682 MEDICAL ENTOMOLOGY 



a pair just behind coxa IV, the metapodia ; a third pair, the metaster- 

 nalia, behind the sternal plate ; these plates may be differently arranged in 

 the two sexes, and are fused together in the male. The female genital 

 opening is usually under the margin of the genital plate, which terminates 

 near coxa III, that of the male just behind the mouth parts, at the anterior 

 margin of the sternal plate. Anal plates usually present. Anal opening 

 as a rule small and situated at or near the posterior margin of the ventral 

 surface, which is smooth and devoid of furrows ; posterior margin not 

 crenulated or festooned. Stigmata between legs III and IV ; the peritreme 

 usually well developed. Eyes absent. Mouth parts formed for piercing ; 

 the hypostome, unlike that of ticks, smooth and not armed with teeth. 

 Legs of the usual arthropod type ; the second pair in the male in many of 

 the species armed with teeth used in clasping the female. 



The members of the Gamasidae, also known as the Parasitidae or 

 Insect mites, exhibit great diversity in habits. The majority are vegetable 

 feeders, some being injurious to cultivated plants, while others are found 

 among decaying leaves and other vegetable debris. A considerable 

 number are predaceous ; others are true blood-suckers, living on the 

 blood of small rodents, bats and birds. As these latter are the most 

 important from our present point of view it will be convenient to take 

 one of them as an example for descriptive purposes. Laelaps, a species 

 of which from the Indian field rat, Gerbillus indicus, is figured on Plate 

 LXXXVII, will serve for the purpose. 



The body is built on the same general plan as that of a tick. The 

 head and thorax are fused together to form the cephalothorax, which in 



this species is oval, though various shapes are met with 

 External anatomy . 

 of Laelaps : The m otner forms. The capitulum projects a little from 



Mouth Parts. the anterior end of the body and bears the mouth parts, 

 late LXX (ix, including the maxillary palps. The mouth parts con- 

 sist of paired mandibles, maxillae and a median hypos- 

 tome. Each mandible is an elongated rod of chitin consisting of two 

 segments, of which the distal one is the longest. The appendage differs 

 'in the two sexes. In the female it terminates in a pair of digits which 

 are articulated to the end of the mandible and oppose one another like 

 the claws of a crab ; each digit is cone-shaped, and is armed with three 

 or more strong teeth, while a long thin hair ^nding in a hook arises 

 from the median surface of the external digit. Attached to the base 

 of the mandible there is a row of hairs arranged in a fail-like manner, 

 those on the outer side being longer than the rest. In the male the 

 mandibles are highly modified and are armed with structures believed 



