Insect identification (SP/Brazil)
Well, it's a type of firefly, belongs to the order of 'Coleoptera' (beetles), suborder of 'Polyphaga', family of 'Lampyridae' (firefly beetles). It's quite similar to the 'Photinus obscurellus' species (and probably the same species too), however it doesn't always have a black head like yours, however, articles have suggested that  firefly species in Brazil (Sao Paulo State) have 'adapted to mesophyl tropical forests, secondary growths, marshy areas, and open fields' (source = https://www.researchgate.net/publication/233574632_Fireflies_Coleoptera_Lampyridae_from_Southeastern_Brazil_Habitats_Life_History_and_Bioluminescence). 

This adaption may have led to species having black heads as oppose the more colourful head of the Photinus obscurellus. However the bodies and the dimensions match yours very closely.

This image is from Google Images (more specifically : http://bugguide.net/node/view/194980/bgimage)