PARTHENOGENETIC PURE LINE OF APHIS AVEN^ FAB. 57 



and at the same time suitable in other respects for use in further 

 selection work. It was noticed that the cornicles (honey tubes) 

 were quite variable both in size and shape. Also it was observed 

 that they were fairly well chitinized, and that they did not ap- 

 preciably shrink while being passed through killing and clearing 

 fluids in preparation for mounting on microscope slides and further 

 that when mounted on microscope slides they could easily be 

 measured. At first I thought that I would take the ratio exist- 

 ing between the length of the cornicles and the length of the body 

 as a suitable character. Then it was noticed that there was some 

 shrinkage of the body due to the action of the killing and clearing 

 fluids, which, at the time, I feared would cause an introduction of 

 serious errors in measurements. For this reason I finally decided 

 to make selections for the purpose of attempting to increase, 

 not relatively, but absolutely, the length of the cornicles. Such 

 selections, it is observed, should not only tend to increase the 

 length of the cornicles in relation to the length of the body, but 

 also to increase the size of the individuals in the line. In other 

 words, two distinct characters are involved in the consideration 

 of variations in the absolute cornicle-length. 



Before going into the details of the selections made in this 

 isolation (isolation No. 3) it is probably best that a few words 

 as to methods be given. In all cases the individuals of a frater- 

 nity were treated alike, they were placed in the same killing and 

 clearing fluids, and each individual was kept in each fluid about 

 the same length of time. The specimens were all mounted in 

 balsam on microscope slides, and measurements were made with 

 an ocular micrometer. In each instance the individual whose 

 cornicles were to be measured was allowed to produce its progeny 

 before being killed. If it was the one which was to be selected for 

 carrying on the strain its progeny were saved, isolated while 

 yet immature, and reared until they each gave individuals of 

 the next generation. This method of allowing each individual 

 to rear its progeny before being killed (in order to examine the 

 characters) being the one used throughout the experiments here 

 reported. I find that it is one that commends itself for several 

 reasons, among them the following: 



(a) Characters can be much better observed, and measured 



