ASCERTAINING THE PARASITES OF HOST-INSECTS. 



77 



Three days after the gall-midges the cone-moths make their appearance, their 

 number increasing rapidly, attaining three days afterwards its maximum (28 per 

 cent.) and subsequently quickly diminishing. Not before the cone-moths have nearly 

 ceased emerging does Torymus azureus begin to appear, and the curve of the latter 

 rises and falls much more slowly than that of the two other species. 



The curves in fig. 2 show the same succession of the three species and almost the 

 same shape of the curves as in fig. 1. On the 14th of April the gall-midge reaches 

 its maximum, on the 18th the cone-moth, and not before the 26th does Torymus 

 attain its maximum. 



Consequently there exists a difference in the time of appearance of the phytophagous 

 insects, and this enables us to ascertain how the parasites group themselves around 

 their hosts. 



The diagram fig. 3 shows the curves of Perrisia strobi, Winn. (No. 1), Laspeyresia 

 strobilella, L. (No. 2), and one of the parasites of the latter, Nemeritis cremastoides, 

 Hlmg. (No. 3) from another locality. We notice that the latter appears three days 



40 % " U 13 U 15 16r 17 18 19 20 21 22 23 24 



30 % 











,-i 



VO 1 













s 



.n:o 



3 



20 "la 



















s'* 1 



:o 2 











JO"/o 









































/ 























Fig. 3. Diagram of Perrisia strobi (No. 1), Laspeyresia 

 strobilella (No. 2) and one of the parasites of the latter, 

 Nemeritis cremastoides, Hlmg. (No. 1), calculated as in 

 fig. 1, bred from spruce-cones from 12th to 24th April 

 1916 ; material from Lillogda, near Asele. 



after its host and ceases emerging one day after the latter. Both the curve of the 

 host and of the parasite have two apices, on account of the number of males, which 

 invariably make their appearance a couple of days earlier than the females, the 

 number decreasing more rapidly than that of the females increases. The difference 

 between the apices of the host and the parasite is only one day. The diagram shows, 

 as a consequence, that the curve of the parasite closely follows that of the host, with 

 an interval of one or two days. 



If we return to the diagram fig. 1 and add to it the curve of Platygaster contortieornis, 

 Eatz., a species very common in spruce-cones, it is evident (fig. 4) that the curve of this 

 species shows the same relation to that of Perrisia strobi as that of Nemeritis cremas- 

 toides to Laspeyresia strobilella, the only difference being that Platygaster appears a 

 little earlier than Perrisia. The curve of the latter has two apices, which is brought 

 about in the same way as in Laspeyresia and Nemeritis. In the curve of Platygaster, 

 on the other hand, the top is cut off, because the females, which in this species are far 



