The Black-bellied Thorntail occurs in Amazonia and in a separate range on the coast of eastern Brazil. Each region has a different subspecies.
In the Amazonian subspecies D. l. melanosternon, males have an iridescent green forehead and throat, a black belly, and a long, narrow, forked tail. Females have a whitish throat and breast with dusky specks, and a shorter tail with white tips.
The species account records it in the canopy of lowland Amazonian forest.
One female of the Amazonian subspecies D. l. melanosternon was observed at Caitaú-Uará on 5 September 1992, and another at Fortuna on 13 December 2007. Both fed at flowering trees below the canopy in forest transitioning between seasonally flooded várzea and unflooded terra firme. One fed at a flowering Inga tree.
At Sani Lodge, Ecuador, on 13 September 2003, one female was found at a lichen-covered cup nest in a roughly 40 m Ceiba tree. The nest lay on a small branch near the outer crown, about 5 m below its top. The female hovered 20–40 cm from the nest on approach and fed chicks inside it on 15 September. On 21 September 2005, another female built a nest in a similar position at Sacha Lodge.
An extinction-risk assessment has not been added yet.