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The Cecidomyiidae ovipositing into cut surfaces of oak are female Xylodiplosis. This genus is well known in Europe and North America although relatively little information has been published. The remarkably narrow and elongate larvae live within xylem vessels, usually in freshly cut or recently broken live wood. The long, telescopic ovipositor, visible in these photographs, is presumably an adaptation fo r placing eggs precisely into the xylem vessels. Adults are active early in the year and there are at least two European species : X. aestevalis and X.nigritarsis. I have seen females ovipositing in Italy (Tuscany) in February. Cecidomyiidae have remarkably diverse biology. Many species are phytophagous (including gall inducers) but others are mycophagous or zoophagous (including predaceous and even endoparasitic species).