Which fly is characterized by a metallic blue or green color and is about twice the size of a house fly?

The blow fly is indeed characterized by its metallic blue or green coloration, which distinguishes it from other types of flies. Typically larger than a house fly, blow flies can be about twice their size, making their appearance noticeable. These flies are often found near decaying matter, as they are attracted to the scent of decomposition, and they play a significant role in the process of decay and recycling nutrients within ecosystems.

The unique coloration of blow flies can serve as an identification feature, as it differentiates them from flesh flies, which may also appear in similar environments but possess a more grayish or blackish hue. Unlike moth flies, which have a more fuzzy appearance and lack the metallic sheen, or cluster flies, which have a distinct olive-brown color and are often associated with indoor infestations in the fall, blow flies stand out due to their vibrant color and size. This background makes the blow fly the most suitable answer for the question posed.