Oecologia (Berl.)'45, 197-201 (1980) Oecologia 9 by Springer-Verlag 1980 The Effect of Proboscis and Corolla Tube Lengths on Patterns and Rates of Flower Visitation by Bumblebees David W. Inouye Department of Zoology, University of Maryland, College Park, Maryland 20742 USA and Rocky Mountain Biological Laboratory, Crested Butte, Colorado 81224 USA Summary. The rates at which bumblebees of different proboscis lengths forage on flowers of a series of corolla tube lengths were determined. The results indicate significant correlations between proboscis length and time spent by bees on flowers. Bumblebees of long proboscis length can forage significantly faster than bees of shorter proboscis length on flowers with long corolla tubes. There is also evidence which suggests that bumblebees of short proboscis length prefer and are more efficient on short corolla tubes. These results support the use of proboscis length as a mor- phological indicator of resource utilization in bumblebees. Introduction The number and relative abundance of species in natural commu- nities are phenomena which have occupied the attention of many ecologists. Studies of resource partitioning provide one method of investigating the limits which interspecific competition places on the number of species that can stably coexist (Schoener, 1974), and biologists have often estimated resource differences by using morphological characteristics that are assumed to reflect differ- ences in resource utilization (Hespenheide, 1973). The commonest such indicators are dimensions of feeding structures, which are usually correlated with mean food size, hardness, or depth in some protective medium. This study was undertaken to determine the actual significance of proboscis length of bumblebees as a morphological indicator of resource utilization. Although previous studies have concluded that proboscis length is related to differences in flower visitation (e.g., Hobbs, 1962; Hobbs, etal., 1961; Cumber, 1949), this study was designed to quantify the relationship for bees in a natural flower community. Methods Although some references fail to define their measurements accura- tely, "proboscis length" of bumblebees is generally accepted to refer to the length of the labium, or the combination of the premen- turn and the glossa. The bumblebee proboscis can be assumed to operate in the same manner as that of the honeybee (Knuth, 1906). The maxillary galeae and the labial palpi are brought to- Present address: Department of Zoology, University of Maryland, College Park, Md 20742 gether around the glossa to form a proboscis for the intake of liquids (Snodgrass, 1956). The glossa is essentially a muscular tube covered with short hairs. When it comes into contact with the nectar at the bottom of a corolla tube, capillary action draws nectar up to its base (Knutz, 1906). The pre-oral food-receiving pocket, or cibarium, functions as a sucking pump to draw liquids up through the tube. When not in use, the maxillae and labium are separated and folded back below and behind the head (Snod- grass, 1956). The data on proboscis lengths of queens and workers presented in Fig. 2 were taken from published measurements of bumblebees collected on the Front Range of the Colorado Rocky Mountains (i.e., east of the Continental Divide) (Macior, 1974). Measurements indicate the sum of the individual lengths of the prementum and glossa. In reality the proboscis length is 5 10% longer, due to the presence of connective tissue. Small samples of workers and queens indicated that there was no significant difference in probos- cis length between the Front Range and western Colorado popula- tions of a species. To determine the actual depth from which nectar can be extracted by a bumblebee ("totaI effective foraging length", Macior, 1978), the length of the head and even part of the thorax must be added to the proboscis length if the mouth of the corolla being visited is wide enough to accomodate them. However, these measured proboscis lengths are still useful for comparing nectar-foraging rates (Macior, 1978). Data on flower sizes and bee visitation rates were collected in the neighborhood of the Rocky Mt. Biological Laboratory, Gothic, Colorado. Corolla tube lengths of freshly picked flowers from different flower stalks and areas were measured to the nearest 0.001 inch with a vernier caliper. The measurements were chosen to reflect the actual distance that the bee must reach in order to extract nectar from the bottom of the corolla tube. Accumula- tion of nectar in the flower would reduce this distance. While distinct constrictions or fringes sometimes provide relatively reli- able clues, it is not always obvious how far a bumblebee can insert its head and thorax into a flower. Some of the variance around the regression lines involving measurements of corolla tube length can be attributed to this measurement error. The amount of time bumblebees spent visiting individual flowers was measured with a stopwatch. For composites (Astera- ceae) the total amount of time a bee spent probing individual florets on a head was recorded and divided by the number of florets probed, to give a value of time per flower. Although this technique thus includes time spent moving between florets, which is not included in measurements for other flowers, the florets are in such close proximity that this component should be neglig- 0029-8549/80/0045/0197/$01.00