Estimation of bee size using intertegular span (Apoidea)

The dry weights of solitary female bees are accurately and readily estimated by a nonlinear, exponential regression equation using the shortest linear distance measured between a female bee's wing tegulae across her thoracic dorsum. The regression equation is y = 0.77(x)0405, where y is intertegular span and x is dry weight. Melittologists often have need of a reliable and direct means to estimate the "size" of a pinned museum specimen without reverting to the cumbersome task of taking dry weights. A size measure is necessary for standardizing comparisons made in studies of bee energetics, foraging ecology, allom etry, anatomy, chemical ecology, reproductive and sexual selection, and nesting biology. Weight, whether wet or dry, is complicated by variations in crop or scopal contents. For example, workers of Apis mellifera foraging at saguaro cacti carry, on average, nearly a fifth of their wet weight in corbicular nectar and pollen (Cooper et al., 1985; Schmidt and Buchmann, 1986). Glandular reservoir contents further complicate weight estimates. Females of the large bee Habropoda (=Emphoropsis) laboriosa will contain from nearly 0 to 6 mg of Dufour's gland lipid secretion (Cane and Carlson, 1984), dependent upon age, time of capture and recency of secretion. Head width has been successfully employed for body size estimation of live male Philanthus wasps (O'Neill, 1983). It is less suitable for comparing the genera of bees, owing to head allometry that may reflect mandibular/labial gland development or nesting biology (e.g., leaf-cutting megachilids have robust mandibular musculature). Body length too has allometric drawbacks, particularly for the more elongate and cylindrical bodies of stem-nesting taxa. An intuitively appealing estimate of bee size would be a measure of thoracic volume. The content of thoracic flight musculature should directly translate into the lift required for flight by a bee of a particular size. The tegulae covering a bee's wing bases provide suitable landmarks between which can be measured a chord distance, the intertegular span, which might satisfactorily estimate bee dry weight. A female of each of 20 species of bees was selected for measures for the regression analysis. None of these species are eusocial, but among them are parasitic taxa, groundand stem-nesting species, which range in size from nearly Drosophila-sized Dialictus to bumblebee-sized carpenter bees. The species measured are: Collet? s inaequalis, Hylaeus rugulosus (Colletidae); Andrena accepta, Calliopsis andreniformis, Per dita coreopsidis (Andrenidae); Protoxaea gloriosa (Oxaeidae); Augochlora pura, Dialictus versatus, Dufourea marginata, Nomia melanderi (Halictidae); Hesperapis carinata (Melitti dae); Anthidiellum notatum, Megachile texana (Megachilidae); Anthophora urbana, Centris atripes, Diadasia olivaceae, Exomalopsis solani, Melissodes agilis, Triepeolus verbesinae, and Xylocopa micans (Anthophoridae). These species together represent the six major (and one minor) non-eusocial bee families and 18 of the tribes. One intact and pinned female (with empty scopae) of each species was first measured using a Wild? dissecting stereomicroscope fitted with an ocular micrometer. The shortest distance between the bases of her tegulae was recorded to the nearest 105 m (Fig. 1, inset). Both tegulae were kept within the shallow focal plane to ensure their alignment. For preliminary comparisons, head widths were also taken, using the greatest perceived width of the head when viewed from the vertex. Bees were then slipped off of their pins (or in the case of smaller species, dry, unpinned specimens were used) and dried for 5 days at 45? C, until they ceased losing weight. They were weighed using a Sartorius model 1712MP8 balance to the nearest 10~5g. Additionally, the weights and intertegular spans of 12 females each of Xylocopa virginica and of Diadasia rinconis were measured as above to assess degrees of intraspecific variability in these measures for a relatively large and a medium-sized bee. They were not included in the regression calculations. The sample of female weights, head widths and intertegular spans of the 20 species were preliminarily submitted to a simple linear regression. Intertegular span linearly increased with bee dry weight (P < 0.01, R2 = 0.945), as did head width (P <0.0l,R2 = 0.904). While significant, these linear regressions yielded unsatisfactory fits for many of the data points when plotted for the smaller bees. This content downloaded from 207.46. 13.158 on Wed, 16 Nov 2016 04:30:06 UTC All use subject to http://about.jstor. org/terms 146 JOURNAL OF THE KANSAS ENTOMOLOGICAL SOCIETY