764 



SCIENCE. 



[N. S. Vol. XXII. No. 570. 



creases (for either or possibly both of the 

 reasons already given) until the condensation 

 takes place wholly on ions. For greater in- 

 tensities of ionization the number of ions 

 must increase further and hence the efficient 

 nucleation rises again while the curve passes 

 through a minimum. 



The curves enable us to make certain in- 

 teresting comparisons, inasmuch as the same 

 nucleation results from radium decaying for 

 a stated length of time, as results from the 

 action of radium at a certain distance from 

 the line of sight. From the importance of 

 secondary radiation in connection with these 

 observations, such comparisons are probably 

 not simple. The essential feature is the pas- 

 sage of the nucleation through the same stages 

 of variation, whether of size or of number, 

 in both cases, no matter how the given suc- 

 cessive intensities of ionization may be pro- 

 duced, or whether they come from within or 

 without. 



My thanks are due to Professor Barus for 

 his suggestions and his assistance throughout 

 the research. 



Lulu B. Joslin. 



Brown Univeesity, 

 Providence, R. I. 



A LACUSTRINE APHID. 



One would suppose that submerged aquatic 

 plants might wholly escape the attacks of 

 plant-lice and scale-insects. In the Feuille 

 des Jeunes Naturalistes, February, 1905, p. 

 62, G. Goury describes a supposed scale-insect 

 which he found on a submerged petiole of 

 Limnanthemum near Fontainbleau, in France. • 

 Unhappily, he put it in an aquarium, and the 

 pond-snails (Limnwa) devoured it during the 

 night. This prevented him from giving a 

 description, and although he names it Lecan- 

 ium limnardhemi, we can not resist the ob- 

 servation that all he says about it would 

 apply to a leech egg. However, it is possible 

 for aquatic plants to be attacked by aphids, 

 thoxTgh these do not inhabit the wholly sub- 

 merged parts. On October 7 of the present 

 year, my wife and I visited a lake in the im- 

 mediate vicinity of Boulder, Colorado. The 

 shallow water contained a large quantity of 

 Myriophyllum veriicillatum L., a submerged 



plant with only small portions projecting 

 above the surface. We were astonished to 

 find that whenever the plant was . not wholly 

 submerged it was infested by aphids, usually 

 in enormous numbers. At first I wondered 

 whether' they could have come from the ad- 

 jacent terrestrial vegetation; but an examina- 

 tion of the narrow-leafed cottonwoods (Pop- 

 ulus) and cockleburs (Xanthium) near by did 

 not reveal any aphids. Closer inspection 

 showed that the insects were thoroughly at 

 home on the Myriophyllum and were under- 

 going all their transformations thereon. We 

 brought some home, fully believing that we 

 had something new, but on looking up the 

 literature it was found impossible to distin- 

 guish them from the European (and doubtless 

 circumpolar) Bhopalosiplium nymphcece (L.), 

 which is said by Buckton to infest water-lilies 

 (to which it is at times very destructive), 

 Alisma, Butomus, Potamogeton, Hydrocliaris, 

 Lemna, etc. 



The following description, from the Colo- 

 rado material, is given because the available 

 descriptions are somewhat incomplete; it will 

 also be useful in case any doubt should arise 

 as to the absolute identity of the European 

 and American forms. 



Winged Form. — Yellowish-olive, with the 

 head, the chitinous plates of the thorax and the 

 antennae black; the middle of the abdomen 

 also suffused with black; legs black, pallid 

 only at extreme base; wings clear, stigma very 

 light-yellowish, nervures black; nectaries in- 

 crassate, with the apical part black, the basal 

 pallid; lateral edges of abdominal segments 

 with alternate light and dark spots, best seen 

 in balsam mount; antennae on frontal tuber- 

 cles ; third joint with several sensoria on outer 

 side, fifth with a sensorium in a notch not far 

 from apex; surface of joints finely imbricated. 

 Measurements : length of body about IJ mm., 

 of wing about 2^- mm. ; the rest in fx — nectaries, 

 255; Cauda narrow and fairly long, its width 

 37; anterior tarsus (excluding claws), 120; 

 antennal joints, (3) 225, (4) 165, (5) 150, 

 (6fl.) 97, (6&) 225. Wings with branched 

 vein having distance from first branch to sec- 

 ond, 620; second to tip of wing, 225. 



Apterous Form. — About 2 mm. long, broad, 



