78 



IVAR TRAGARDH. 



more numerous than the males, increase in number to the same extent as the males 

 decrease, so that the percentage of hatched insects during two days is the same. 





"/, 



18 



19 



20 



21 



22 



23 



24 



25 



26 



27 



28 



29 



30 



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Fig. 4. Diagram of Perrisia strobi, Winn. (No. 1), Platygaster contort icomis,Tla,tz. 



(No. 2), Laspeyresia strobilella, L. (No. 3), Torymus asureus, Bhn. (No. 4), and 



Aprostocetus strobilanae, Katz. (No. 5), bred from spruce-cones from 17th. April to 



7th May 1916 ; material from Kungsor, Koping. 



The diagram, fig. 5, shows the same relation still more plainly, the curves of 

 Perrisia and of Laspeyresia being more separated one from the other. 



This close relation of the curves of Platygaster and Perrisia argues strongly in 

 favour of the assumption that the former is the parasite of the latter, which was 

 previously suspected, all other species of the genus Platygaster of which the food 

 habits were known being parasites of gall-midges. 



An examination of material from which a great number of both Perrisia and 

 Platygaster had been hatched enabled me to make certain that the above conclusion 

 as to the relation of Platygaster to Perrisia was true, a dead Platygaster being found 

 in the inflated larval skin of a Perrisia. 



40 "/o 





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10 



11 



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16 



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Fig. 5. Diagram of the same species as in fig. 4, bred from 9th to 30th April 1916 ; 

 material from Forshem, Kinne. 



There remain Torymus azureus, Bhn., and Aprostocetus strobilanae (Ratz.), the 

 latter being one of the commoner insects hatched from the cones. As pointed out 

 above, the former makes its appearence only when the majority of Perrisia and 

 Laspeyresia have emerged, and its curve is of a quite different shape. While the 

 curves of the two latter both rise rapidly to 30 per cent., the emergence of the majority 

 of them taking place within a few days, the curve of the former hardly rises above 



