THE WESTERN GRASS-STEM SAWFLY. 9 



larvae. Occasionally an unusually vigorous plant of a slender- 

 stemmed grass, like Hordeum juhatum, affords stalks with, diameter 

 sufficiently great to be attacked by Cephus. 



* Small grains, such as wheat and rye, readily serve as hosts to this 

 insect, because they are of suitable size and the length of their grow- 

 ing season coincides with the growth of the larva. Even if harvest time 

 should happen to come before the maturity of the larva, the reaping 

 machine probably would sever the stem far enough above ground 

 to leave the larva below the sickle cut, where it could house itself 

 safely before the end of the season. 



Judging the future by the recent past, it seems probable that this 

 fly, before another decade is past, will be found attacking practically 

 all of our native and cultivated grasses and most of our grains. 



It must be remarked in this connection that up to the present time 

 this species has confined itself entirely to the West and has been 

 found in only a few localities east of the Mississippi River. Its choice 

 of wheat for food has taken place, so far as known, only in North 

 Dakota and western Canada, although it is probable that Montana 

 wheat fields have been invaded. From present appearances its 

 attacks probably will be confined to vegetation growing within the 

 area where spring wheat is sown. 



THE EGG 



The egg of Cephus ductus is, when newly laid, decidedly crescent- 

 shaped, glassy in appearance, milky-white in color, usually quite 

 symmetrical, the ends of the crescent tapering and rounded. It is 

 marked by very faint, short, longitudinal lines or wrinkles, placed 

 without regard to order or pattern. 



The size of the egg varies with the size of the female that pro- 

 duced it and measures from 1 mm. to 1.25 mm. in length. The 

 greatest breadth is about one-third the length. 



The covering membrane is hyaline and transparent. Although 

 very thin and delicate it is sufficiently strong so that the egg may be 

 safely lifted and moved by the aid of a fine brush. The egg always 

 lies free within the stem of the host plant, either in the stem cavity 

 or in a hollow excavated by the ovipositor of the female that placed 

 it. This cell is always a little larger than the egg, so that it is com- 

 paratively an easy matter to remove the egg to a moist cell or else- 

 where for study. 



The number of eggs distributed by each female appears to vary 

 but little. Dissections of a number of adults taken in the field and 

 of others reared in captivity agree in most cases in giving a count of 

 about 50 eggs in the ovaries, these eggs being, as a rule, equal in 

 size and apparent maturity. 

 150056°— 20— Bull. 841 2 



