List of the Insect Fauna of the County. 117
flies, should certainly be carefully studied. The agamous
genera Aphilothrix, Neuroterus, Dryophanta, and Biorhiza are
but aestival parthenogenetic generations of the sexual genera
Andricus, Spathegaster, and Trigonaspis, but Dr. Adler still
leaves four species of Aphilothrix (A. seminationis, Gir., A.
marginalis, Schlecht., A. quadrilineatus, Hart., and A. albo-
punctata, Schlecht.) whose generations, he tells us, are ex-
clusively parthenogenetic—"the alternate sexual generation
does not exist." Cynips is still untouched. This varying
biography and linking of structurally different forms into one
species is deeply interesting, and leads to most important
considerations and results which we cannot now further
follow out.
Amongst the cynipideous gall-makers the genus Rhodites
limits itself to the rose, and other species produce galls on
the bramble, ground-ivy, Hieracium, Potentilla, poppy, &c.
The larvae of the Cynipidae are fat, fleshy, apodal, whitish
grubs ; they pupate in the galls, without exception.
We have but few British sawflies which produce galls, and
I am only able to include three species in our Essex list.
These all occur on willows, and are produced by species of
the Nematidae. The very common bean-shaped gall of
Nematus gallicola, which is so commonly seen projecting from
both sides of the leaf of many willows, must be well known
to all. Its manner of reproduction is very commonly
parthenogenetic, the male N. gallicola being excessively rare.
The late Mr. F. Smith did not meet with it until 1873, the
year before his death; and he worked assiduously at the
Hymenoptera for upwards of fifty years. It is somewhat
remarkable that a beetle (Balaninus Brassicae) is a very general
inquiline, or lodger, in these galls. Unlike the Cynipidae, the
gall-making sawflies do not undergo their metamorphoses
within the gall, but the larvae leave it when full-fed and pupate
in the ground. The Cryptoeampi are an exception; their
cocoons are formed within the woody galls.
In the Diptera, as in the Hymenoptera, we have one large
gall-producing family—the Cecidomyidae, whose numerous
species are commonly known as gall-gnats. Seventy-six