THE WESTERN GRASS-STEM SAWFLY. 



15 



If the same amount of moisture had entered the stem where the 

 larva was hibernating it probably would have caused its death. 

 This experiment, taken in connection with others that were not so 

 directly conclusive, seems to prove that the porous plug in the stub 

 must prevent in some way the admission of an undue amount of 

 moisture into the chamber below, although water readily penetrates it. 



The gallery below the plug is always entirely free from debris, 

 forming a hibernation chamber and later a pupation cell. Within 

 this chamber the larva lies with its head up and usually pressed 

 against the barrier at the top, always on the alert to retreat down- 

 ward at any sign of disturbance. It descends by alternately flexing 

 and straightening the body, bracing itself first by the jaws, then by 

 the caudal horn as it hitches its way down In ascending, the caudal 

 horn is thrust against 

 the side of the gallery 

 or the cocoon, the 

 body is straightened, 

 the j aws obtain a pur- 

 chase to hold the dis- 

 tance gained, when 

 the body is again 

 drawn up until the 

 caudal horn is applied 

 to the side wall for 

 another push. 



Late in the summer 

 or during the autumn 

 the larva spins for it- 

 self within the hiber- 

 nation chamber an 

 almost transparent 

 tube of filmy silken 

 tissue. This silk tube is sometimes several times the length of 

 the larva, is closed at both ends, and is free from the sides of the 

 chamber, so that often it can be readily withdrawn entire. When 

 first constructed this fabric is comparatively strong and pliant but 

 after some months it grows more brittle and is easily ruptured. As 

 a rule it remains intact" until the emergence of the adult. Even the 

 presence of half a score of parasitic larvae often fails to wreck the deli- 

 cate structure during the winter. 



The longevity of the sawfly larvae is remarkable and is worthy of 

 mention. September 8, 1911, a number of stubs of Elymus conden- 

 satus containing Cephus larvae were gathered and set upright in sand 

 within doors. From time to time this sand was moistened but 

 finally was allowed to stand perfectly dry. During October, 1912, 



Fig. 10.— Western grass-stem sawfly: Infested wheat stubs, enlarged 

 3 diameters, the two left-hand ones opened to show hibernating 

 larvoe in si'.u. 



