Britain of some imdescribed Aphides. 325 



Thelaxes hetulina. 



I think Mr. Darwin remarks that some suspicion 

 attaches to a genus which is represented by a single 

 species. Unless the characters differentiated rise to a 

 high significance in classification, hesitation must be in- 

 volved in the creation of such a genus ; due regard being 

 paid to the difficult and obscure subject of variation. 



The genus TJtelaxes was constructed by Prof. Westwood 

 in 1840, but some continental authors have ignored it 

 in favour of the genus Vacinia, which has been thought 

 to have priority. Vaciina was established by Van Hey- 

 den in his paper entitled, "Entomologische Beitrage," 

 published in lb37 in the second volume of the 'Museum- 

 Senkenbergianum,' pp. 289 — 291, and he there describes 

 Vacuna coccinea, which is a true Phylloxera ; as appears 

 evident from a drawing of the wing, kindly made for 

 me by Prof. Westwood ; and taken from one of the type- 

 specimens sent to him by Van Heyden himself. No 

 notice seems to be published as to the character of the 

 lower wing. Westwood says " the very minute species 

 Vacuna coccinea, Heyd., has its wings very similar to 

 those of Phylloxera of Fonscolombe, in which the cubital 

 vein is unforked; whilst in Thelaxes of Westwood the 

 cubital vein has a distinct furcation." Prof. Westwood 

 moreover thinks that his T. quercicola, which has never 

 been described, may possibly be identical, or at least 

 congenerical, with Aphis dryophylla, Schr. ; and he 

 writes, " In this particular it agrees with Thelaxes dryo- 

 phylla oi the ' Mon. British Aphides.'" He quotes his 

 former diagnosis : — " Wings flat on the back whilst at 

 rest, anterior with three oblique discoidal nervures, the 

 anterior bifid; antennae short." — See ' Introd. Mod. 

 Class, of Insects,' vol. ii., p. 41:1. From the foregoing 

 it would appear that the natural position of Thelaxes is 

 between Glyphina and Pemphigus. 



I am indebted to the courtesy and observation of the 

 Piev. E.N. Bloomfield for the opportunity of describing 

 another species of Thelaxes. At first, the discoverer of 

 this insect thought that it had some affinit}" with Gly- 

 phina. The insect inhabits the birches round Guestling, 

 near Hastings, and it congregates in clusters, not so 

 much on or under the leaves, as near the ends of the 

 shoots. Towards the middle of June he found both the 



