THE HALTERES 83 



median vein at the wing margin ; the proximal one of these, not having 

 divided as it does in the urotype, represents the third and fourth. The 

 cubital vein arises just below the median, but is not connected with it ; 

 it divides at a point just behind the first bifurcation of the median vein, 

 the two branches proceeding direct to the wing margin ; the anterior 

 of these is connected with the posterior division of the median vein 

 by a short cross-vein, the median cubital. There is only one anal vein, 

 which arises independently behind the cubital, and reaches the wing 

 margin at the same point as the second division of the cubital vein. 



The humeral and axillary cross-veins are well marked, the latter 

 connecting the radial, median, and cubital veins, which are derived from 

 the anterior trachea, with one another at the wing base. The radial 

 median cross-vein connects the radial vein, just distal to its first division, 

 with the anterior branch of the first division of the cubital vein. The 

 median cross-vein forms the distal boundary of the discal cell, and the 

 median cubital cross-vein connects the posterior division of the median 

 vein with the anterior division of the cubital. 



The cells will be recognized by the veins which form their anterior 

 boundaries. The cell behind the fused second and third radial veins is 

 the third radial cell, that behind the fifth, and limited above by the radial 

 median cross-vein, the fifth radial cell. There are four median cells, the 

 distal one of which is the first, while the second is divided into two parts 

 by the median cross-vein. The anal cell corresponds to the anal lobe. 



In the Muscoidean flies the venation is rendered simpler by the 

 coalescence and suppression of many of the veins. Much importance is 

 attached to the curvature of the fourth vein, which after a downward 

 and outward course for the most part of its length turns upwards towards 

 the apex of the wing, either terminating at the same point as the third 

 vein, and thus closing the first posterior cell, or approaching very near it. 

 It should be noted that in these flies the first longitudinal vein occupies 

 a position corresponding to that in which the subcosta is found in 

 Haematopota. 



The halteres, or balancers (Plate XV, fig. 2) are characteristic of the 

 Diptera, and are believed to be homologous with the metathoracic 



wings of other insects. They are small club-shaped 



. The Halteres 



bodies, situated behind the wings and behind and 



above the posterior spiracles of the thorax. Each arises from a small 

 raised cushion, the rounded end being supported on a slender stalk. 

 Both the raised base and the distal end are well supplied with nervous 

 tissue, the cushion consisting mainly of a highly complex arrangement 



