MITES AND TICKS 185 



eludes the cheese-mites and other Tyroglyphidae which are often 

 pests of stored products, as well as the itch-mite Sarcoptes scabei, 

 and the Oribatei or beetle-mites. Generally speaking, the Acaridiae 

 are soft and the Oribatei more or less armoured, but exceptions 

 occur in both groups. 



Finally, the Tetrapodili or Eriophyid mites are an aberrant sub- 

 order of the Acari as they possess only the front two pairs of legs: 

 the two posterior pairs are lacking or reduced to fine hairs. These 

 include the gall-mites and other plant parasites, all of which have 

 elongated, worm-like bodies in which the original segmentation is 

 indistinguishable, for the ring-like appearance of the posterior part 

 of the body is only an epidermal feature. A general characteristic is 

 the possession of two long sinuous setae arising from the posterior 

 part of the body. 



The Acari are widely distributed all over the world from the 

 north of Greenland to the antarctic. Terrestrial forms occur from 

 sea level to the mountain tops, while marine and fresh water species 

 are not uncommon. The distribution of parasites and commensals 

 is intimately related with that of their hosts, but even free-living 

 mites show marked degrees of adaptation to environments far 

 removed from their original biotope. 



General behaviour 



Mites are found everywhere: many are present in damp soil and 

 moss in shady places, where they live on small fungi, others live on 

 plants, sucking the sap and sometimes causing gall-like tumours, 

 while still more are the predators or parasites of insects, verte- 

 brates and other animals. Whereas insects are numerically the 

 leading group of animals inhabiting arable soils, in forests their 

 place is taken by the Acari which, in humus are easily the most 

 abundant invertebrates. It has been shown that they constitute 

 2-3% of the total weight of the invertebrate fauna in Danish beech 

 woods and 5-7% in spruce woods. This difference, due to vast 

 numbers of Oribatid mites which are practically limited to humus, 

 appears to be a constant one. 



Many mites are cavernicolous, others are myrmecophilous: of 

 these some are scavengers, others eat the ants' food and still more 

 feed on the ants themselves. ^i^'t » » P V^T-^--. 



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