20 



BULLETIN 841, IT. 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- 

 nal vibration so common 

 among the smaller chalcids 

 and many other Hymenop- 

 tera. She gives no evidence 

 of being in search of any 



Fig. 13.-Western grass-stem particular point, but goes 



sawfly: Female oviposit- straight down the stem. 



ing. About life size. _. . _ _ . . n 



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 They are frequently partly withdrawn and their direction 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 thrust in several times, as far as they will go, then are withdrawn, 



Fig. 14.— Western grass-stem 

 sawfly: Saws, greatly en- 

 larged. 



