Body size but not age influences phototaxis in bumble bee (Bombus terrestris, L.) workers

We studied phototaxis, the directional movement relative to light, in the bumble bee Bombus terrestris. We first developed and validated a MATLAB based system enabling reliable high-resolution tracking of a bee and a measurement of her distance relative to a changing LED light source. Using this system we found in all our experiments that workers show positive phototaxis. The strength of the phototactic response was influenced by body size but not age, and this effect was significant when the light source was weak. In a separate experiment, foragers showed stronger phototactic response compared to nurses only in one of two trials in which they were larger and tested with weak light intensity. The evidence that phototaxis is associated with size-based division of labor in the bumble bee and with age-related division of labor in the honey bee, lends credence to response threshold models implicating the response to light in the organization of division of labor in cavity dwelling social insects.