Anemone, either
A. virginiana or
A. cylindrica, collected on Aug 14, 2011.
In previous years I had noted the blackish streaking but this time I teased it apart and found Diptera puparia.
These may be the larvae of the fly

but I was not able to rear them to adulthood.
O. Lonsdale says: "I did the dissection and it's definitely
Phytomyza anemonivora Spencer.
Good find! As far as I can tell, this is a new record for the USA! It's only otherwise known from Ontario."