ' . , .. . 



membranes, the usual method of articulation m the 



Arthropoda. The coxa or basal segment is immovable, and is often 

 armed with a single spur, when it is said to be dentate ; or it may be 

 armed with two spurs with a deep incision between them, when it is said to 

 be bidentate. The next segment, the trochanter, is shorter and may be 

 broader than it is long. The femur is much longer and is jointed to 

 the trochanter by a pseudo-articulation. The fourth segment or tibia 

 and the fifth or protarsus are also elongated. The sixth segment or 

 tarsus may also form a pseudo-articulation at its proximal end ; at its 

 extremity there is either a long or a short stalk, to which are attached two 

 well-developed claws. In the Ixodini the tarsi are nearly always armed 

 with one or more ventral spurs ; their presence or absence, and the 

 manner in which the tarsi end, are of importance in distinguishing the 

 species. In the Argatini the tarsi are never armed with spurs. On the 

 ventral surface of the claws there is a disc-like expansion, the pulvillum, 

 which is well developed in the Ixodini, but is rudimentary or absent 

 in the Argatini ; the length of the claws and that of the pulvillum in 

 relation to them should be noted. 



On the first pair of tarsi there is a remarkable structure known as 

 ' Hallers organ ' ; it consists of several cup-shaped pores containing 

 sensory hairs and dermal cells, and is situated on the 

 Spiracle dorsal surface of the tarsus. It has been fully de- 



scribed by Nuttall in collaboration with Cooper and 

 Robinson ; according to Lahille and others its function is olfactory. 



The spiracles or stigmal plates are seen as small raised plates of 



