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STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 413 



postscutellum. It covers the haltere, as in all tf calyptrate " 

 Muscidas. 1 



The Halteres. The halteres or balances (fig. 6, hal.) are 

 generally considered to represent the rudimentary meta- 

 thoracic wings. They are covered by the squamse, and are 

 situated on the sides of the thorax above the posterior 

 thoracic spiracles. Each consists of a conical base on which 

 are a number of chordonotal sense-organs, and on this base 

 is mounted a slender rod, at the end of which a small spheri- 

 cal knob is attached. The wall of the distal half of this 

 sphere is thinner than the proximal half, and in preserved 

 specimens is generally indented. Experiments show that the 



1 The nomenclature of Comstock and Needham has not yet been adopted 

 by dipterologists in general ; but, on account of its great morphological value, 

 it will no doubt in course of time replace the present confused system. It 

 may therefore be useful if the nomenclature employed in the foregoing 

 description be compared with those most usually employed. 



Longitudinal nervures. C v Costal. Sc r Mediastinal. auxiliary. R^ 

 Subcostal, 1st longitudinal. R. 2 + 3. Radial, 2nd longitudinal. R. 4 + 5. 

 Cubital, 3rd longitudinal; ulnar (Lowne). M. 1 + 2. Median, 4th longitu- 

 dinal; discal (Verrall). M . 3 + Cu v Submedian, 5th longitudinal; postical 

 (Verrall). A r Anal, 6th longitudinal. Pseudonervure, axillary, 7th longi- 

 tudinal. 



Transverse nervures. h. Humeral, 1st transverse; basal cross vein 

 (Verrall). rm. Discal, 2nd transverse; middle cross vein (Verrall); medial 

 transverse; anterior transverse (Austen), m-cu. Anterior basal transverse 

 (Austen); lower cross vein (Verrall); postical transverse (Lowne). m. Pos- 

 terior transverse (Austen) ; postical cross vein (Verrall) ; discal transverse 

 (Lowne). cu-a. Posterior basal transverse (Austen) ; anal cross vein (Verrall); 

 anal transverse (Lowne). 



Cells. C. Costal. 1C. Second costal. Se. Third costal (Lowne cor- 

 rectly calls this "sub-costal"). IR. Marginal. 3R. Sub-marginal; cubital 

 (Lowne). 5R. First posterior cell (Austen) ; sub-apical (Lowne and Verrall). 

 2JIP. Second posterior cell (Austen); apical. \Cu. Third posterior cell 

 (Austen and Verrall) ; patagial (Lowne). 2 M 1 . Discal (this term is used also 

 in Lepidoptera, Trichoptera, and Psocoptera, and in each family refers to a 

 different cell!). R. Anterior basal cell (Austen); upper or 1st basal or 

 radical (Verrall); prepatagial (Lowne). M. Posterior basal cell (Austen); 

 middle or 2nd basal or radical (Verrall); anterior basal (Lowne). Cu. Anal 

 cell (Austen) ; lower or 3rd basal or radical (Verrall) ; posterior basal (Lowne). 



