14 BULLETIN 265, U. S. DEPARTMENT OF AGRICULTURE. 



No food is taken by the larva after passing the fifth molt and 

 entering the sixth or last instar. The larva usually remains quiet 

 for a time after this molt, as after the others, and then begins to 

 search for a suitable place in which to pupate. This, as a rule, is 

 a dry, pithy stem, such as the flowering stalk of the dock or pigweed, 

 or a dry alfalfa stem. It will also bore into rotten wood or soft 

 bark, if nothing better is available, and, under laboratory conditions 

 at least, will occasionally burrow into the moist earth at the base of the 

 plant. Upon finding a suitable stem the larva at once commences 

 to burrow into it. If the stem has been cut, as may frequently be 

 the case with alfalfa, the larva burrows into the pith at the cut end 

 and soon disappears. Otherwise, it burrows directly into the side 

 of the stem, cutting out a circular hole and then hollowing out the 

 pith within the stem, working the pieces back to the opening with 

 the feet (fig. 1). 



The larva burrows an inch or more from the opening and hollows 

 out a pupal cell somewhat larger than itself. When this is accom- 

 plished the larva turns around within the cell, returns to the opening, 

 and caps it over with a regurgitated, translucent, mucilaginous sub- 

 stance, mixed with bits of pith. This substance is smeared around 

 the edge of the opening, the mouth parts and fore legs aiding in th^ 

 process, and the head is moved back and forth until the hole is closed. 

 After this the larva can be seen for a time through the cap, but as the 

 latter is thickened from within the actions of the larva become indis- 

 tinguishable. The larva then gives its attention to the pupal cell, 

 cleaning it out and often filling up the burrow behind it with pith. 

 A specially compacted mass or plug of pith is formed at the entrance 

 to the pupal cell, as an additional protection, and after completing 

 this the activities of the larva are over, and it soon becomes shrunken 

 and capable of only a slight squirming motion. 



The amount of time consumed in making this burrow and forming 

 the pupal cell naturally varies with the condition of the wood, but it 

 usually occupies six or eight hours, unless the larva bores into the 

 cut end of a stem, when the time is considerably shortened. One 

 larva under observation started to bore into the side of a stem at 

 11.15 a. m., and began to cap the opening at 4.35 p. m. The opening 

 was closed at 5 p. m., and the larva undoubtedly took some time sub- 

 sequently to complete the burrow. Another larva started its second 

 burrow at noon, after having worked three and a half hours on the 

 first, only to find the stem already occupied, and completed it by 

 6 p. m. In the breeding cages, where pithy stems were always avail- 

 able for the larvae, they almost invariably had completed their bur- 

 rows within 24 hours after the last molt. The larvae of the summer 

 generations remain in this cell six or eight days before pupating, while 

 the larvae of the last generation pass the winter here, remaining in 

 the cell six months or more. 



