INTRODUCTORY. 5 



There is at least one species of Tyroglyphidas which 

 must be looked upon as a commensualist or mntualist ; 

 this is Tyroglyphus Wasmanni ; which, up to the present 

 time, has only been found in ants' nests. 



If the species of Tyroglyphida3 be few they make up 

 for it not only by the number of individuals, as before 

 stated, but also by the immensely wide distribution of the 

 principal species. Acari generally are apt to have but 

 few local species compared to the number which extend 

 over a very wide range, but the Tyroglyphidse pro- 

 bably carry this further than almost any other family. 

 It is difficult to say whether this is an original character 

 or to what extent it is due to the fact that their chief 

 habitat is in substances which are carried far and wide 

 by man; wherever such products of civilisation as 

 cheese, flour, grain, dried fruits, or drugs go, there the 

 Tyroglyphidse go with them, and spread from the 

 centres which they thus reach. To such an extent is 

 this wideness of range carried that a species which is 

 abundant all over Central Europe was brought home 

 by the Jackson-Harmsworth expedition from the 

 Franz Joseph Archipelago, and the collector assured 

 me that he obtained it from the talus far from camp ; 

 the same species is found far south. Probably if ever 

 we know more of the Acari of tropical and southern 

 countries the distribution will be found to be even 

 wider than it is now known to be. The minute size of 

 the creatures, and the difficulty of destroying their 

 eggs and Hypopi doubtless greatly favour this ex- 

 tended range. 



The Tyroglyphidse are almost all soft-bodied crea- 

 tures, the very few which have hardish, chitinised 

 cuticle, such as Chortoglyphus arcuatus, being quite 

 exceptional ; they must be considered as amongst the 

 simplest, and probably amongst the most primitive, of 

 Acarine families as regards organization ; they are 

 without special breathing organs, they have not any 

 heart such as is found in some Gramasidse and Ixodidas ; 

 they have not any eyes such as are found in Trombi- 



