12 
BULLETIN 841, U. S. DEPARTMENT OF AGRICULTURE. 
frequently found several inches above the lower end of the boring, 
moving through the solidly packed "sawdust." As the larva ap- 
proaches maturity it is doubtful if it ventures into the upper and 
slender part of the stem, but it still reworks the frass farther down, 
enlarging the bore in places. 
The number of instars is difficult to determine, owing to the larval 
practice, just referred to, of passing all the frass several times through 
the digestive apparatus. Nearly all the cast skins disappear com- 
pletely under this treatment, only the heavily chitinized parts such 
as the jaws and caudal horn being recognizable in the burrow. Care- 
Fig. 7. — Western grass-stem sawfly: Mature larvae removed from their galleries. Enlarged 4 diameters. 
ful investigation of these, fragmentary remains appears to establish 
the fact that there are four molts. The contents of innumerable 
stems have been examined with scrupulous care and with varying 
results. In a few cases as many as four sets of jaws and in others 
four caudal horns have been found, mixed with the frass within the 
stems. Seldom were more than four sets removed from a single 
stem; usually only three were found. As is stated elsewhere in this 
paper, it is no uncommon thing to discover two and even three 
larvae mining a single stem, although but a single individual can 
possibly reach maturity with the amount of nutriment contained in 
