280 Benj. D. Walsh, on the Insects inhabiting the Galls 



are true insects, and the gall-making Acariclfe (mites) which are not 

 true insects, out of the question — the gall-bearing genera of plants are 

 themselves exceedingly limited in number. Take the different genera 

 of North American trees and woody shrubs, for example, and — exclud- 

 ing all galls made by Grall-gnats and by Mites — count up all the other 

 North American galls which are met with thereupon, entirely omitting 

 all exotic galls. Celtis (Hackberry), as it will be found, has three 

 galls, all Psylladous, and two of them and probably all three produced 

 by what is probably an undescribed genus of the Homopterous family 

 Psylladee. (Proc. etc. II, pp. 461 — 2.) Ulmus (Elm) has one gall, 

 produced by Thelaxes (?)* a genus of the Homopterous family Aphi- 

 dse. Populus (Poplar) has at least three galls produced on P. angu- 

 lata by Pemphigus (^Aphidse)^ and two more, populiglobxdi Fitch and 

 populicense, Fitch, produced on P. balsamifera by the same genus; be- 

 sides two new species, evidently Aphidian, which I have found respec- 

 tively on P. treuiuloides and P. grandidentata (?), after the gall-makers 

 had deserted them — thus making in all seven galls. Hamamelis (Witch- 

 hazel), which is not found near Rock Island, 111., has, according to 



* Thelaxes ulmicola Walsh. I suspect that I have erred in referring this in- 

 sect to Westwood's genus Thelaxes, which is said to have the "anterior" one of 

 the three discoidal veins bifid. In ulmicola it is the posterior one, or what may 

 be less ambiguously termed the terminal one, that is bifid. Possibly, however, 

 '•anterior" may be a clerical error for "posterior." The European type of The- 

 laxes inhabits the Oak and not the Elm. 



f I have hitherto erroneously referred these three species to Byrsocrypta, a 

 genus founded by Haliday and apparently synonymous with Tetraneura Hartig, 

 and which differs from Byrsocrypta as limited by myself by having only one, 

 instead of two discoidals in the hind wing. I was led to separate generically 

 these gall-making Pemphigus from certain root-inhabiting Pemphigus which I 

 have described, 1st, because their antennal structure differed somewhat, and 

 2nd, because I was unwilling at that day to believe, that the same genus could 

 contain both gall-making and non-gall-making species. But, 1st, I am inform- 

 ed by Baron Osten Sacken that the European Pem})higus bursarius, which also in- 

 habits Poplar galls, has, according to Koch, antennae like those of my root-inha- 

 biting Pemphigus; and 2nd, as has been already observed, (ante, p. 237,) there 

 are several groups, both Hymenopterous and Dipterous, that contain both gall- 

 making and non-gall-making species, and there are even some groups, such as 

 Nematus and Cecidomyia, that contain both Gall-makers, Guest-gallflies, and Ex- 

 ternal Feeders.— I have described the gall of Pemphigus [byrsocrypta'] pseudobyr- 

 sa Walsh, as "entirely open below, the sides of the leaf bending down together 

 so as to touch each other and conceal the opening." (Proc. etc. I, p. 306.) This 

 18 applicable only to the mature gall, when, as is usual with Aphidian galls, it 

 opens out to allow the winged insect to escape. On May 20 the immature gall is 

 completely closed, but, as usual, with a slit below ; and at this date it contained 

 one large apterous Aphidian and a few small larvae. Hence this is a true gall, 

 and not, as I inferred, a false or spurious gall. 



