This identification was made by comparing the specimen to material in the NHM in London – many thanks to Erica McAlister & Kim Goodyear for giving me access to the collections.
EDIT (21/2/2010): The genus Cordyligaster hasn’t been revised since Guimaraes wrote a key to the neotropical species and published a new species (C.townsendi) in 1971* and there appears to be a bit of synonomy in the genus. The NHM collection contains seperate sections for C.fuscipennis and C.petiolata but, as far as I can see, fuscipennis is just a synonym of petiolata now and the 2 “species” look remarkably similar in the collection.
I now have specimens from Peru that also conform to Guimaraes’ idea of C.petiolata.
* Guimaraes, J. H. 1971. Notes on the genus Cordyligaster Macquart, with the description of a new species from Brazil (Diptera, Tachinidae). Pap. avuls. Zool. 25: 99-103.