HORTICULTURAL SOCIETY. 163 



After the third moult they show rudiments of wings, and are then 

 known as pup?e; after the fourth moult the wings are fully ac- 

 quired, a short time after which they all leave the gall and take 

 to flight. 



The galls are all empty about the first of August and some turn 

 black and dry up though remaining on the twigs through the 

 winter. 



The species has no further connection with the gall after the 

 second generation has taken to flight but continues under very 

 different conditions. What these conditions are is still a mystery. 

 Not even the late Lichtenstein who has given years of patient 

 study to this group in Europe has been able to give any satisfac- 

 tory explanation. But that many of the gall making species actually 

 leave the galls and pass a part of their existence under very dif- 

 ferent conditions has not only been put beyond reasonable doubt 

 by the observations of Lichtenstein but is also carried out by my 

 own. 



Many entomologists, though have doubted this migration of 

 aphids and actual observations are still wanting, except for the 

 hop-aphis which belongs to a high ground that do not make any 

 galls, but it has been shown by Prof. Riley to migrate from the 

 plum to the hop plant. This at least points in the same direction. 



From the great number of galls found on young poplars in the 

 citv of Minneapolis during last season millions must have taken to 

 flight after the middle of July. But although the characters of 

 this second generation is so marked that there can be no difficulty 

 to recognize it wherever found, I have never succeeded in coming 

 across a specimen outside of the gall, unless in a spiders web from 

 which no conclusions could be drawn. I have examined almost 

 every plant in a radius of several miles of this city and I am fami- 

 liar with more than a hundred species but none of which is the 

 one in question. 



Lichtenstein, from this study of a closely related species in 

 Europe, thinks that it migrates to the roots of certain plants where 

 it continues until the true sexes are produced in the fall which 

 couple and the females return to the poplar to deposit the fecunda- 

 ted eggs. Careful observations and experiments can alone estab- 

 lish this. 



Benjamin Walsh, who first described one species, also found at 

 other times two species belonging to the same genus, in nests of 

 ants, which he has described as distinct. In case that Lichten- 

 stein's supposition of migration to roots of other plants should 

 prove correct, it would, at least, be near at hand to suppose that 

 Walsh's Pemphigus formicarius is nothing but the missiDg link 

 in the life history of Peonphigus vagabundus. This will at least 

 deserve the careful attention of the aphiologists of our country. 



My observations in nature goes no further than to the flights of 

 the second generation on deserting the gall. But by confining 

 some of these on branches of the poplar, I found that they would 

 not feed., but wander restlessly about trying to escape, and a few 

 that were pressed by hunger to attempt the food, evidently found 



