June, '10] THOMrSON: TACHINID PUPATION 289 



puparia is ranch thicker than in niagnicornis and nidicola. If these 

 forms pupate loosely within the skin of the caterpillar, within the 

 empty pupa of the latter or within its cocoon, they are very liable to 

 the attacks of secondary parasites, as the studies of the parasites of 

 the tussock moth have demonstrated. It is almost certain to avoid these 

 secondary parasites and such predators as ants, which often work 

 great havoc among unprotected puparia, that they sometimes descend 

 into the earth to pupate, as we have observed that Tricholyga grandis 

 occasionally does. On the other hand we have noticed that fresh mag- 

 gots of Pales pavida Rdi., when placed upon earth often manifest no 

 desire to descend into it but pupate upon the surface. The short 

 duration of the pupal period among some of these forms probably 

 makes pupation in the soil of rather minor importance since they are 

 only for a short time exposed to drying influences and to secondary 

 parasitism. 



Still other species such as Front ina frenchii Will., as a parasite of 

 cecropia pupate well protected within the thick cocoon of the host 

 from which they make no effort to emerge. The parasite mentioned 

 is frequently found in the spring in very large numbers within the 

 cocoon of cecropia, the puparia sometimes occurring to the number of 

 forty or more. These puparia are in some cases very thin-walled and 

 light in color. This phenomenon we have observed to varv' directly 

 as the niunber of puparia present, and there can be little doubt but 

 that it is due rather to the inadequacy of the food supply, than to 

 such an adaptation of the species to the environment as is found in 

 the case of magnicornis. This is rendered the more probable since 

 in a study of the reproductive habits of this species we have noticed 

 that these thin-walled and light-colored puparia produce flies which 

 are short-lived and which do not generally develop to maturity. 



Hibernation, The factors which restrict hibernation among Tachi- 

 nid parasites to a definite stage in the life historj^ or permit of a 

 variation in this particular appear to be the habit of the host, and 

 individuality of the parasite. As a general rule parasites with but 

 a single generation annually hibernate in a certain stage and in 

 a definite manner; on the other hand, those which have several gen- 

 erations in a season may pass the winter in various ways. Among the 

 single-brooded species there are some with which the specific habit of 

 the parasite is the controlling factor rather than the habit of the host. 

 Such forms usually hibernate in the nymphal stage within the pupa- 

 rium. The condition of the hibernating nymph may be either unde- 

 veloped, as in Varichceta aldrichii and Parexorista chelonice, or ad- 

 vanced, as in Bleplianpa scutellata and Crossocosmia sericarice. This 

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