20 
BULLETIN 841, U. S. DEPARTMENT OF AGRICULTURE. 
When she has made her selection of a suitable stem, the female 
usually alights about half way up and runs briskly to the upper end, 
halting almost imperceptibly every few steps. The gait of an ascend- 
ing fly is so characteristic that it determines with much certainty if 
the individual is a female intent on oviposition. 
Arriving at the apex of the stem, after a care- 
ful survey of its condition, she frequently makes 
an elaborate toilet, preening herself most care- 
fully, until she is in perfect condition. She then 
descends, exaggerating slightly the hesitating 
step by which she had ascended. The antennae 
are held horizontally in front of the head as she 
moves, and she occasionally 
touches the surface of the 
stem with their tips. There 
is none of the rapid anten- 
na! vibration so common 
among the smaller chalcids 
and many other Hymenop- 
tera. She gives no evidence 
of being in search of any 
particular point, but goes 
straight down the stem. 
When satisfied that she has 
gone far enough she halts abruptly, usually an 
inch or less above the second node from the top 
of the stem, slowly arches her abdomen and 
clasping her hind pair of feet around the stem 
as far as they will reach begins to drive the 
saws into the hard outer tissue. Figure 13 
shows the attitude taken at this time. These 
saws are exquisitely fashioned, curved like a 
scimitar, double, very thin with serrated edges. 
(See fig. 14.) They are used to split the outer 
coating of the stem rather than to cut it, and 
they make an opening so exceedingly small that 
it is almost impossible to find the scar after the 
wound has healed. These saws are gradually 
forced into the stem, the operation occupying a 
minute or more. In the field the female always 
heads downward during oviposition and the curve of the saw blades 
brings them, when fully inserted, in a line parallel to the axis of the 
stem. They are frequently partly withdrawn and then* direction 
slightly changed. When the stem is in proper condition the saws are 
thrust in several times, as far as they will go, then are withdrawn, 
Tig. 13.— Western grass-stem 
sa'wfly: Female ovoosit- 
ing. About life size. 
Western grass-stem 
Saws, greatly en- 
