Appearance
"B. atratus" are predominantly black in color. The throat of the bee is completely black while the abdomen is black with a small amount of lightening/discoloration towards the end. Small, dense fir bristles cover almost the entire body of "B. atratus".Like most bumblebees, the abdomen of the bee has a rounded tip and a distinct forewing and hindwing. "B.atratus" workers have a long tongue that facilitates the process of retrieving nectar from within deep and tubular morphologies of flowers.
The antenna of "B. atratus" are also relatively long. Workers have a wing length that ranges between 4 and 10 mm. Queens are usually two to three times the mass of workers.
Distribution
"B.atratus" are found throughout South America, including Columbia, Ecuador, Brazil, and Argentina and Brazil.Behavior
The social dominance hierarchy for "B. atratus" is in large part due to the polygyny colony cycling. Dominant queens that have ascended the dominance scale have much greater access to nest territories on the brood clump.Less dominant queens are often forced to the outside of the brood clump and sometimes are forced out of the nest/brood clump area entirely. While subordinate queens become the equivalent of workers, dominant queens have the most reproductive success and also usually live longer than their subordinate counterparts.
Queen dominance is correlated with aggressive behaviors. One theory for why these subordinate remain in the nest relates to the possibility for egg resorption.
Habitat
"B. atratus" nests are usually found above the ground. Nests have been found in an array of different locations, including warm, tropical areas and cold, high altitude environments.Such a diverse geographic distribution illustrates how "B. atratus" has the unique ability to adapt to a wide range of conditions and pollen sources. Although most nests are built on the ground, nests have been found elevated in trees above the ground.
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