THE WESTERN GRASS-STEM SAWELY. 9 
larvae. Occasionally an unusually vigorous plant of a slender- 
stemmed grass, like Hordeum jubatum, 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 tho 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 
