THE CANADIAN KNTOMOLOGIST. 35 



prolong the victim's life to its very utmost. The adaptation, of course, 

 always exists, but here we probably see it at its highest level." 



This retardation is by no means confined to Braconid parasites, but 

 also occurs with other parasites, and, as Mr. Poulton suggests, in other 

 than hot latitudes. I recall very well some observations which Mr. 

 Schwarz and I made on the larvie of Plusia brassiae in cabbage fields in 

 Washington in iSSr. It was in the autumn, and full-grown larva? of this 

 insect were rather abundant in the fields. Fifty or more specimens were 

 taken to the laboratory, and showed an almost complete percentage of 

 parasitism by Copidosoina truncatellum. 



Some of the observations made at that time were recorded by me in 

 the American Naturalist for February, 18S2, pp. 150-1, and also in the 

 Annual Report of the Department of Agriculture for 1883, p. 121. I 

 believed then, and I think Mr. Schwarz concurred (although neither of us 

 ever published the statement), that this practically complete percentage of 

 parasitism was not necessarily indicative of the whole percentage for the 

 season, but indicated that the parasitized larvae remained longer in the 

 field, and without ocular evidence of parasitism for a considerable time 

 after the unparasitized individuals had spun up and transformed to 

 chrysalids. 



In fact, it frequently occurs with lepidopterous larvae, and, of course, 

 with other insects as well, that parasitized individuals grow more slowly 

 than the rest, and often may be identified by their smaller size. Many, as 

 we know, are destroyed before reaching full growth by certain parasites, 

 but many others reach the full larval size and linger on, sluggish it may 

 be, but apparanily unharmed for a considerable time after their unstung 

 mates have crawled away and hidden themselves for transformation, or 

 even perhaps, in case of multiple-brooded species, until individuals of a 

 succeeding generation have approximated their stage of growth. 



The practical feature of all this in work witli parasites comes from 

 the fact that we must take this retardation strictly into consideration in 

 estimating percentages of parasitism. Should larvae in considerable 

 number be collected at the end of the season and kept for rearing pur- 

 poses, it appears that through this retardation of parasitized individuals it 

 may easily happen that an apparently almost complete percentage of 

 parasitism will be observed which will by no means indicate the true per- 

 centage with the generation as a whole. 



