OENOTHERA APHIDS. 7 



here the smaller chromosome appears always to be the one to lag 

 behind. Figure 35 shows the partial resting stage between the two 

 spertnatocyte mitoses. No second spermatocytes were found in divi- 

 sion. The same side to side pairing of chromosomes as in the other 

 species was observed. 



This species has the smallest number of chromosomes of any of 

 those studied, and it apparently has the smallest amount of chroma- 

 tin, if we may judge by comparison of the metaphase of the first 

 spermatocyte in the various species. 



The Oenothera Aphids. 



As the spermatogenesis of Aphis oenotherae was fully described last 

 year (Stevens '05, pi. iv, figs. 28-42), only two new figures of the first 

 and second spermatocyte will be introduced for comparison with 

 otherspecieshavingthesamenumberofchromosom.es (figs. 36 and 

 37.) The size relation resembles quite closely that of the brown rose 

 aphid, with which it was compared in my former paper, but the largest 

 chromosome is usually more elongated and the difference between the 

 two largest is greater. (Compare figs. 3 and 36.) In external appear- 

 ance Aphis oenotherae is very different from Aphis rosae. The females 

 are a bright red and the males and their apterous mothers green . 

 This aphid was not found on the young plants at Bryn Mawr in 

 spring, or on the Oenotheras at Harpswell, Maine, or Woods Hole, 

 Massachusetts, during the summer. It may therefore have a second 

 host,* from which it migrates to Oenothera biennis in the late summer 

 or autumn. 



Another smaller, green aphid was found in abundance on the foliage 

 of the Oenotheras at Harpswell, Woods Hole, and Bryn Mawr. The 

 parthenogenetic form of this aphid is light green, the winter egg-lay- 

 ers darker with a blackish longitudinal stripe on the back, and the 

 apterous males brownish and very small. This was the only species 

 in which apterous males were observed. The equatorial plate of the 

 maturation spindles of the parthenogenetic egg, with eight chromo- 

 somes, is shown in figure 38, the metaphase of the first spermatocyte 

 in figures 39 and 40, the anaphase in figures 41 and 42, and a spermatid 

 in figure 43. 



* Aphis oenotherae has recently hatched in considerable numbers on rosettes of 

 Oenothera biennis, where the eggs were laid in the greenhouse in October. The egg 

 layers were red, but their offspring are both red and green, and individuals of either 

 color, isolated, have produced offspring of the same color in the second and third 

 generations. If they continue to breed true to color an attempt will be made to keep 

 several lines of each color isolated until the sexual generation appears in the autumn. 



