14 
BULLETIN 841, U. S. DEPARTMENT OF AGRICULTURE. 
bore just below the groove and in this manner plugs the upper end 
of the stub that is to be left in the ground after the upper stalk has 
been broken away (figs. 9, 10, and 11). This dry frass in some manner 
is packed firmly into its place, perhaps by means of pressure rather 
than by being cemented with a liquid furnished by the larva, since 
the plug is readily penetrated by moisture. This is somewhat 
remarkable in view of the fact that an undue amount of moisture 
appears to have a disastrous effect upon the mature larva. One 
would suppose that these stubs, often whoUy submerged in water- 
soaked earth for weeks at a time, would absorb, during the long 
period of hibernation, a fatal amount of dampness from the rain or 
melting snow. 
But there is no evidence that this ever happens. 
Fig. 9. — Western grass-stem sawfly: Infested wheat stubs from Bottineau County, N. Dak. 
September 16, 1911, one of the larvse was removed from the hiber- 
nation chamber and placed in a small vial, still inclosed within the 
silken tube or cocoon, which was unbroken. For months this larva 
remained passive and motionless except when the vial was exposed 
to bright sunshine. Because of the light or heat, or both, when 
placed in the sunlight it would become active at once, and travel up 
and down within its cocoon in its efforts to escape. January 20, 
1912, to prevent the air in the vial from becoming too diy a small 
drop of water was introduced and the vial again corked tightly. An 
hour later it was noticed that the silk tube had collapsed and the 
larva within was limp and apparently dying. The surplus moisture 
was removed quickly, whereupon the larva revived almost at once. 
