THE WESTERN GRASS-STEM SAWFLY. 17 
their position, but the adults which issue are probably agile enough 
to turn about and escape. 
The duration of the pupal period is not known certainly, but is 
believed to be very brief, not more than a week at the most. After 
the first day the legs and body darken until they become a lustrous 
black within the transparent, almost invisible filmy membrane in 
which they are inclosed. This membrane is often lacking and may 
occasionally be destroyed by the movements of the pupa within the 
chamber. 
When fully mature the pupa changes within the cell to an active 
adult. This adult remains imprisoned until some unknown impulse 
compels it to force its way upward through the plug of frass placed 
at the upper end of the chamber by the larva 9 months before. The 
writer, by splitting stubs of grass or grain in June, has liberated 
adults repeatedly, which, when free, were able to take instantly to 
wing without any preliminary process of drying 
or other preparation. These adults were evi- 
dently resting, in perfect condition, waiting for 
some secret signal from the outside world before 
taking the final step for liberation. 
A very few die within the cell, possibly because 
of lack of vitality needed to break through the 
stopper of frass above them. In cases where 
the girdling of the stem was inefficiently done, 
so that the grass stalk did not break off during 
the winter season, the adult dies as a matter of 
course, since these flies apparently are not fitted 
with iaws capable of biting through the woody fig. 12. -western grass-stem 
-■ , 00 j sawfly: Pupa. Three and 
Stems Ot dry grass. one-half times natural size. 
THE ADULT 
The adult CepTius ductus is a beautiful insect with a polished 
black body marked by three prominent yellow bands across the 
abdomen. The legs are yellow and the wings smoke-colored. 
The description, by S. A. Rohwer, follows: 
Length 7 to 12 mm. Head shining, polished; anterior margin of clypeus truncate 
with angles prominent and sometimes slightly denticulate; antennae usual for the 
genus; thorax shining but with setigerous punctures on scutum; sheath nearly paral- 
lel-sided but a little broader at base, apex truncate with corners rounded; hypopy- 
gidium rather narrowly sub truncate apically. Black marked with bright lemon 
yellow, amount and extent of yellow markings varying greatly; head of female usually 
black but more rarely with face entirely yellow or having yellow spots ; head of male 
black but always with yellow on face; thorax black, the upper angle of mesepister- 
num, parapteron, and scutellum (usually) yellow; legs yellow with coxae, trochanters 
(occasionally both of these having yellow marks), bases of femora more or less, apices 
of tibiae and tarsi sometimes, black; hind tibiae and tarsi sometimes reddish yellow; 
abdomen black, spot or band on second tergite, band on third, fifth, sixth, and eighth 
