Genus ISOSOMA Walker. 
There is no longer any question as to the phytophagic habits of 
Walker's genus Isosoma, although they were questioned by E. A. Fitch 
as late as 1882. The careful observations of early American writers, 
particularly Harris, Asa Fitch, Walsh, and Riley, had fixed the status 
of Isosoma hordei as a plant-feeder beyond all perad venture, but their 
conclusions were not accepted by certain English entomologists until 
Westwood had published his careful studies on Isosoma orchidearum 
and until Wcyhenbergh had called attention to his earlier observations 
on a Dutch species. (See the writer's paper on the Biology of the 
Chalcididre, Proc. U. S. Nat. Mus., vol. xiv, p. i)$i~>.) In addition to I. 
hordei Harris, the fact of phytophagy was established by Asa Fitch for 
his I. secale^ tritici, hordei. and fulvipes. The last of these, as we have 
shown by comparison of the type specimens, is synonymous with 
Harris's I. hordei, and the third, therefore, through preoccupation of 
the name, needs a oew designation. Eiley has also proved conclusively 
that his I. tritici (here treated as I. grande) is phytophagic. Of the 
other species which we treat below, the plant-feeding habit is perhaps 
not absolutely proven in each case, but the insects are structurally 
so closely related that, taken in connection with the proof at hand, 
this habit can not be doubted. The true European Isosomas have 
now practically all been accepted as plant-feeding species. Schlech- 
tendal, in his "Die (iallbildungen der deutschen Gefiissptlanzen" 
(Jahresbericht des Vereins far Naturkunde zu Zwickau, 1890), gives 
thirteen species of this genus as true gall makers in Europe. 
Owing to the fact that an alternation of generations connected with 
a peculiar dimorphism has been established in the case of I. tritici 
Riley (= I. grande, form minutum) and 7". grande Riley, it is quite 
likely that a similar phenomenon will be found to occur with more than 
one of the following species, in the. majority of cases, however, the 
males have been associated with the females, and where species have 
been described from the females alone, these have been large and well- 
formed individuals, with no trace of degradational characters, such as 
the absence of wings, which would indicate that they are parthenoge- 
netic forms. Moreover, I. tritici Riley has a somewhat different facies 
from the other members of the genus, consisting largely in its smooth 
aspect and one or two other minor points, which practically associate 
it with Haliday's genus Philachyra. While the writer is not inclined 
to give Philachyra generic importance, and considers that its species 
should be still associated with the true Isosomas, these feeble charac- 
ters may still be associated with the phenomenon of dimorphism, and 
1 Walsh considered Fitch's species as synonymous with hordei since they were dis- 
tinguished almost entirely by the coloration of the legs. Careful study, however, 
of Fitch's types, now iu the possession of the United States Department of Agriculture, 
shows them to be distinct in other characters. 
