Date Published: June 7, 2019
Publisher: Public Library of Science
Author(s): Annalie Melin, Harald W. Krenn, Rauri C. K. Bowie, Colin M. Beale, John C. Manning, Jonathan F. Colville, James C. Nieh.
An appreciation of body size allometry is central for understanding insect pollination ecology. A recent model utilises allometric coefficients for five of the seven extant bee families (Apoidea: Anthophila) to include crucial but difficult-to-measure traits, such as proboscis length, in ecological and evolutionary studies. Melittidae were not included although they are important pollinators in South Africa where they comprise an especially rich and morphologically diverse fauna. We measured intertegular distance (correlated with body size) and proboscis length of 179 specimens of 11 species from three genera of Melittidae. With the inclusion of Melittidae, we tested the between family differences in the allometric scaling coefficients. AIC model selection was used to establish which factors provide the best estimate of proboscis length. We explored a hypothesis that has been proposed in the literature, but which has not been tested, whereby body and range sizes of bees are correlated with rainfall regions. We tested this by using body size measurements of 2109 museum specimens from 56 species of Melittidae and applied the model coefficients to estimate proboscis length and foraging distance. Our results from testing differences across bee families show that with the addition of Melittidae, we retained the overall pattern of significant differences in the scaling coefficient among Apoidea, with our model explaining 98% of the variance in species-level means for proboscis length. When testing the relationship between body size and rainfall region we found no relationship for South African Melittidae. Overall, this study has added allometric scaling coefficients for an important bee family and shown the applicability of using these coefficients when linked with museum specimens to test ecological hypothesis.
Bees play a key functional role in almost all terrestrial ecosystems in pollinating both wild flowering plants [1–3] and agricultural crops [4,5]. Bees have also been identified as a potent co-evolutionary force in the diversification of plants [6–10] and in promoting gene flow (pollen transfer) among plant populations . Key to these crucial ecological interactions is an understanding of their feeding preferences as determined in part by proboscis length and foraging distance, both of which have an allometric relationship with body size [12,13].
We measured 179 specimens from 11 species belonging to the three genera Meganomia, Rediviva and Redivivoides (Melittidae). Melittidae are well-represented in South Africa, with 67 species in eight genera  and we were able to obtain a representative sample for the region (~ 40% of genera). We provide the mean IT, glossa, prementum and proboscis for each species in supplementary information (S5 Table).
This study established the body size allometric scaling relationship for the bee family Melittidae, thereby expanding the work by Cariveau et al.  to include six of the seven extant bee families. We then applied this family-specific allometric equation, with an estimation for the scaling coefficient between IT and proboscis length, to body size measurements obtained from museum specimens to estimate the proboscis length and foraging distance for Melittidae. In addition, using body size measurements we estimated foraging distance following Greenleaf et al. . Using this trait information for South African species of Melittidae, we were able to examine body size, proboscis length and foraging range patterns in relation to rainfall seasonality.