This study characterized the morphological traits of 111 bee species from five families collected in northern California. The researchers found that bee body size was correlated with traits like thorax depth and wing length. Body size also differed among families, with Apidae bees significantly larger than other families. Bee families also varied in the amount of hair covering their bodies, from less than 20% in Colletidae to over 60% in Apidae. These morphological differences among bee families could impact plant-pollinator relationships and floral evolution more than the pollination syndrome hypothesis suggests.

1. Abstract
Methodology
Results
Discussion
Literature Cited
Acknowledgements
Introduction
Bees, the most important group of insect pollinators, coevolved with angiosperms. They feed upon the
pollen and nectar produced by plants, and in exchange, transfer pollen among flowers. The quality of pollination
services provided by bees is related to their size and morphological traits. Using an extensive plant-pollinator
interaction database, we recorded the morphological traits of approximately 111 bee species that were collected
while visiting flowers in northern California. We found that morphological traits were highly correlated, but that
bee families differed in their size and the distribution of hair covering their bodies. Further research will explore
the morphological “fit” between bee and flower traits.
We characterized bees collected as part of a regional study of 10 plant-pollinator communities along a
northern California transect (Moldenke, 1976). To update the nomenclature, and gather information on bee size
traits, we used eol.org, discoverlife.org, and bugguide.net. Images of female pinned bees containing scale bars were
used to measure overall length, thoracic width, thoracic depth, intertegular distance and right forewing length using
ImageJ (v1.48) software (Fig. 1). We also categorized hair density by examining these images and scoring bees on
a 5 point scale, with 1 representing ≤ 20 % of the body covered in hair, and 5 representing 80-100 % hair
cover. All statistical analysis was performed using IBM SPSS Statistics (v21) software.
We attempted to characterize 111 species distributed across five bee families: Andrenidae, Apidae,
Megachilidae, Colletidae, and Halictidae (Fig. 2). Across species we found a strong correlation between the
overall length of the bee and thorax depth (Spearman’s Rho = 0.817, p < 0.0001, Fig. 3). Bee length was also
positively correlated to right forewing length (Spearman’s Rho = 0.466, p < 0.001, Fig. 4) and the intertegular
distance (Spearman’s Rho = 0.501, p < 0.015). An ANOVA showed that overall body length differed among bee
families (F4,69 = 8.857, p ≤ 0.001). Dunnett’s T3 post-hoc tests showed that bees in the family Colletidae were
significantly smaller than bees in the families Megachilidae and Apidae (Fig. 5). Bees in the family Apidae were
significantly larger than bees in the other four families, whereas the families Halictidae and Andrenidae were not
statistically different from bees in the families Colletidae and Megachilidae (Fig. 5).
Bee families also differed in the amount of hair covering their bodies (Chi-square 72.471, df = 16, p <
0.001; Fig. 6). Bees in the family Colletidae had ≤ 20 % of their bodies covered in hair, while the Halictidae had 0-
40 % of their bodies covered in hair (Fig. 6). In contrast, most of the bees in the family Apidae had ≥ 60 % of their
bodies covered in hair (Fig. 6). Megachilidae displayed the greatest variation in the amount of hair covering their
bodies, but most were 40-61 % range, while most bees in the family Andrenidae had 21-40 % of their bodies
covered in hair (Fig. 6).
As with earlier studies, we found that overall body size is correlated with various morphological traits.
For example, Cane (1987) demonstrated that intertegular distance is proportional to bee size. Our results showed a
similar pattern, although we detected a stronger relationship between overall body length and thoracic depth. Wing
length, which is commonly used to estimate body size in insects, was also positively correlated with body size. As
body size evolves, wing morphology must correspondingly change to maintain flight performance (Kingsolver and
Koehl, 1994).
We also found evidence that bee families differ in the amount of hair covering their bodies. Plumose hairs,
a unique trait of bees (Michener, 2007), are involved in collecting and carrying pollen. It is possible then that bees
with more hair are more efficient at transferring pollen. If so, plants visited by bees in the family Apidae, would
receive better pollination services; however further investigations are needed to demonstrate if this is true.
Although the pollinator syndrome hypothesis suggests that floral traits are adapted to specific groups of
pollinators, such as hummingbirds, this study suggests that bees differ morphologically at the family level. Such
variation in body size and hairiness has the potential to impact pollination, and in turn plant-pollinator coevolution.
It is possible that flowers are not broadly adapted to bees as the pollination syndrome hypothesis would suggests,
but rather flowers are to particular bee families. Further morphological analyses of plant-pollinator communities
are needed to determine how finely adapted flowers are to their pollinators.
Cane, J. H. 1987. Estimation of bee size using intertegular span (Apoidea). Journal of the Kansas Entomological
Society 60: 145–147.
Kingsolver, J. G. and M. A. R. Koehl. 1994. Selective factors in the evolution of insects wings. Annual Review of
Entomology 39: 425–451.
Michener, C.D. 2007. Bees of the World, 2nd ed. Baltimore, John Hopkins University Press.
Moldenke, A. R. 1976. California pollination ecology and vegetation type. Phytologia 34: 305-361.
Proctor, M., P. Yeo, and A. Lack. 1996. The Natural History of Pollination. Portland, Timber Press.
We would like to thank Dr. Andrew Moldenke for sharing his data with us and Bethany Wynne for assistance with
data entry.
Biologists have long recognized that different pollinator groups generally visit flowers with divergent
morphologies. These observations led to the development of the “pollination syndrome hypothesis,” which states
that plants evolved a suite of floral traits, including color, scent and shape, in response to selection by particular
groups of pollinators. For example, plants pollinated by hummingbirds evolved long red corolla, whereas moth
pollinated flowers are generally white, open at night and emit a strong sweet fragrance (Proctor et al., 1996).
Biologist still frequently use the pollination syndrome hypothesis to assign pollinators to plant flora without
observing them in the field because of the assumed pollinator-floral trait associations. However, this hypothesis
does not consider the morphological differences among species within a group of pollinators, nor the implications
for floral evolution. Bees, the most important group of pollinators, have evolved various morphological traits to
collect and transfer pollen, yet vary substantially at the genus and family level (Michener, 2007). For example,
bees in the family Apidae are densely covered with plumose hairs, to which pollen readily adheres. Bees also vary
in size, from less than 2 mm to nearly 40 mm (Michener, 2007). Such variation undoubtedly impacts their ability
to transfer pollen. In order to determine if morphological differences among bees could influence plant-pollinator
relationships, we measured bees collected from several communities in California and compared their traits at the
family level.
Variation in Bee Morphology
Caitlin Hatmaker, Kelly McAfee, Nicholas Salinas, Sarah Scrivano, and Dr. Ruben Alarcón
Biology Program, California State University Channel Islands, Camarillo, CA 93012
Figure 3. Relationship between body length and thorax depth.
Figure 4. Relationship between body length and right forewing length.
Figure 1. Measurements of Andrena duboisi were taken using ImageJ software: thorax width (A),
intertegular distance (B), thorax depth (C) and right forewing (D).
Figure 2. Bee families from left to right:
Andrenidae (Andrena barbilabris),
Apidae (Bombus californicus),
Colletidae (Hylaeus mesillae),
Megachilidae (Hoplitis fulgida), and
Halictidae (Lasioglossum trizonatum).
http://www.discoverlife.org/
Figure 5. Mean body length (±SE) for bee families. Lower case letters denote significant differences
between means.
Figure 6. Distribution of bee body hair across bee families.