1922 ] Uichcinco — Notes on Parthenogenetic Macrosiphum 73 
cases have been found to be identical: The aphids sooner or 
later return to their former position, that is, with the abdomen 
uppermost and the head directed toward the ground. The degree 
of ability to orient itself in this manner apparently differs with 
the age and morphological characters of the individual. The 
alate adults turn around almost instantly after their former 
position is reversed. The nymphs in the first instar are the 
slowest to respond to the treatment. The later nymphal instars 
of the future alate and apterous individuals and the apterous 
adults do not react at once, as in the case of the winged adults, 
although the repsonse takes place much more quickly than in 
the first-instar nymphs. 
The following interpretations are offered for the foregoing- 
behavior: the tansy aphid, like the parthenogenetic forms of the 
other species of this family, feeds practically all the time. It is 
interrupted in this activity only when it changes its feeding 
location, after the supply of food material in a given part of the 
plant becomes temporarily used up. In a healthy plant, where 
there is an abundance of succulent tissue at the growing region, 
this change in location takes but a short time. The feeding oper- 
ations, of course, require that the aphid remain stationary in one 
poistion during the process, and, consequently, it has to stand 
still almost all the time. Now, under these conditions, prolonged 
exposure to the direct rays of the sun, especially at midday, 
when they are very intense, is undoubtedly uncomfortable, if 
not ruinous, to the eyes of the aphids. The tansy plants generally 
grow in unsheltered places and the growing tips on which the 
aphids feed are fully exposed to the sun. The habitual position 
of Macrosiphum tanaceti is probably an adaptation to that en- 
vironmental condition. By locating itself on the plant in such a 
way that the aphid’s head is directed downward, the rays of 
bright sunlight from above do not strike the eyes directly. With 
this view in mind, however, it is difficult to find an explanation 
for the maintainance of the same behavior by the insect in the 
morning, when the rays of light are not so intense, and in the 
evening when it is almost dark. One probability is that the 
heliophobic reaction of the aphid has brought about secondary 
