1922] Uichanco — 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 



