Recently I was attempting to photograph Aculeates around an area of flowering Heather (Calluna vulgaris).It was sunny and warm and several female Mellinus arvensis wasps were around the heather, apparently hunting the many flies feeding on the flowers. These are a species that provision the nest cells with various flies. I noticed one Mellinus, carrying something, come into land on a bare earth bank next to where I was standing. When I focused through the camera viewfinder I was surprised to see that she was in the process of eating a fly (probably a Greenbottle, Calliphoridae). The head of the fly had already gone, and over the period of a couple of minutes she ate the muscles of the thorax and part of the abdomen. Eventually she flew off leaving behind the cuticle and some chewed remains.
According to all the literature adult wasps, whether solitary or social, take nectar from flowers and honeydew to feed themselves, reserving animal prey for the larvae, although some have been observed to occasionally malaxate their prey and ingest the body fluids. Spradbery in ‘Wasps’ (1973) references Rau as having observed an Odynerus dorsalis consuming the entire body contents of a caterpillar. However the female Mellinus I watched and photographed actively chewed and ingested muscle and other body tissue.