THE CODLING MOTH IN MICHIGAN. 7 



depression is made in the bark, the walls along the exposed sides are 

 constructed from fragments of bark held in place by silken threads, 

 and the inside is finally lined with a thin layer of silk. Within the 

 small space of the cocoon the larva will be found in a doubled-up posi- 

 tion. In the spring, previous to pupation, the winter cocoon is partly 

 remodeled by the larva (see PL I, fig. 4), and provision is made for 

 the issuing moth by the construction of an exit tube (see PI. I, 

 fig. 5). This is partly made from fragments of the original wall of 

 the cocoon and partly by the addition of new fragments of bark. 

 Depending upon the location of the cocoon, the exit tube varies in 

 length from one-fourth of an inch to over 1 inch. The purpose of the 

 tube must be to provide a safe exit at the critical period of the emer- 

 gence of the moth. Within the cocoon over the opening to the exit 

 is placed a thin sheath of silk which is ruptured by the pupa at the 

 time it wriggles out (see PL I, fig. 6) to give issuance to the moth. 



The transforming larvae of the first brood also make their cocoons 

 with an exit tube. The cocoons of these larva?, however, are only 

 used for a short time and are hence of a more primitive construction. 



Variation in size of wintering larvse. — In size the wintering larvae vary 

 considerably- (see PL I, fig. 7). There exists naturally a certain 

 amount of individual variation, but in addition there are climatic 

 factors which tend to increase tins variation. The wintering larvae 

 of the first brood are for the most part fully developed. There seems 

 to be a tendency for undersized larvae to transform the same season, 

 as if less fit to pass the winter. Of the second-brood larvae there are 

 always a number that fail to attain full growth in the fall, and others 

 totally fail to enter hibernation before frost sets in. Larvae para- 

 sitized by Ascogaster carpocapsse are seldom more than half grown 

 and lack the pink color of the healthy larva. 



Judging from the uniformity of head measurements, the wintering 

 larvae, though variable in size, are probably to a great extent of the 

 last or sixth instar. (See p. 78.) 



Winter-killed larvse. — In earlier studies of the codling moth the 

 writer has noted that killing due to cold occurred more or less fre- 

 quently among wintering larvse. During the spring of 1910 and 1911 

 more definite data were obtained showing a rather high percentage 

 of winter killing. Thus, of the total number of larvse from the band 

 records of 1909 (Tables III and IV), 27.6 per cent failed to develop. 

 A mortality of about 4 per cent may be ascribed to injury from the 

 handling of the insects, while the rest, 20 per cent, succumbed mainly 

 to injury from cold. Under normal conditions in orchards the per- 

 centage of larvae killed from cold is undoubtedly lower than the 

 above figures because a proportionately large number is always 

 destroyed during the whiter by woodpeckers and nuthatches and in 

 the spring, summer, and fall by predaceous insects and parasites. 



