266 ENTOMOLOGY FOR MEDICAL OFFICERS 



and used for grasping small prey, they may be remarkably 

 exsertile. 



The mouth is generally a chink, deep-seated at the base 

 of the chelicerse ; sometimes the bases of the pedipalps are 

 united to form a lip. In addition to the chelicerse other 

 mouth-structures may exist — in particular a needle-like or 

 rasp-like " tongue," or hypostome, formed by a prolongation of 

 the ventral edge of the " beak " aforesaid. The pharynx is 

 suctorial, since mites live chiefly on juices, and the capacity 

 of the stomach is generally increased by paired gastric cseca. 

 The Malpighian tubules, when present, open into the hind part 

 of the intestine, near the anus ; sometimes there is no open 

 connection between this part of the intestine and the stomach. 



When special breathing- organs are present they have the 

 form of tracheae, and the manner in which these open is used 

 for classifying mites. 



The genitalia open ventrally, sometimes between the last 

 pair of legs, or (as in ticks and their kind) near the front end 

 of the body, or at the after end of the body. The female 

 produces eggs, generally in great number, and the larva 

 usually differs from the parents in having only three pairs of 

 legs ; after moulting, the larva becomes a nymph with four 

 pairs of legs, but still sexually immature ; and after another 

 moult the nymph is transformed into the adult. This series 

 of metamorphoses may be gone through directly and quickly, 

 or may be strangely modified and arrested, or may — as in 

 many ticks — occupy several months if circumstances are 

 adverse. 



This Order is one of very great importance to the medical 

 officer. Many of the species are troublesome ectoparasites of 

 domestic animals and man ; others live entirely on the blood 

 of vertebrates (including man), and incubate and disseminate 

 various Haematozoa dangerous to their hosts; others do 

 indiscriminate damage to man's stored provisions ; and 

 others, again, may work havoc with his growing crops. 



The distinctions between the more important families of 

 Acarina are shown in the following table. The families that 

 chiefly concern the medical officer are the Ixodida, Sarcoptidm, 

 and TyroglyphidcB ; the TrombidiidcB and Tarsonemidce, and 

 perhaps also the Gamasidce occupy points on the medical 



