82 BRITISH APHIDES. 



Before Passerini's time the number of ascertained 

 species was not large, but he increased the Hst of 

 genera, and described, in 1863, twenty-one species 

 inhabiting Italy alone. 



Until quite recently our knowledge of the life-history 

 of this group was very incomplete, but within these 

 last ten years, through the labours of Lichtensteiu, 

 Riley, Low, Monell, Courchet, and Kessler, a large 

 mass of information has been accumulated, memoirs 

 have been written, and excellent drawings made. For 

 those who wish to follow the metamorphoses of these 

 insects closely, I have, at the end of the diagnosis of 

 species, added a Hst of memoirs in the order of their 

 appearance. Where so many have worked, it is not 

 easy to decide upon priority in discovery, and indeed 

 many important points have been discovered simul- 

 taneously in different countries. 



The remarkable phases assumed by Phylloxera, 

 first made known by Planchon and Lichtensteiu, 

 have suggested a somewhat similar economy in these 

 higher groups. Yery little was known of the mode of 

 existence of the wiuged forms of the ScMzoneurince, 

 which issue by thousands from the galls above alluded 

 to ; and nothing was known of the egg, or the true 

 sexes, or whether hibernation took place ; or if the 

 constructor of the gall survived the winter, or was 

 hatched in spring from the presumed egg. A great 

 deal of this uncertainty has now been cleared away. 

 Professor Charles Riley, in America, has proved that 

 all the elm-inhabiting Pempliigince west of the Missis- 

 sippi give rise to perfect sexes in the autumn, and that 

 the impregnated egg is consigned by the female to some 

 sheltered portion of the trunk, where it rests secure 

 till the following spring. The issue from this egg con- 

 stitutes the stem mother (the Pseudogyne fundatrice) of 

 Lichtensteiu, and she is the constructor of the gall, just 

 as a Schizoneura mother commences the rolhug of the 

 leaf as a protection to her future progeny. 



Professor Riley claims to have made the discovery in 



