282 Dr. H. Adler’s Researches on 
of gall-flies the males are much less numerous than the 
females ; among certain of these Hymenoptera they appeared 
even to be altogether absent. Several hypotheses had been 
put forward to account for these anomalies. It is useless to 
recall them here, as they have disappeared before the results 
of observation. It was eight years ago that the first true 
notion of the phenomena of the reproduction of these insects 
was introduced to science. In 1873 Mr. Bassett pub- 
lished in the ‘Canadian Entomologist’ some observations 
upon gall-flies, amongst which was one of great importance. 
He demonstrated that one species living on Quercus bicolor 
produced upon the peduncle of the leaf and upon its median 
nervure swellings whence emerged, in the month of June, 
insects among which the male sex and the female sex were 
represented in equal numbers. Galls of another form appeared 
at the end of summer upon the extremities of the young 
branches ; the insects which were developed in their interior 
and hibernated there were all females, differing only from 
those of the preceding generation in being slightly larger. 
Mr. Bassett concluded from these facts that the two genera- 
tions proceeded one from the other, and succeeded one another 
in the course of a year. Although based on a bare supposi- 
tion and wanting in direct proofs, this theory has been verified 
by the observations of M. Adler of Schleswig, who, in 1875, 
began his numerous and persevering researches without being 
acquainted with the opinions of the Canadian entomologist. 
The investigations of M. Adler extended to all the species 
of oak-gall-flies which he has been able to observe in his native 
country, North Germany. He took the greatest pains to 
follow the various phases of development of each species, and 
the successive forms in which it appears. ‘The description of 
the methods employed in the rearimg inspires absolute con- 
fidence in the results. . 
M. Adler divides the Cynipide of the oak into four groups, 
which include all the species observed by him, viz. :— 
I. Group Neuwroterus ; 
II. Group Aphilotriz ; 
III. Group Dryophanta ; 
IV. Group Biorhiza. 
The insects belonging to the first of these groups may furnish 
us with an instance of the singular phenomena of reproduc- 
tion discovered by M. Adler. 
Neuroterus lenticularis produces, on the under surface of the 
