PURE LINE INHERITANCE AND PARTHENOGENESIS. 29 



she was killed, and the head, with the antenna*, was severed 

 from the body, cleared in Gage's mixture, and mounted in 

 balsam on a microscope slide. This method I found to be the 

 only satisfactory one, for two reasons. First, measurements of 

 the segments of the antennae could be made accurately; and 

 second, the fecundity of the individuals could be tested before 

 the selection was made, a very important factor, since it fre- 

 quently happens that some adults have very few or no offspring. 



Having made the measurements of the antennal segments, 

 and obtained the index for the same, I selected from each fra- 

 ternity the individual or individuals desired. These were usually 

 those whose antennal index was either the lowest or the highest 

 in the fraternity, provided they had left a sufficient number of 

 offspring. Now the offspring of the individuals selected were 

 separated and placed on individual plants; those of the other 

 individuals of the fraternity were discarded. 



In making the measurements of the segments of the antennas 

 a mechanical eye micrometer was used. Both antennae were 

 measured, and an average obtained from the two measurements 

 in order to get the index for the individual. From these indices 

 ratios were obtained, segment four being taken as unity. 



Having obtained the ratio of the length of segment three in 

 each case to the length of segment four, I have plotted (Fig. i) 

 the results of the selections for ten generations. For the first 

 six generations random selections were made in order to be 

 sure of a pure strain and to determine the average mean index 

 for this strain. The mean thus obtained gave a ratio of 1.80:1, 

 that is, segment three of the antennae was found to be one and 

 eight tenths times as long as segment four. This ratio estab- 

 lished for the pure line is represented by the heavy horizontal 

 line running through about the middle of the plot in Fig. I. 



RESULTS. 



On April 9 the first individual was isolated, a wingless asexual 

 female, and she became the mother from which all the succeeding 

 individuals were descended. This stem mother gave birth to 

 twenty-nine offspring. She was now killed and her antennal 

 index taken. It was found to be 1.77:1. From the twenty- 



