In fish, the retrorse (backward) inclination of the myomeres forms a precursor of the differentiation between a deep and a superficial musculature of the trunk.
As the bee's proboscis is inserted into the flower it pushes past the retrorse anthers to the nectar at the base of the tube.
Origin
Early 19th century: from Latin retrorsus, contraction of retroversus, from retro 'backwards' + versus 'turned' (past participle of vertere).