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constitute a very numerous Order which includes both 

 terrestial and aquatic forms. The popular impression 

 which regards them all as parasitic is incorrect. A large 

 number are so, but the majority are free living. Many 

 which are found upon the bodies of various kinds of 

 animals are not parasitic but are there temporarily for 

 purposes of transportation only. These generally exhibit 

 special adaptations such as adhesive discs or special 

 claws for clinging purposes. These mites are usually 

 immature stages, and much confusion has arisen in the 

 past from their having been described as distinct species. 

 The Order includes several important types, of which 

 naturally the parasitic forms have called for serious 

 attention from man, e.g., the itch and mange mites of 

 man, domestic animals and birds. The well-known 

 Dermanyssus sucks the blood of poultry and cage birds ; 

 other forms attack the feathers only. Ticks are Acarina 

 which are exclusively blood-sucking and are important 

 not merely on this account but because they are the 

 only natural agents for the spread of disease such as 

 red water in cattle, malignant jaundice in dogs, etc., 

 and tick fever in man in Central Africa. Some are 

 destructive to vegetation, such as the familiar currant 

 mite and the pear leaf blister mite, whilst gall forming 

 mites are common on wild plants. Certain species, 

 when introduced to houses and other buildings, some- 

 times multiply in such numbers as to constitute a 

 veritable plague, and there are some forms related 

 to Tarsonemus normally feeding upon plant juices 

 which, should they get upon man, cause a highly 

 troublesome itch accompanied by erythema. Within 

 this large Order there are certain very definite 

 differences in the manner of breathing, and the 

 structural arrangements associated with this have 

 been made the basis for the establishment of sub- 

 orders. It is unnecessary to discuss these differences 

 here, but it is desirable that they should be mentioned, 

 since the respiration of Tarsonemus is a matter of 

 practical importance and also as the division it belongs 

 to presents a peculiar respiratory feature. Let me give 

 one or two illustrations in classification. The familiar 

 cheese mite and itch mites have no special respiratory 

 tubes and appear to breathe through the soft delicate 



