This is one of my favourite French Guianan tachinids! At first glance it looks fairly standard but when you examine the face you will see that antennal segment 3 (normally rectangular) has been formed into a feather-like structure, similar in design to moth antennae! This structure is recessed in a deep concavity in the face and fringed by a complete line of facial-ridge bristles.
I have nicknamed these ‘feather-face’ tachinids for my own personal reference but they are more properly called multifissicorn tachinids. The feature has only been found so far on males, which suggests that it is an adaptation for finding the females but this is just conjecture and is impossible to determine without a study of their lifecycles. The feature has been found on 2 taxa so far – the other being a tiny (4-5mm) taxon with a black body and golden dusting pattern on the thorax (fg-taxon #11 – Borgmeiermyia sp.).
EDIT (23/2/2010): After some investigations I found Arnaud (1963)* and the keys in there describe a few genera of multifissicorn tachinids (Cryptocladocera & Cerotachina). This taxon looks like Cryptocladocera (Cryptocladocera have branches on the antennae that can be 1/3 of the length of the antenna while Cerotachina should have branches of no more than 1/10 then length) but I am having difficulty tracking down any specimens to compare mine against. See also fg-taxon #97 (Cerotachina).
EDIT (Monty Wood, 26/9/2010): These are all Pelecotheca sub-genus Cryptocladocera.
* Arnaud P.,H., (Jr) (1963) Systematic Studies in the Tribe Neominthoini (Diptera, Tachinidae). American Museum Novatates. number 2135 p. 1-55.