RESULTS OF CROSSING. 323 



spot in the hybrids is due to the female variolarius and therefore 

 the results from this cross are less decisive, on this point, than 

 the results from the cross with servus. 



The intromittent organ, however, is a chitinous spiral which 

 can be dissected out and accurately measured, making it possible 

 to determine the exact size relations in the hybrid. We shall, 

 therefore, first describe the results obtained from a study of the 

 inheritance of this organ and shall then more briefly state the 

 less definite results obtained from a study of the genital spot. 



Dr. Eltringham's discovery of the difference in the length of 

 the intromittent organ between the two species E. variolarius 

 and E. servus (Foot and Strobell, '15) led us to expect a differ- 

 ence in the length of this organ between the two species E. vari- 

 olarius and E. ictericus. The three species do, in fact, differ 

 greatly in this character. At a magnification of twenty diam- 

 eters, the mean length of the intromittent organ of E. servus 

 is 166.41 mm. of E. variolarius, 96.70 mm. and of E. ictericus, 

 60.64 mm. 



Our cross-breeding experiments with variolarius by ictericus 

 differ in important details from our experiments with E. vario- 

 larius by E. servus. In the latter we succeeded in raising to 

 maturity, eleven males and 16 females of the FI generation, 

 and from these we were able to raise a large number of the F 2 

 generation (204 males and 249 females). In the variolarius X 

 ictericus cross, on the contrary, we succeeded in raising only one 

 specimen of the FI generation (a female). We were successful, 

 however, in crossing this FI female with a pure ictericus male 

 and we raised from this back-cross 76 males and 73 females. 

 These results are a gratifying supplement to our cross-breeding 

 experiments with variolarius and servus, for in the latter, a back- 

 cross was commenced so late in the season that we were able to 

 secure only 18 males and 8 females. 



Details of Crossing Euschistus variolarius X Euschistus icteri- 

 cus. The three female variolarius used for this cross were 

 hatched in our laboratory, and immediately after the fifth moult 

 were caged with three male ictericus which were also hatched 

 and reared in the laboratory. The parents of the three females 

 were two of ten specimens of variolarius collected at White 



