INTRODUCTION. 15 



scales, covering sometimes extensive parts of the body or legs, are 

 common in many genera of BOMBTLIID.S and TIIEREVID^E. 



The vestiture of the legs varies from practical bareness, through 

 the whole gamut of soft and fine to a long and ragged pubescence, 

 bristles, spines, scales, and tooth-like serrations (the latter generally 

 confined to the underside of the femora), all of them often forming 

 good generic or specific characters. As a rule, aerial species (that 

 is, those that are frequently on the wing and addicted to hovering) 

 possess soft pubescence on the body and weak legs devoid of 

 strong bristles, as they are presumably employed mainly for 

 alighting ; whereas, on the contrary, pedestrian (which frequently 

 means predatory) species mostly possess strong bristles or spines, 

 and powerful legs capable of running swiftly and surely and of 

 holding their prey when captured, and possessing comparatively 

 reduced powers of flight. Compare, for instance, a Bombylius 

 with an average Asilid. A peculiar character called " touch 

 hairs " is present in certain genera of LEPTID.E, TABANID^;, and 

 some other widely divergent groups, and this is dealt with under 

 the genus Leptis.* 



The genitalia in the BRACIIYCBRA are usually more or less con- 

 cealed in most families or are of simple form, but they afford in 

 the <5 good taxonomic characters in ASILID.I: and DoLicHOi?ODiD^E, 

 being fully free in the latter family ; but the genitalia are always 

 more uniform in the $ and of little value lor classification in 

 that sex. 



Life Histories.-^ 



Although the metamorphoses of the Diptera in general are still 

 but imperfectly understood, considerable progress in their study 

 has been made since the older writers (Keaumur, JJe Geer, etc.) 

 first broke the ice, and the early stages of a comparatively large 

 number of common species have been worked out. Taking the 

 order as a whole, the larva may be described as maggot-like, 

 though extreme forms of shape are far from uncommon. It 

 generally consists of a head segment and twelve others, of which 

 the three anterior ones are presumed to represent the thorax 

 of the perfect insect. The head segment is often so indefinitely 

 presented that Brauer sometimes uses the expressions " head 

 capsule" and "jaw capsule." Even when obviously present the 

 head is sometimes most disproportionately minute or placed below 

 the anterior end of the body so as to be invisible from above ; 

 also not infrequently in the flesh-feeding maggots no head can be 

 detected with certainty. These latter forms are called acephalous, 



* These notes on vestiture are compiled chiefly from Verrall. 



t These remarks on the early stages are wholly culled from Dr. Sharp's 

 paper (v. Verrall, p. 31). This article should certainly be studied by nil 

 interested in the subject. In addition to a preliminary general resume of 

 brachycerous larvae, it offers a translation of Brauer's classification of the 

 fumilies in BRACHYCERA. based on larval characters, and, lastly, more detailed 

 information is given on each family or group of families separately. 



