1891.] 2G9 



He regarded this emigration ratbcr as the rule than the exception 

 in the genera comprised in this section of the family. Moreover, in 

 a letter written to me in 1883, and subsequently published, he states 

 his conviction that emigration may take place from one plant or tree, 

 to another plant or tree of an entirely different botanical family, and 

 he states, amongst other examples, that the winged forms of Tetraneura 

 uhni hibernate on Triticum I'epens. 



Passerini has shown that the immature forms of Peinphigus Boyeri 

 {Aphis radicum of Fonscolombe) feed below ground on the roots of 

 various plants, such as Zea, Sorghum, Loliuvi, &c. M. Lichtenstein 

 considered that the period of two years is necessary to complete the 

 evolution of Pemphigus lursarius. The galls made by this Aphid are 

 formed on the petioles of the leaves of the Lombardy and the black 

 poplar tree. They are pear-shaped, and when ripe show reddish 

 apertures at their summits, out of which, from late in May to the 

 end of July, the winged insects emerge, and then take their flights. 



M. Lichtenstein, in conjunction with M. Courchet, placed a 

 number of these insects with growing grasses under glass bells to note 

 if they made any descent to the earth round the roots ; but up to 1881 

 no direct proof could be adduced that such were the hibernating habits 

 of this species. Oviposition of some PemphigincB has been proved to 

 take place under the bark of trees by Messrs. Eiley and Mouell ; but 

 they thought it probable that certain species might be subterranean 

 and root-feeders during some portion of their existence. (See " Notes 

 on Aphidid?e of U. S. America, Washington, 1879"). 



One requisite of a scientific thinker consists in being a good 

 sceptic or doubter ; that is to say, although the mind must be receptive 

 of all fair deduction from undoubted premises, theory and hypothesis 

 must continue in abeyance until such premises rest unquestioned. 



In 1881, 1 hesitated to accept as proved some of M. Lichtenstein's 

 deductions, and I preferred to take up a neutral position as to the 

 question of emigration of Aphides, such as that of Tetraneura, from 

 the elm tree to the roots of grasses. 



Through the kindness of Mr. Mosley, of Huddersfield, I am able 

 to add another item to the proof that emigrating forms of an aerial 

 species feeding on leaves of a forest-tree may retire to the roots of 

 low herbs, with a view to hibernate ; and that they again issue in the 

 spring, previous to the development of the perfect sexes. 



On the 13th July of the present year, Mr. Mosley, entomolo- 

 gizing near a wood in the neighbourhood of Huddersfield, saw some 

 plants of the common buttercup {Ranunculus rcpcns), quite white 



