PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
129 
sects emerge in winter or in early spring. There are, it 
is true, a few that hardly can be said to fall properly into 
either of these groups, but the exceptions are so few that 
they may be left out of view for the present. I am not 
aware of any case in which two broods of a gall are de¬ 
veloped in the same summer. The insects reared from all 
the galls described above are sufficiently different in ap¬ 
pearance and in structure to be recognisable with certainty 
by an adept; and even to allow of their being divided into 
genera by means of certain structural characters, for the 
most part dependent on differences in the structure of the 
ovipositor and adjacent parts, though extending to other 
parts also, as, for example, to the development or function¬ 
less state, or even the absence, of the wings. It will have 
been observed that the insects reared from many of the galls 
are known to be of both sexes in the same brood, but that 
in the case of many of the galls, insects of one kind only 
have been reared, belonging to the form that I have 
throughout referred to as asexual, i.e., capable of laying 
eggs without impregnation, from which the next genera¬ 
tion are developed in the usual course of metamorphosis. 
The question arises at once whether it is probable that 
this is the constant condition of any species of insect, inas¬ 
much as it may be regarded as a law seldom, if ever, 
broken, that, in the development of organisms of so high a 
type as insects, a continued process of reproduction with¬ 
out a possibility of, at least occasional, crossing, is un¬ 
known. This is one of the problems to be solved. 
Another problem at once meets us, when the matter is 
looked at from the side of the galls, and that is this. We 
have already seen that many of the galls are developed only 
early in spring or in summer; and that the insects emerge 
not later than July. There is no reason to suppose that 
the imagos continue to exist for any length of time, cer¬ 
tainly not till the following spring. How, then, are the 
galls produced at the proper season in the following year ? 
For example, the currant galls of the oak are produced on 
structures that can scarcely be said to have been even 
beginning to be formed at the time of flight of the gall¬ 
flies in the previous year, nor is it conceivable that the 
parent insect could have punctured the part on which the 
gall is to be formed in June of the previous year. Even 
could the insect have done so, it is necessary to suppose 
that the egg lay dormant, and that no effect followed for 
many months; a supposition entirely unsupported by any 
kind of proof. The same difficulty meets us of course 
with the autumn galls, such as the “oak-spangles,” and 
other common species, though at first the difficulty here 
may not seem so great. Both these problems were 
felt to be unsolved in the life-history of the gall-flies pre¬ 
vious to the discovery of the “ dimorphism ” of both the 
insects and their galls, but that has solved them for many 
species already, and probably will do so for all ultimately. 
The method of investigation has been as follows. Galls 
of any given kind have been collected, and the insects reared 
from them, and placed either on young oaks in pots, or 
upon branches of older trees enclosed in gauze for the pur¬ 
pose of preventing at once the escape of the insects under 
observation, and the possibility of access of other species 
that might lead to false conclusions by forming galls of a 
kind different from those belonging to the species in 
question. The insects are watched while laying their 
eggs, and the buds, branches, or leaves are at once marked 
in some way so as not to injure them. They are then 
watched to see what result may follow. Erom these ob¬ 
servations the life-histories of many of the gall-flies have 
been traced by Dr Adler; and the conclusions have been 
verified alike by himself and by others, by the frequent 
repetition of the experiments, under precautions and 
variations such as to preclude error. He found that in 
many, indeed in most, cases the gall that followed the 
puncture of any given species (using the term as it was 
understood in the group of Cynipidse before his experi¬ 
ments) was not like that from which the insect had 
emerged; but that it belonged to some distinct form, in a 
few cases different from any previously known, but 
generally belonging to what had been regarded as a per¬ 
fectly distinct species; and in every case this second form 
of insect had been referred to a different genus from the 
former. This will be rendered clearer by tabulating the 
results for our Scottish Cynipidae and their galls. They 
are as follows, the forms under each not yet found in 
Scotland being given in brackets; these ought to occur 
with us also, assuming Adler’s conclusions to be correct, 
and should be looked for wherever the corresponding form 
has been already met with:— 
.4 sexual. Emerge. 
Sexes distinct. 
Emerge. 
Aphilothrix radicis Spring. 
Andricus noduli 
Autumn. 
it 
corticis „ 
( ,, gemmatus) 
,, 
>» 
autumnalis „ 
,, ramuli 
July. 
it 
oollaris ,, 
,, curvator 
June. 
»» 
gemm® ,, 
( ,, pilosus) 
it 
( 
globuli) „ 
„ iuflator . 
June, July. 
Dryophanta folii Winter. 
(Spathegaster Tascken- 
bergi) 
June. 
»» 
longiventris Nov. 
( ,, similis) 
1) 
it 
divisa „ 
„ verrucosa 
i i 
Neuroterus lenticularis Spng. 
„ baccarum 
J» 
( 
laeviusculus) ,, 
,, albipes 
a 
Jf 
fumipennis ,, 
,, tricolor 
July. 
it 
numismatis „ 
,, vesioatrix 
June. 
it 
ostreus Winter. 
( „ aprilina) 
May, June. 
