Size determines antennal sensitivity and behavioral threshold to odors in bumblebee workers

Abstract

The eusocial bumblebees exhibit pronounced size variation among workers of the same colony. Differently sized workers engage in different tasks (alloethism); large individuals are found to have a higher probability to leave the colony and search for food, whereas small workers tend to stay inside the nest and attend to nest duties. We investigated the effect of size variation on morphology and physiology of the peripheral olfactory system and the behavioral response thresholds to odors in workers of Bombus terrestris. Number and density of olfactory sensilla on the antennae correlate significantly with worker size. Consistent with these morphological changes, we found that antennal sensitivity to odors increases with body size. Antennae of large individuals show higher electroantennogram responses to a given odor concentration than those of smaller nestmates. This finding indicates that large antennae exhibit an increased capability to catch odor molecules and thus are more sensitive to odors than small antennae. We confirmed this prediction in a dual choice behavioral experiment showing that large workers indeed are able to respond correctly to much lower odor concentrations than small workers. Learning performance in these experiments did not differ between small and large bumblebees. Our results clearly show that, in the social bumblebees, variation in olfactory sensilla number due to size differences among workers strongly affects individual odor sensitivity. We speculate that superior odor sensitivity of large workers has favored size-related division of labor in bumblebee colonies.

Estudo comparativo das sensilas antenais de operárias de Melipona scutellaris (Hymenoptera: Apidae) de diferentes altitudes

“…A função dessas sensilas é descrita como sendo olfativa em alguns Hymenoptera (oChieNg et al 2000;sPaethe et al 2007) e foi estudada em detalhes na espécie de abelha Bombus terrestris Linnaeus (Hymenoptera: Apidae), mostrando sua importância para a percepção de odores. Nesse caso, dentro de uma mesma colônia, os indivíduos com maior densidade dessas sensilas exibiam melhor capacidade de reposta a odores do que os indivíduos com antenas pequenas e menor densidade de sensilas (sPaethe et al 2007). Apesar da função de quimiorrecepção ainda não ter sido confirmada para M. scutellaris, o fato de essas sensilas estarem mais abundantes na extremidade antenal sugere que as mesmas podem atuar como quimiorreceptoras.…”

Antennal sensilla of cleptoparasitic and non-parasitic bees in two subfamilies of Apidae

“…A strong correlation between the abundance of sensilla and the capacity to locate a food source for the colony has been reported in workers of Apis mellifera L. and Bombus terrestris (L.) (Spaethe et al 2007; Riveros and Gronenberg 2010). Morphological…”

Morphological and morphometric studies of the antennal sensilla from two populations of Atta robusta (Borgmeier 1939) (Hymenoptera: Formicidae)

“…There are also several different types of sensilla, which are classified according to their shape and size. Generally speaking, the most common types of sensilla of hymenopterans are trichoid and basiconic, which are involved with mechanical and chemical perception (Renthal et al, 2003;Ozaki et al, 2005;Rocha et al, 2007;Spaethe et al, 2007;Onagbola et al, 2008).…”