566 MEDICAL ENTOMOLOGY 



collectively as the rostrum or beak, and may either project beyond the 

 camerostoma or lie concealed below its dorsal wall, which is called the 

 epistoma. The mouth parts consist of a pair of mandibles situated dorsal- 

 ly, which may either be armed with teeth or claws or may be styliform ; 

 they are often enclosed in sheaths covered with minute spines. The 

 maxillae are situated ventrally and are usually shorter than the mandi- 

 bles ; they are narrow flat blades of chitin. Situated in the middle line 

 of the ventral surface of the rostrum there is a flat rod of chitin, the 

 hypostome, which is armed with teeth. In many acarina, as for instance 

 the ticks, the maxillae are wanting, and it is generally believed that the 

 hypostome, which in this case is well developed, represents the fused 

 maxillae ; hence the origin of the term labio-maxillary dart, which is 

 often applied to it. 



The palps consist of at most five joints, the terminal one of which 



is often armed with sensory hairs. Banks groups the various forms 



of palpi as follows : ' (1) Where they are simple, 



'filiform and have a tactile function ; (2) where they 



1 are modified for predatory purposes, being provided with spines, hooks 



' or claws ; (3) where the last joint is opposable to the preceding, so that 



' the mite may by its palpi cling to some object ; (4) where they have 



' become obsolete, and are more or less united to the rostrum '. 



The adult acarian has four pairs of legs, as also the nymph, but the 

 larva has only three pairs. Banks states that in the embryo of Gamasus 

 . there are four pairs of legs before birth, one of 



which is aborted but develops again later at the nymphal 

 stage ; this, as he points out, shortens the gap between the Acarina 

 and the Arachnida. Each leg consists of from five to seven seg- 

 ments as follows : coxa, femur, tibia, protarsus and tarsus ; sometimes 

 a trochanter and patella may be recognized. Each leg is articulated 

 either directly into the skin or into special skeletal enlargements 

 spoken of as epimera. The tarsus ends either in one or three claws or 

 ungues, and the number of these may vary in the different stages ; they 

 are seldom toothed. All the joints may be smooth or covered with hairs 

 and spines; the latter often afford valuable taxonomic characters. At 

 the base of the claw there is usually a pad-like sucker, the pulvillum, 

 caroncle or ambulacrum, which is often rudimentary. In some acarians 

 the first pair of legs have a peculiar sensory organ on the tarsus. 



Eyes may or may not be present ; when present they are as a rule 

 situated either at the margin of the anterior end of the scutum, or on 

 folds on the ventral surface. 



