XXVI INTRODUCTION. 



with the habits of the mature insects, yet just what the rela- 

 tions are is not yet well understood. Osten Sacken has ob- 

 served that the eremochsetous flies (i. e. diptera in which 

 there is a general absence of bristles, as for example the 

 Stratiomyidae, Leptidse, and Tabanidse) are for the most part 

 holoptic in the male sex and at the same time are principally 

 aerial flies, flying swiftly and with the habit of hovering, 

 using their legs only for alighting. On the contrary the 

 chsetophorous flies (as the Muscidse, sens, lat., Phoridae, Doli- 

 chopodidae, Asilidse, etc.) iise their legs as much as, sometimes 

 more, than the wings for locomotion, and rarely have the eyes 

 contiguous in the male sex. Probably the development of the 

 macrochsetae reaches its highest extent among the Tachinidse, 

 as for instance in Dejeania, Saundersia, etc., and the Dexiidae 

 (Hystrisiphona, etc.), where the abdomen may be almost 

 wholly covered with long and erect, very rigid spines. 



As concerns other forms of covering, the usage of writers 

 is not very exact ; the terms hair, pile, pubescence and tomen- 

 tum are used with a wide degree of latitude. In general, how- 

 ever, pile should be restricted to indicate close, thickly set, 

 fine hair, as in the pile of velvet, while hair may mean longer, 

 and less abundant. Pubescence should mean very short, fine 

 hairs, while tomentum can only be correctly applied to recum- 

 bent, flattened scale-like or stubble-like hairs, which gradually 

 merge into dust or pollen, which is so generally present in 

 flies, and upon which the determination of many species must 

 largely depend. 



INTERNAL ANATOMY. 



For the following brief account of the internal anatomy of 

 Diptera I am indebted to Prof. V. L. Kellogg. 



The special features of the internal structure of the Diptera 

 are the high degree of concentration of the nervous system 

 attained in some of the members of the order, the expansion 

 of the two main tracheal trunks in the base of the abdomen 



