MUSCULATURE OF DIPTERA — SMART 359 



prised in group 4 of table 4. Contemplating these, one notes that the 

 groups lacking the TDT muscle are quite isolated from each other 

 and that their apparent immediate relatives are among the groups 

 possessing the TDT muscle and not those lacking it. 



5. The bulk of the Cyclorrhapha possess a TDT muscle as is seen 

 by a glance at the families, etc., comprised in group 5 of table 4. 



As in the case of Brachycera, the groups lacking the TDT muscle 

 are isolated from each other and, except for the very specialized 

 parasitic pipunculids, they are groups small in number of species and 

 find their apparent immediate relatives among the groups possessing 

 a TDT muscle. 



When these facts are contemplated one begins to feel that : 



A. The possession or lack of the CS muscle indicates a major 

 systematic division in the Diptera and one of a very ancient evolu- 

 tionary date, having taken place at a time when all Diptera possessed 

 a TDT muscle. This condition of presence or absence of the CS 

 muscle does, in fact, divide the Diptera into the two main groups, the 

 Nematocera on the one hand and the Brachycera with the Cyclor- 

 rhapa on the other. 



B. The TDT muscle has been lost by diverse groups at different 

 times in the evolutionary history of the Diptera. 



a. In the Nematocera the loss of the TDT muscle is probably of 

 ancient date. It is possible that the group lacking the TDT muscle 

 is monophyletic, i.e., originating from ancestors possessing a CS 

 muscle but which lost the TDT muscle. 



b. The nematocerous families in group i of table 4 are probably 

 representative of the ancient original Diptera stock possessing both 

 the CS muscle and the TDT muscle. 



c. In the Brachycera the loss of the TDT muscle seems to have 

 taken place several times. In some cases this must have happened 

 at a distant evolutionary date since whole families have evolved which 

 lack the muscle. Among these groups lacking the TDT muscle are 

 flies with very specialized habits. 



d. Comparatively few Cyclorrhapha lack the TDT muscle. Looking 

 at them from an evolutionary standpoint one feels that probably the 

 loss is of comparatively recent date. The loss also seems to be corre- 

 lated with specialized habits of one kind or another as in the pipuncu- 

 lids, the tsetse flies (Glossina) , and Hippobosca. 



Further comparative studies may confirm the singularity of the 

 Diptera comprised in group i of table 4 and convince dipterists that 

 they do in fact represent, albeit in a much modified form, the ances- 



