MITES AND TICKS 193 



be obtained from living animals and plants or from decaying 

 organic matter. Some are entirely parasitic upon plants or other 

 animals, others attach themselves to animals in their larval stage 

 but are free when adult, while again others live an entirely inde- 

 pendent and predaceous life. In the predatory forms the mandibles 

 are chelate and masticatory, but in the parasitic forms the mouth 

 parts are modified for sucking the blood or juices of the host. The 

 largest and smallest examples of the order are found amongst the 

 parasitic species. 



The food and feeding habits of mites are closely related to their 

 general behaviour and have already been discussed in some detail. 

 Some species of mites rarely occur free living but are found in 

 association with mammals: here they feed, not on the host, but on 

 other mites and insects living on it. It can readily be imagined that 

 after a time, instead of living a blameless existence, some of these 

 found an easy way to obtain food by sucking the blood of the 

 animal on which they happened to find themselves, and thus 

 developed the parasitic habit. In this way many families have 

 become ecto-parasitic on birds and mammals. 



The majority of the mite-borne diseases of man were originally 

 diseases of animals closely associated with humans, such as 

 domestic rats and mice. At present some of these diseases, which 

 include dermatitis, rickettsiosis, plague, various typhus and other 

 fevers, phthiriasis, scabies and gastro-enteritis, tend to be en- 

 countered only in certain localities, but there is always a risk that 

 they may be spread either by their original animal hosts, or by 

 man, so that new foci of the disease are created. This important 

 subject, somewhat beyond the scope of the present volume, has 

 recently been reviewed by Zumpt and Graf (1950) to whose 

 publication the reader is referred. 



Halacaridae are either predaceous, lichen feeders or are para- 

 sitic (Newell, 1947). The basis of the behaviour of fresh water 

 mites is a random locomotory activity in search of food such as 

 Crustacea and aquatic insects which are detected by touch. A 

 simple action system based on the principle of trial and error seems 

 to be correlated with a simple mode of life. The predatory habit 

 is often of economic importance and Cheyletus eruditus is the 

 commonest predator of Tyroglyphid mites in stored products. Not 



N S.S.C.M. 



