Midge larva on Raspberry rust. Coloured scanning electron micrograph (SEM) of Mycodiplosis larvae on rust. Cecidomyiidae is a family of flies known as gall midges or gall gnats. As the name implies, the larvae of most gall midges feed within plant tissue, creating abnormal plant growths called galls. Mycodiplosis species feed primarily on spores of rust fungi (Basidiomycota, Pucciniales). Raspberry rust (sometimes called raspberry yellow rust) is a disease of the foliage of raspberries, caused by the fungus Phragmidium rubi-idaei. Magnification: x260 when printed 10 centimetres wide.