1909] Toxoptera graminum and its Parasites 75 



extends southward beyond the boundaries of the United States. 

 So far we have not found the sexual forms south of southern Kan- 

 sas, about latitude 37°, but north of this latitude it can certainly 

 pass the winter ordinarily in the egg state and in mild winters as 

 viviparous 9 $ either wholly or in part developed. South of 

 Lat. 37° there really seems to be no vital necessity for sexual 9 9 

 in order to perpetuate the species or to enable it to pass the winter. 

 Indeed, the greatest need would seem to be for some means of 

 bridging over the dry period, during w^hich there is a serious lack 

 of food and in a temperature far too high to induce torpidity thus 

 rendering food unnecessary. 



DEVELOPMENT DURING SUMMER. 



Beginning July 8, 1907, Mr. W. J. Phillips, at Richmond, Ind., 

 began a study of the continuous generation of this species, follow- 

 ing it through to December 10, besides securing sexual 9 9 and 

 eggs therefrom on blue-grass in the fields in October. With these 

 eggs, he began March 27, 1908, to again follow out continuous 

 generations, commencing with the first stem mother that hatched 

 from eggs deposited the previous autumn. From the wintered 

 over egg to the oviparous 9 , or from one egg stage to the other, 

 the maximum was twenty-one generations of viviparous 9 9 

 when the first bom were used in each case, but only ten genera- 

 tions where the latest born was used in a parallel experiment. 

 The experiment ended December i, 1908. It is not unlikely that 

 there are a greater number of generations produced in the south, 

 where the warm period is more prolonged, though in the north 

 this period while shorter is not usually so excessively dry. 



Ordinarily, the viviparous 9 will reach full development and 

 herself begin reproduction about eight days from birth, but in late 

 autumn this period becomes very irregular. In one case Mr. 

 Phillips had a viviparous 9 under observation, under outside 

 conditions from the day of her birth October 18 to December 10. 

 Though apparently fully developed she produced no young and 

 w^ent into winter in a most vigorous condition. It will be noted 

 that it was just such as these that survived the freezing experiment 

 previously described. The young, when born, are enveloped in 

 a membraneous sack, from which they disengage themselves 

 almost the instant they leave the mother. A viviparous 9 may 

 produce as many as 70 young scattering them throughout a period 



