X INTRODUCTION. 



dent to understand and appreciate, not only the present work, 

 but all other systematic works upon diptera. I have not 

 thought it desirable to consider at length many interest- 

 ing subjects connected with them, such as the internal anato- 

 my, embryology, etc., as being rather apart from the object of 

 the work, an introduction or, aid to the study of systematic 

 dipterology. 



HEAD. 



The head in diptera is extremely variable in shape, reach- 

 ing its most remarkable development in the Diopsidae and 

 Nycteribiidee. It is frequently more or less hemispherical, but 

 more often the posterior surface or occiput, is flattened or 

 even concave, giving a more hemispherical form. Often it is 

 flattened and widened transversely as wide or wider than the 

 thorax. In the Nycteribiidse it may be folded back into a 

 groove on the dorsum of the thorax, but with those exceptions, 

 it is always attached to the thorax by a freely movable neck. 

 Next to the wings, the head offers the most important char- 

 acters for classification. 



Eyes. The large compound eyes are present in all diptera 

 save some Pupipara. In the great majority of males they are 

 contiguous on the upper side of the head for a greater or less 

 distance; in such cases the insects are called holoptlc. In 

 many males, however, (all the Acalyptratse and several fami- 

 lies of the Orthorrhapha, as well as in numerous genera of 

 other families) and in all females with but few exceptions 

 (some Acroceridae, Blepharoceridse, Bombyliidae and Platype- 

 zidae, etc.) the eyes are separated more or less broadly by the 

 front ; such insects are called dichoptic. Barely the eyes may 

 be contiguous below the antennas or both above and below 

 them, as in the Acroceridae. In not a few flies, especially 

 those of the aerial eremochsetous kinds, the upper facets of the 

 eyes are larger and more conspicuous than the lower ones, 

 sometimes separated by a distinct line, or even entirely di- 

 vided. In these flies especially, the eyes in life are often 



