Friday, February 26, 2010

Bumblebee Colors

Colors? What colors? Bumblebees are black and yellow, right?

Well yes, many of them are. But lots of bumblebees have brown, red, and white colors as well.

And even when black and yellow, most bumblebees do not fit our popular, oversimplified image of alternating black and yellow bands around the body.

In fact the patches of color that decorate the bumblebee's generally black body can appear in a vast variety of patterns of varying complexity.

Central to this issue of bumblebee color patterns is the fact that there are many different kinds, or distinct species, of bumblebees. Who knew? When I was a kid, even though I was wise to the different species of butterflies that I saw visiting flowers, I thought all bumblebees were the same. And I'm sure this is true for most people. Scientists, however, have discovered about 250 species worldwide and around 50 in North America alone. The first clue to understanding this tremendous bumblebee diversity is noticing the variety of color patterns they display.

Students of the bumblebee have discovered that most individual species can be identified primarily on the basis of the color pattern. That means that the color patterns of different species are different from each other most of the time. There are a number of factors that tend to confuse the issue, but we'll get to that in a moment. For now, all we need to recognize is that most books, websites and identification guides to bumblebees use color patterns as the most important means of distinguishing one species from another.

Here's how the observation of color patterns works. First we start out with a silhouette of the upper surface of the bumblebee body in diagrammatic form. The silhouette is all black, as that is the basic color of the exoskeleton in these insects. The main body parts shown are the head, thorax and abdomen. The individual segments of the abodomen, six in queens and workers, are numbered from front to back as T1 through T6. In this numbering system, the 'T' stands for 'tergite,' the anatomical term given to the exoskeletal plate that forms the upper surface of each abdominal segment.

Then, on top of the basic silhouette are added a number of colored areas... various shades of yellow, red, brown and white. These colored areas correspond to dense patches of hairs, referred to as pile, occurring on the body. The shape, color and location of these patches is what determines the overall color pattern.
Using this system to take a quick look at some of the color patterns of different bumblebee species not only provides a useful starting point for identification, it is a breathtaking reminder of the diversity of ecologies and lifestyles these pollinators have evolved.

Postscript: The simple correspondence between a single color pattern and a single bumblebee species is complicated by three facts of life in the bumblebee world - polymorphism, Mullerian mimicry and cryptic species. More about these issues in later posts to this blog. [Images copyright (c) Dave Barr, 2010]

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