|A quick pictorial reminder of C. fungosa - a little parasitic green jewel.|
|A. quercuscalicis - close-up of the front half.|
However, already knowing the identity of this species as it emerged from a gall in a sealed container, I do not wish to dwell on the finer points of diagnostic morphology (not this time anyway...); instead I'd like to look at the biology of the Knopper Gall. Knoppers are asexual galls and (despite only arriving here in the 1950s) are common in Britain, mainly on pedunculate oak Quercus robur and rarely on sessile oak Q. petraea. Fresh knoppers are found on acorns in summer and autumn, starting green and sticky but becoming brown and woody (like the one illustrated below). The thick wall contains an air space with an ovoid inner gall inside.
|An old, woody knopper gall, around 2cm across|
In May the sexual males emerge and wait for females to chew their way out - the males fight to secure the best mating locations. Mated females then fly to Q. robur trees and lay eggs (in late May) into newly pollinated female flowers; each egg is positioned precisely between the developing acorn and cup. In September, the galls drop to the ground (before ungalled acorns) and overwinter, along with the larvae inside, within the leaf-litter. Most (in Britain, 70-80%) of asexual females emerge the following February or March, the remainder staying in the knopper for up to three years and joining a different cohort. In either case, they then fly to Q. cerris trees to lay eggs in male flower buds and begin the life cycle once more.
To finish (for now), I have retained the knoppers and will be interested to see if anything else emerges although it seems unlikely unless further adults remain. So, there may be a third instalment covering internal knopper structure and possibly more inhabitants. I would also like to recommend the following book which has recently been published - it is excellent and was the source for the life cycle summary given above:
Redfern, M. (2011). Plant Galls. Collins, London. [vol. 117 in the New Naturalists series]