174 PROC. ENT. SOC. WASH., VOL. 23, NO. 8, NOVEMBER, 1921 



well known, widely distributed, very abundant, of fair size and 

 has several generations in a year. 



The data for this paper were chiefly obtained directly from the 

 study of living larvae, prepupae, pupae and adults of Pteronidea 

 ribesli. Large numbers of these insects were examined every 

 day, and the progress of metamorphosis was recorded by notes 

 and drawings. Specimens showing changes in areas or folds, 

 were preserved for confirmation of the observations and to aid 

 in the final preparation of the notes. Dissections of living 

 larvae and prepupae were occasionally made. 



The body folds of the larval skin were studied from both the 

 exterior and interior; and the arrangement and points of attach- 

 ment of all of the larger muscles were determined. The arrange- 

 ment of the areas of the larva and the newly formed prepupa 

 were found to be identical. But as there are great differences 

 between the prepupa and the pupa, and since it is in the pre- 

 pupal stage that profound changes occur, the areas of the pre- 

 pupa were followed to the pupal stage by frequent observations 

 on living specimens which had been removed from their cocoons. 

 Changes of body contour together with the relaxation of the 

 mandibles and the gradual movement of the optical centers 

 inwards and dorsad in the head served as an index to the prog- 

 ress of the prepupa in its development. The relation of the 

 sclerites of the pupa to the prepupal areas and the changes in 

 body contour caused by the formation of the pupa within the 

 prepupal skin has suggested the homologies herein described. 

 The removal of the skin of the prepupa, before the pupa had 

 developed sufficiently for shedding lent further weight to these 

 suggestions. The evidence obtained by this last method was 

 often very satisfactory, because on the sclerites of the incom- 

 plete pupa there were indications of divisions corresponding 

 with the folds and areas of the enclosing prepupal skin. 



The pupa skin seems merely a sack within which the adult is 

 developing and while in general structure it is sufficiently similar 

 to the adult, as to make unnecessary any such elaborate method 

 as is necessary to trace homologies between prepupa and pupa, 

 considerable attention was given to the development of the 

 sclerites and abdominal appendages. 



The interpretations of the body folds as thus determined in the 

 larva of Pteronidea ribesii have been successfully applied to 

 several other sawfly larvae belonging to well separated groups, 

 the principal of which are: Neodiprion lecontei (Fitch), Cimbex 

 americana Leach, Arge salicis Rohwer. 1 



'A few drawings showing the satisfactory way in which these interpretations 

 can be applied to larvae belonging to well separated groups have been included 

 in this paper. (See Plate XIV, figs. 2, 4, 7 and 8.) 



