THE THORACIC EXO-SKELETON. i8i 



the larvae of the Lepidoptera have no spiracles on the segments 

 corresponding with the meso- or metathorax. 



Weismann [2] assigns the anterior spiracle to the prothorax 

 on developmental grounds. Hammond [78] adopts the same 

 view. Brauer [80], on the other hand, says it is meso- 

 thoracic. 



Gosch [79] pointed out the verbal character of the discus- 

 sion, and his statements may be summarised as follows : It is 

 the rule to describe inter-segmental abdominal spiracles as 

 belonging to the segment immediately behind them, and this 

 rule is apparently justified by the fact that when the 

 abdominal spiracles are segmental they are always near the 

 anterior border of the sclerite on which they occur. In the 

 case of inter-segmental thoracic spiracles the rule has been 

 reversed, and they have been ascribed to the segment in front 

 of them. Hence the anterior spiracle has been named pro- 

 thoracic. 



It has been asserted that the anterior spiracle in the 

 Coleoptera always remains attached to the prothorax when 

 the latter is separated from the mesothorax. In my ex- 

 perience it as often remains on the mesothorax. I do not, 

 however, think that such evidence is admissible. 



Weismann's statements are far more important ; the anterior 

 spiracle of the nymph is certainly prothoracic, but it is by no 

 means certain that the anterior spiracle of the imago, which is 

 developed behind that of the nymph, is also prothoracic, as 

 Weismann believed. 



It will probably be argued by some that the spiracles are 

 segmental appendages. I regard them as fissures developed 

 in relation with the internal tracheae, and hold that such 

 fissures may be either segmental or inter-segmental. I have 

 already shown that the anterior spiracle in the Blow-fly lies 

 between the pro- and mesothorax. 



The posterior spiracle is between the metasternum and the 

 tympanic bulla, and the bulla is undoubtedly mesothoracic. 

 It is therefore situated between the meso- and metathorax. 



The Pleural Kegion is bounded above by the wing roots, in 



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