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 wholly 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 larvae 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 dry 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. 



