MILKWEED AND OAK APHIDS. 9 



daughter plate of the first spermatocyte. Figures 63 and 64 are slightly 

 different stages in the pairing of the chromosomes as a prophase of the 

 first maturation mitosis. In the second spermatocyte the two smallest 

 chromosomes are as widely separated as any other two (fig. 65). One 

 individual in this material had nine instead of seven chromosomes in 

 the spermatocytes, the two smallest closely associated in the center 

 of the plate and seven in the outer circle. This was probably a stray 

 specimen of another species, perhaps closely related, for here the 

 arrangement of the chromosomes seems to be as characteristic of the 

 species as their form and size. One day's collection of this material 

 contained several individuals which, on dissecting, showed both em- 

 bryos and winter eggs from the same ovaries, thus confirming what I 

 found once among my rose aphids in the greenhouse; but the fact 

 that I collected and examined a great deal of the same kind of mate- 

 rial from the same locality on the following days, and found no more 

 cases of this kind, shows it to be of rare occurrence. 



In the black and the orange milkweed aphids we have two conspic- 

 uously different aphids with chromosomes of the same number and 

 not markedly different size relations. A similar looking aphid to the 

 black milkweed aphid was found on the garden nasturtium, and here 

 the number and size of the chromosomes is near enough the same to 

 make it probable that they are the same species. Figures 66, 67, and 

 68 show the prophase and metaphase of the first spermatocyte of the 

 nasturtium aphid. 



The Oak Aphids. 



On the red oak were found some peculiarly marked red and yellow 

 aphids, the males having spotted wings. These have seven chromo- 

 somes in the spermatocytes (fig. 69). Figure 70 is a daughter plate 

 of a first spermatocyte, figure 71 an equatorial plate, and figure 72 a 

 daughter plate of a second spermatocyte. One specimen in this 

 material had eight chromosomes, the arrangement being similar to 

 that in the pale milkweed aphids, but the number different (figs. 73 

 and 74). 



On another species of oak was found a white aphid with seven chro- 

 mosomes, varying considerably in size from those of the red oak 

 aphid. First spermatocytes are shown in figures 75 to 78. These figures 

 show no uniformity of grouping. In fact, the only point of uniformity 

 so far discovered in any species is the close association of the same 

 two chromosomes in the first spermatocytes of the pale milkweed 

 aphids and the one stray red oak aphid (figs. 58 and 73). 



