286 



Ist that Ergates, as examined by Boas, cannot be considered 

 as showing the general rule. 



2nd that the prothoracic stigma of recent insects originally belongs 

 to the intersegmental membrane or to the praesegmental zone of the 

 mesothorax and that it can be shifted unto the prothorax. 



3i'cl that the second thoracic stigma was originally situated in 

 the intersegmental membrane between the mesothorax and meta- 

 thorax, and that commonly it lies more ventrally than the other 

 stigmata. 



4th that the abdominal segments (1 — 8) possess a praesegmen- 

 tal stigma, 



Janet (1909) mentions a 9th abdominal stigma for Lepisma^ 

 Brauer (1851) for Panorpa a stigma on all the 13 segments, 

 except on the mesothorax and metathorax. 



Accepting the probability. that the homoiomery, whieh shows 

 itself so strongly in Insects, originally also ruled the tracheal 

 system, the conclusion logically' follows, that in the beginning the 

 stigmata on the thorax were also situated praesegmentally. But then 

 we must also assume that the second pair of thoracic stigmata in 

 reality belongs to the metathorax and the prothoracic stigma to 

 the mesothorax. From this follows a shifting of the stigmata in 

 the direction of the head sometimes over a considerable distance. 

 Through this shifting the metathoracic stigma is pressed a little 

 towards the ventral side, whilst the first thoracic stigma may at 

 the same time turn to the dorsal side. 



If this hypothesis is correct, then the whole thorax has been 

 modified and therefore cannot have preserved a primitive struc- 

 ture. This assertion is supported by the different arrangement of 

 the muscles, already described by Lijonet. Yon Voss also has 

 pointed out the secondary structure of the thorax in his papers 

 1911, 1912, 1913. If therefore the primitive structure of a segment 

 is the object of our research, we must study the abdominal instead 

 of the thoracic segments. Herein lies a strong argument against 

 Fracker and ïsou, who take the prothorax as a starting point. 



