Manchester Memoirs, Vol. lix. (191 5), No. 9. 3 



and it is of hardly less interest from the economic. We 

 know, for instance, in a well-studied case such as Aphis 

 avence, one of the common apple aphids, that, once the 

 winged forms have left our apple trees in the spring, we 

 shall not be further troubled with this species until its 

 return from the graminaceous secondary host in the late 

 summer or autumn. No parallel race appears to remain 

 on the apple. But in cases such as Macrosiphum rosce on 

 roses, and Aphis rumicis on beans and many other garden 

 plants, we must expect that though winged forms are 

 developed, which carry over the infection to fresh plants, 

 unless some unexpected tragedy overwhelms them, there 

 will remain a sufficiency of apterous individuals to main- 

 tain the strain on the plants under observation. 



In the two cases to which I wish to draw the attention 

 of the Society to-night, the production of parallel lines has 

 been made known before in each instance, but I venture 

 to think that the confirmatory evidence, which I have 

 been able to obtain, may be of some interest to an English 

 society, especially so since one of the species, Hamameliste^ 

 tullgreni, has not previously (so far as I know) beeh 

 known to occur in this country. 



Both species belong to the sub-family Pemphigince, 

 Mordw. 



(i.) Thecabius affinis, Kalt 



The migration of this species from poplar to Ranuncu- 

 lus was made known by Mordwilko, who thus established 

 the identity of what had previously been known as 

 Pemphigus affinis, Kalt., with Pemphigus ranunculi, Kalt. 

 The stem-mother hatches from an ovum laid on poplar 

 and produces a " gall " at the edge of a leaf. Within this 

 gall the second generation are produced and in due time 

 develop wings. These are the migrant es which fly from 



