292 Mr. W. Wesche on systeiiudic affinities of the Phoi'idse 



but differs in sliape, in the valve, and in being more highly 

 chitinised in the outer envelope. The appearance of this 

 organ in Asilus and Doliclwinis is so singular and so 

 similar as to indicate a very near relationship between 

 these three families. The AsilidaB alone among the 

 families of the Brachycera preserve the ciliated hypo- 

 pharynx, which is so constant a character in the 

 Nematocera.* 



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

 relationship to the Mycetophilidse ; I think that while the 

 Mycetophilida:;, Asilidas, Empidae, Dolichopodida^, Loncho- 

 pteridte, Phorida^ and Leptida3 have had a common ancestry, 

 the Dolichopodidai and the Phorida3 have each inherited 

 a more than normal tendency to vary, resulting in the 

 curious contradictions in detail that are found in the 

 Phoridse, and the wide departure of the Dolichopodida) 

 from the usual 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 Hippo'bosca, which 

 we know by the character of the mouth, 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?e and Simulidse, 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 

 Lonchopteridce the mouth has closely preserved the 

 characters found in Lcptis, and except for the remarkable 

 wings and their sexual differences, developed no striking 

 peculiarities, and is probably a family much less liable to 

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

 though the transverse veins have disappeared. A parallel 

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

 apterous or semi-apterous species exist, belonging to the 

 Phoridffi, Dolichopodidre, and Hii^poboscidge, bears on my 

 argument, and demonstrates the superior value of the 

 mouth parts as tests of affinity. 



* The inouth parts of the Nemoceia. W. Wesche, Jour. Eoy. 

 Micr. Soc. 1904, pp. 28-47. 



