25 



KfMMETH again went into this question thoroughly in 1914 and 

 has added very accurate figures to his text (PI. V, fig. 1 — 25). He 

 has exaniiniHl various orders of insects, but unfortunately, mostly 

 as imagines. His principal results are: 



The thoracic shield is formed by three thoracic segments and 

 one abdominal segment, which, however, retains an abdominal 

 structure. These four rings bear three pairs of stigmata, one ab- 

 dominal i>air as usual situated in the praesegmental zone (some- 

 times more dorsal : Pulicidae, sometimes more ventral : Khynchotae) 

 and two thoracic pairs, situated postsegmentally or intersegmen- 

 tally. The first pair always breaks through the connective mem- 

 brane between ,the prothorax and mesothorax, sometimes more 

 prothoracally (Coleoptera, Uhynchota, Hymenoptera), sometimes 

 more or less in the intersegmental connective membrane (Plecop- 

 tera, Lepidoptera). In Odonata, Neuroptera and Panorpata it is 

 forced by the strong reduction of the prothorax against the prae- 

 segmental edge of the mesothorax. It is able to move consi- 

 derably in the dorso-ventral direction. 



The second pair of thoracic stigmata is found between the 

 mesothorax and metathorax, mostly on a line with the first pair. 

 In Hymenoptera and Lepidoptera it is situated directly under 

 the root of the back-wing. This is confirmed by the larvae. 



Zander found in 1910 that the first pair belongs to the post- 

 segmental zone of the prothorax, the second pair to the inter- 

 segmental membrane of the mesothorax and metathorax. An 

 exception is formed by Di/tiscus and Ergates, where the first pair 

 is situated in the praesegmental part of the 2Qd segment. 



There is not the least indication that formerly there were three 

 thoracic pairs of stigmata, and that in one group one of these 

 became obliterated, in another group another. The only thing which 

 is certain, is that the functionizing stigmata of different insects 

 belong to diflferent segments. The second stigma mostly lies a 

 little ventrad. 



Without making any claim to finality in this important ques- 

 tion, I think it may be accepted as quite certain: 



