10 



and alternation of generations by him in the case of Tsosoma tritici 

 Riley, as it was then known, and /. grande was without a parallel, in 

 this genus, and so remains in this country. Among the ten or twelve 

 American species that I have reared, none of the others, so far as I 

 have been able to determine, enter the pupal stage in the fall and 

 winter in that condition, « and thus the greater wheat straw-worm {Iso- 

 soma grande) is one stage in advance of the others in spring, and 

 the spring form, rninuta, is developed at the time when other species 

 are entering the pupal stage. This is also the only species that I have 

 not succeeded in rearing from food plants other than wheat, with the 

 possible exception of hosoma loehsterl^ which might have been reared 

 from young cheat plants, though I hardly think this probable. The 

 fact that 1 have only found this latter species in spring, and then only 

 females, is indicative of a dimorphism and alternation of generations; 

 but unless it be an undescribed species reared from stems of Trknspis 

 sederoides^ which is very late to mature, being even later than any 

 other species known to me, I do not think such alternation can be 

 connected with any other species that I have studied. On the other 

 hand, and at the other extreme in the matter of food plants, the 

 Elymus Isosoma (Z eJymi French), has never been with certainty 

 reared from wheat, though abundantly from the stems of cheat grow- 

 ing among wheat and from Elymus growing along the margins of 

 wheat fields. 



I also find, much to my surprise, that I have reared Fitch's Isosoma 

 tritioi aside from its known food plant, wheat, only from Elyiims vir- 

 ginicus. Even where this latter grass and the closely allied E. cana- 

 densis have grown side b}^ side, the joint worm {Isosoma tritici 

 Fitch) has held strictly to the former. The white-spotted Isosoma 

 {I. aJhomacidata Ashmead), perhaps the most closely allied to I. grande 

 of any of the species known to me, and which we should suppose 

 would more than any other incline to dimorphism and alternation of 

 generations, seems, however, to show no such tendency, and, more- 

 over, I have reared it from both cheat and Elymus virginicus^ the life 

 cycle, so far as I have been able to follow it, being parallel with those 

 of Isosoma elymi, I. tritici, and /. hordei. I do not, of course, wish 

 to obscure the possibility of an alternation of generations among these 

 insects, with a diflerent food plant for each generation. On the 

 opposite page is given in tabulated form the food plants of the spe- 

 cies of Isosoma known to attack grains and grasses in North America. 



« Should the observations of Dr. Andrew Nichols, given under Isosoma hordei, 

 prove correct, this may in future prove erroneous as to I. grande, unless the latter 

 also attacks barlej'. — F. M. W. 



