Photo taken at the 2013 Havering bioblitz
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This is the sexual generation gall of Biorhiza pallida, the one correctly called the 'oak apple'. Each gall contains dozens of either male or female larvae (not both in any one gall) together with, no doubt, their parasitoids and 'lodgers' properly known as inquilines.
Lat/Lng: 51.5505, 0.2827
OS grid ref: TQ583859