2!)2 ]\Ir. W. Wesclie on sijdciiiid'ic (ij/initics of the Phond/e 



but differs in shape, in the valve, and in being more higlily 

 chitiniseil in the outer envelope. The ajipcnuance of this 

 organ in AsUhh and DnHcliopns is so singular and so 

 similar as to indicate a very near relationship between 

 these three families. The Asilida3 alone amons the 

 families of the Brachycera preserve the ciliated hypo- 

 pharynx, which is so constant a character in the 

 Neinatocera.* 



Weighing these facts, I must extend Becker's idea of 

 I'clatiunship to tlie JMycetophilida? ; I think that while the 

 ]\Iycetophilida', Asilida', Empida', Dolichopodidu', Loncho- 

 ptcrida', Phorida^ and Leptida; have had a common ancestry, 

 the Dolichopodida' and the Phorida; have each inherited 

 a more than normal tendency to vary, resnlting in the 

 curious contradictions in detail that are found in the 

 Phorida-, and tlio wide departure of the Dolichopodida; 

 from the usnal type of mouth, the character of the tracheae 

 in most genera being unique. 



It will be noticed in the families enumerated above, that 

 the wing-venation is fairly complex in the larger insects, 

 and simpler in proportion to the size of the others. 

 Taking as an example the venation in Ilippohosca, which 

 we know by the cliai'acter of the moutli, has degenerated 

 from a Muscid form, we there see a tendency for the 

 transverse veins to shorten or be lost, and for the veins 

 to leave the lower part of the wing. This is what has 

 also happened in the Phorid;ii and Simulida;, and all three 

 groups appear to have undergone parallel degeneration. 

 As the parasitic habit increased, or size decreased, the 

 wings, being less used or having less weight to bear, 

 became more simple as regards their venation. In the 

 Lonclioptcridre the mouth has closely preserved the 

 characters fouud in Lejjtis, and except for the remarkable 

 wings and their sexual differences, developed no striking 

 peculiarities, and is probably a ffxmily much less liable to 

 vary than Phora ; so in the wings loss simplification is seen 

 though the transverse veins have disappeared. A parallel 

 case appears to exist in the Psychodida'. Tlic fact that 

 apterous or semi-apterous species exist, belonging to the 

 PhoritUc, Dolichopodida', an<l Hippoboscida^ bears on my 

 argument, and demonstrates the superior value of the 

 mouth parts as tests of affinity. 



* The iiioutli jiarts of llie NoiDOcera. \V. Weselu'', Join-. Roy. 

 Micr. Soc. 1904, ]i].. 28-47. 



