16 



upper joint. A large number of these adult females were secured, and 

 these constituted the types upon which the description of Isosoma 

 grande was based/' 



DISCOVERY OF DIMORPHISM AND ALTERNATION OF GENERATIONS. 



At harvest I arranged with the owner of the Held near Oxford to 

 allow a small area where I had witnessed the oviposition of the female 

 Isosomas to remain uncut, and I afterwards secured these straws, a part 

 being kept out of doors and the remainder kept within doors during 

 the following winter. Some conception of the extent to which these 

 straws were tenanted b}^ the larva? of this species ma}^ be gained by the 

 fact that of 90 straws from the same field 81 were infested and con- 

 tained 136 larva?. These straws were cut close to the ground, and, 

 therefore, the contained larva^ represented the total number. Of 90 

 straws as cut by the harvester, there were a far less number of larvse 

 present, only 25 being found in the entire lot, the remainder having 

 been left in the stubble. 



By October all of the larvte had pupated, and my first adult was 

 obtained December 7 from the lot of straws kept indoors. From 

 this time on till June I continued to secure adults issuing from these 

 straws, but everyone of them were Imsoma trltic! Riley. All of the 

 straws were now split open in order to determine whether or not any 

 individuals still remained, but none were found. 



My first adult from the straws kept out of doors appeared March 

 23, and others continued to appear up to the first week in April, all, as 

 with the straws kept indoors, being Tsosoma triticl Riley. These 

 straws were now split open and examined, but there was no trace of 

 Isosoma grande, which I knew had deposited eggs in these ver}'^ straws. 

 Despite all this, on June 1, in sweeping the grass along the borders 

 of a wheat field at Lafayette, Ind., only about 20 miles from where 

 I had found them the previous year, I captured Isosoma grande, and 

 on the following day found them present in the wheat fields. 



During the fall of 188.5 I took the precaution to sow a small plat of 

 wheat and so protect it that no insects could reach it. The cover was 

 renewed in spring, and some of the Isosoma tr'tticl emerging from 

 straws taken from the field the previous summer were placed in 

 the inclosure where the young protected wheat plants were grow- 

 ing. The adults were placed on this young wheat April 12, and the 

 utmost care taken to prevent any other insects from reaching them, 



« The records and material in the files and collections of the State Laboratory of 

 Natural History show that what is probably the larvee of this species Avas found in 

 abundance in wheat straw in the fields in southern Illinois, in July, 1884, and adults 

 of the summer form {grande) were collected by Mr. Garman, at that time an assistant 

 of Dr. Forbes, in various localities in southern Illinois, during late May and early 

 June, 1884, or just about the time that I began to observe it about Oxford, Ind. 



