the Gall-flies of the Oak. 283 
leaves of the oak, galls which appear in July and fall to the 
ground in September or October. ‘The larva is at this period 
very small; and the perfect insect does not come out till April 
or the beginning of May. Scarcely has it escaped from the 
gall in which it was developed, when it deposits its eggs 
upon the buds of the oak. Around these eggs are formed, 
upon the leaves and the peduncles of the male flowers, galls 
differing from those which had nourished the Neuroterus. 
The insect which emerges from them is no Newroterus at all, 
but had been classed in another genus under the name of 
Spathegaster baccarum, L. This, in its turn, will deposit eggs 
which will produce Neurotert. 
The same alternation has been observed in three other 
species of Newroterus corresponding to three distinct species of 
Spathegaster. 
Not only do the two generations live in galls differing in 
form, size, colour, and situation, and the insects exhibit among 
themselves differences of size, proportions, and structure, 
but what renders the contrast more striking is, that the 
Neuroterus generation 1s only represented by females, whilst 
the Spathegaster generation presents individuals of both sexes. 
We have therefore here a new form of alternating genera- 
tion*. 
The genus Aphilotrix contains a great number of species of 
Cynipide, of which only the female individuals were known. 
M. Adler observed in nine of these an agamic and sexual 
alternation of generations ; these last are represented by species 
belonging to the genus Andricus. 
Three species of Dryophanta which were investigated by 
the same observer exist only in the female state; the sueceed- 
ing generation is formed, as in the case of Neuroterus, of 
species belonging to the genus Spathegaster. 
The fourth group, that of the Beorhiza,is the most interesting 
of all, on account of the differences of form and habits which 
the insects of the two consecutive generations exhibit. 
Biorhiza aptera, which exists only in the female form, is a 
little wingless insect, from 4 to 7 millim. long, known to 
form upon the roots of the oak galls at first soft and of a red- 
dish-white colour, which assume a brown colour on arriving 
at maturity, and become tolerably solid. M. Adler observed 
that the insect which comes out from them does not lay its 
eges on the roots, but climbs the oak to attack the young 
shoots, and, above all, the big terminal buds. From the galls 
which are developed round the punctures an insect comes out, 
* This case belongs to the category which Prof. Balfour, in his treatise 
on embryology, calls heterogamy. 
