Oak Marble Gall.

New galls found on oaks in the spring can be bright green but quickly become brown and very hard.

 

Each gall normally houses one larva of the gall wasp, Andricus kollari - and possibly, several inquiline (foreign squatter species) intruders. When mature they vacate the hard woody sphere through tiny holes

www.bugsandweeds.co.uk/galls p1.html

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