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 FlG - 12 - ~ Western grass-stem 



, -, ■ • sawfly: Pupa. Three and 



Stems Ol dry grass. one-half times natural size. 



THE ADULT 



The adult Cephus 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; antennse 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 subtruncate 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 tijise 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 



