Februarj^, '17] PHILLIPS: ISOSOMA INVESTIGATIONS 143 



infesting the crop becomes seriously threatened. Heretofore it has 

 been generally accepted as a fact that the different species of Isosoma 

 infesting the small grains could and did infest the wild grasses. All 

 evidence now shows that if a species of Isosoma is to maintain its 

 existence, it must be supplied continually with its own particular host. 

 Therefore, when planning control measures in the future for a parti- 

 cular grain-infesting Isosoma, we do not need to concern ourselyes 

 about the presence or absence of any other grain or grass crop or any 

 of the wild grasses. 



Oviposition of Isosoma is an interesting operation from many points 

 of view. In the first place, the position the insect assumes in relation 

 to the stem at the time of oviposition clearly indicates whether it is a 

 gall-forming species or one that inhabits the center of the stem. The 

 gall-forming species always takes a position facing downward (See 

 PI. 9, figs. 5 and 6) while those inhabiting the center of the stem 

 always face upward when ovipositing. /. tritici, a gall-forming species, 

 inserts its ovipositor almost parallel to the long axis of the plant stem 

 instead of perpendicular as would appear from external observations. 

 Three to five minutes are required to deposit one egg by this species. 

 The ovipositor is then almost withdrawn and reinserted' in a different 

 channel. This performance may be repeated until seven or eight eggs 

 have been deposited without the ovipositor ever having been entirely 

 withdrawn from the original point of entrance, and the entire opera- 

 tion may require as long as thirty to forty minutes, or even longer. 

 The individual may then proceed to the opposite side of the stem and 

 insert several more eggs. Occasionally there may be two individuals 

 ovipositing within a quarter of an inch of each other. As compared 

 with the gall-forming species, /. grande, form grande, a species occupy- 

 ing the center of the stem, inserts its ovipositor almost at right angles 

 to the stem, the egg often being placed within the cavity. This species 

 requires three to six minutes to complete oviposition, then with- 

 draws her ovipositor entirely and usually goes to another internode. 

 Grande, however, does not insert more than one egg at a point, but 

 occasionally two may be inserted in the same internode about a quar- 

 ter of an inch apart, in which case one larva proceeds to destroy the 

 other. 



Isosoma tritici, when ovipositing, forces the ovipositor between the 

 fibrovascular bundles, and the egg, which has a long petiole and a 

 very flexible chorion, is then forced through the ovipositor in the 

 form of a long thread. As soon as the point of the egg reaches the 

 enlarged cavity made by the ovipositor its contents flow to that point 

 and the egg soon begins to assume more nearly its normal shape. The 

 petiole is then apparently gradually forced in, as the ovipositor is 



