A midge causing swollen flower buds on Hawthorn.  The buds fail to open.  Inside are several pale creamy-white larvae that are able to jump. 

Dasineura oxyacanthae has similar galls, but inside are orange non-jumping larvae

host/larvae

Photograph the gall on the host plant; note the host species in your comments.  You must examine and photograph the larve clearly.

Hawthorn

Spring/early summer