In 1927 Keilin described the fauna of a rot-hole and a slime-flux in a horse-chestnut tree (Aesculus hippocastanum) standing on the Downing site, Cambridge. For several consecutive years prior to 1927 he took there the larvae of the mosquito Orthopodomyia pulchripalpis Rondani (described as 0. albionensis by MacGregor), this being only the second record for Britain. In 1926 the chestnut tree was cut down and the part of the trunk containing the rot-hole was placed standing on the ground close to the Molteno Institute on its south side. As the rot-hole, after a time, ceased to be water-tight, its bottom was cemented and the “reservoir” kept filled with water. (For further details see Keilin in this issue, p. 280.) Three to four weeks later larvae of Orthopodomyia were found in the reservoir. It was not clear how this recolonisation had taken place as no other breeding place was known in the vicinity.