Really bad pictures, but what a cool fly!
I went searching for Phthiria's (soort wolzwever) last week, but the weather was so bad I found nothing. Today was better weather, and I found some 30 individuals. Phthiria's are characteristic for dunes, often (always?) found on yellow flowers. They are tiny animals and very chill. I will certainly try to take some better pictures.
In Oostduinkerke, a big field of Crepis (=the yellow flower) is present, so it was the ultimate place to search. Tephritis crepidis and Ensina sonchi also present in big numbers :-) I assume this species is more restricted to Oostduinkerke/de Panne whichs has more natural dunes. In Ostend I have searched for it last year, but unsuccesfull!
The top picture is a female, the bottom one a male which is way more dark. I only saw one male that day..