1 88 THE INTEGUMENTAL SKELETON OE THE IMAGO. 



The elastic recoil of the thoracic wall acts in antagonism to 

 both sets of muscles, which only counteract each other to 

 a certain degree, both sets increasing the transverse diameter 

 of the cavity. 



The Median Segment of Latreille. — Before concluding this 

 somewhat lengthy section, I would say a few words on a 

 subject which has been a fertile source of error in relation 

 to the morphology of the sclerites of the thorax in the 

 Diptera. 



Latreille discovered that the ventral portion of the first 

 abdominal segment enters into the composition of the thorax 

 in the Hymenoptera. This is especially well seen in those 

 with a sessile abdomen, and anyone who will examine the 

 thorax of a Cimbex or Trichosoma can easily verify the fact. 

 Latreille calls this segment the ' segment mediaire.' 



The median segment of Latreille bears a spiracle, but there 

 is a second spiracle in front of this in Cimbex, between the 

 metasternum and the dorsum, corresponding with the posterior 

 spiracle of the Diptera. 



This spiracle was unknown to Latreille ; it was discovered 

 by Professor Schiodte in 1856 [76] . 



Latreille erroneously supposed the posterior thoracic spiracle 

 of the fly to be the first abdominal one, and thought that the 

 first abdominal segment enters into the composition of the 

 dipterous thorax. He was, however, more consistent than 

 his followers, for he denied the existence of any homology 

 between the halteres and the second pair of wings. He 

 clearly saw that his theory fell to the ground unless the 

 halteres are regarded as abdominal organs. 



At the present day no homology has been more clearly 

 established than that of the halteres and the posterior wings, 

 yet many who smile at Latreille's idea, that they are abdominal 

 appendages, have readily adopted his view of the existence of 

 a median segment in the dipterous thorax. I have shown, I 

 think clearly, that so far from this being the case, the meta- 

 thoracic tergum is abdominal in these insects, the metapleura 

 being on the abdominal side of the mesophragma. 



