23/01/2008
Today I checked some literature on Aphylla species in the library of natural history museum Naturalis. I am now fairly certain that the dragonflies on these pictures are Aphylla theodorina.

Literature checked
Belle, 1992 - A revision of the South American species of Aphylla Selys, 1854 (Odonata: Gomphidae). Zoologische Mededelingen, Leiden 66(12): 239-264
Belle, 1994 - Four new species of Aphylla from Brazil (Odonata: Gomphidae). Entomologische Berichten 54 (7): 138-144.

Belle (1992) contains a key for the South-american Aphylla species. I think I was a little bit lucky that I was able to identify this gomphid as it is the only (known) Aphylla species in South America with such an obvious yellow Costa. So... already at the start of the key I ended at Aphylla theodorina. Afterwards I checked Belle (1994) if any of the 4 species described after 1992 could look the same. It turned out one of them could have some yellish at the costa, but not to the extent of A. theodorina. Checking available descriptions and plates seemed to confirm this.

Afterwards I searched for Aphylla theodorina on the internet and found that Dennis Paulson has a picture of a specimen on his website. This (male)specimen looks extremely similar to the male on the photo. Check it out:
www2.ups.edu/biology/museum/Aphylla_theodorinaM.jpg