360 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



tors of all Diptera. If this happens, then it may be necessary to 

 detach them from the Nematocera and set them apart as a new sub- 

 order of Diptera. 



The Diptera other than the above group fall clearly into two groups 

 representing two different evolutionary streams. These are, first, the 

 Nematocera in group 2 of table 4 which have retained and specialized 

 the CS muscle but have lost the TDT muscle, and second, the Brachy- 

 cera together with the Cyclorrhapha, which stream originally lost the 

 CS muscle but retained the TDT muscle. 



The existence of these two streams would favor classifications of 

 the Diptera which link the Brachycera and the Cyclorrhapha to- 

 gether rather than those which, relying on the condition of the pupa, 

 etc., attach the designation Orthorrhapha to the Brachycera and thus 

 link them with the Nematocera rather than with the Cyclorrhapha. 



The loss of the TDT muscle within the Brachycera and the Cyclor- 

 rhapha seems to have been sporadic and to have taken place at various 

 times, and the character cannot be used to effect major subdivisions 

 of either of these groups. Whether subfamilies found to differ from 

 their alleged relatives in this character of the possession or lack of 

 the TDT muscle should be raised to family status or merely confirmed 

 in their subfamilial position is a matter for further consideration 

 when either more fossil evidence is available or comparative studies 

 of both imagines and larvae have been carried out. 



9. ACKNOWLEDGMENTS 



The studies of the musculature of Anisopus leading up to the 

 preparation of this paper were commenced while I was the fortunate 

 recipient of a Smith-Mundt grant from the U.S. Department of State 

 and a Fulbright travel grant and was working in the Department of 

 Entomology and Parasitology of the University of California at 

 Berkeley. The latter parts of the work, especially those dealing with 

 the comparative aspects, have been carried out at Cambridge. 



I should like to acknowledge the help derived from discussions 

 with numerous workers, more especially with the late Prof. O. W. 

 Tiegs, F. R. S., who was working at Cambridge in 1953-1954, and 

 Dr. T. Weis-Fogh, also in the Cambridge Zoological Laboratories. 



10. SUMMARY 



The foregoing paper consists of a discussion of some points of the 

 comparative myology of the mesothorax of Diptera. The discussion 

 is based on a preliminary description of the mesothoracic musculature 



