MUSCULATURE OF DIPTERA — SMART 355 



is a perfectly good TDT muscle in the metathorax ; the prothorax has 

 an extra coxal depressor of the trochanter that has its origin in a 

 "pleural" situation. In connection with the latter, however, it may 

 be noted that it is very difficult, if not impossible to decide whether 

 the "pleural" part of the prothorax is a downward extension of the 

 tergum or a true pleuron f used-up to the tergum. The prothoracic 

 "pleural" depressor of the trochanter in Anisopus may very well be 

 the homologue of the real tergal depressor muscle. There is no such 

 doubt about the metathoracic TDT muscle ; its origin is dorsal to the 

 articulation of the halter. Maki (1938) should be referred to for 

 further information on the occurrence of what appear to be TDT 

 muscles in the thoracic segments of various insects. 



The physiological reactions of the depressor muscles of the tro- 

 chanter of the American cockroach (Periplaneta americana Linn.) 

 have been commented on by Becht and Dresden (1956) ; there are 

 indications that the different fasicles have different physiological 

 properties. 



Boettiger and Furshpan (1952) have suggested that the function 

 of the TDT muscle in Diptera is to act as a "starter" for the general 

 flying mechanism of the thorax ; they were considering Sarcophaga, 

 a. genus in which the TDT muscle is conspicuous. 



When well developed, the morphology of the TDT muscle is very 

 suggestive of its use for jumping, and as such it might conceivably 

 be used at the commencement of flight. This suggestion receives rein- 

 forcement when one notices that all the long-legged helicopter-like 

 Nematocera, which certainly do not jump at the commencement of 

 flight, lack the TDT muscle. On the other hand such Nematocera 

 have a remarkable capacity for flying up and down vertically in 

 swarms, though their general mode of flying lacks the rapidity and 

 directional accuracy that is associated with the flight of many Brachy- 

 cera and Cyclorrhapha. 



Not enough is known of the flight habits of other Diptera to be 

 able to comment on the fact that many heavy-bodied, normal-length- 

 legged Diptera also lack the TDT muscle. 



Smart (1958) suggested that the mesothoracic TDT muscle in 

 Diptera may be used for distorting the configuration of the thorax 

 and so altering the characteristics of the flight in flies possessing the 

 muscle. It could be used for this purpose when the legs of the fly are 

 hunched up when in flight, as they are in most of the flies that possess 

 the muscle in a well-developed condition. The legs would be hunched 

 up by the levators of the trochanter, and if the point of insertion of 



