190 MITES AND TICKS 



and debris, wherever there is an abundance of small insects and 

 other mites. They are predaceous in habit and some species are 

 said to capture their prey by squirting on to them a liquid silk 

 which enmeshes their appendages while the mite sucks their body 

 contents. 



A few mites are littoral and run about the sand between tides, 

 while others, the Halacaridae, are dredged from the sea. Newell 

 (1947) records a total of 41 species, sub-species and varieties from 

 North America and Greenland and has studied their ecology. He 

 found that the fauna of the sub-tidal zone is more uniform than 

 that of the inter-tidal zone, especially from a qualitative standpoint. 

 There is a well-developed sand fauna, a fauna based upon molluscs 

 and various algicolous faunas, while habitats characterised by low 

 salinity have their own peculiar mite fauna containing but a small 

 number of species. 



Numerous species are found swimming in fresh water. These 

 water mites constitute a far from negligible proportion of the fauna 

 of all normal and permanent waters. Biologically and ecologically 

 two groups can be distinguished: the eurythermic forms which live 

 mostly in standing waters naturally subject to wide variation of 

 temperature, and the stenothermic forms which dwell in cold 

 waters and are able to endure only slight variations of temperature. 

 The latter are largely restricted to spring waters, streams and over- 

 flowings, but in addition to the temperature factor governing the 

 choice of habitat, there is also a necessity for fast-flowing, richly 

 oxygenated water. Viets (1940, etc.) has studied the distribution of 

 some 219 species in Europe and has drawn some extremely inter- 

 esting conclusions regarding the probable pre-glacial fauna and the 

 possible routes and times of migration that took place after the ice 

 age. About 220 species are represented in the British fauna, and 

 their distribution is often peculiar. The dispersal of many of them 

 depends upon the larvae attaching themselves to the bodies of 

 various aquatic insects for transport. 



TheThrombidiidae are extremely numerous in both numbers and 

 kinds. They are of world-wide distribution and have been found 

 on every land mass except those permanently covered by ice and 

 snow. They are more abundant in the tropics than in temperate 

 climates and constitute a conspicuous portion of the mite faunas 



