Metabolic energy use by honeybees in flight and butterflies at rest

Four studies were undertaken of different aspects of metabolic energy use by honeybees and butterflies. In the first, the controversial question of whether flying honeybees maintain thermal stability by varying heat production as well as heat loss was examined. Metabolic rate, water loss, wingbeat frequency, body segment temperatures and behavior were recorded for honeybees flying in transparent containment outdoors, where flight quality was markedly better than indoors. During periods of voluntary uninterrupted flight, metabolic rate was independent of air temperature between 19 and 37°C. Thorax temperatures were more stable than in indoor studies, with a slope of thorax temperature on air temperature of 0.18. Evaporative heat loss increased sharply at a lower air temperature than in indoor studies. Metabolic expenditure per wingbeat diminished at higher air temperatures, and reanalysis of previous studies suggests this may be a general phenomenon. Lower metabolic rate at higher and lower thorax temperatures appears to be a passive property that contributes to limiting body temperature elevation at higher air temperatures. In the second study, the standard metabolic rate (SMR) of Manataria maculata , a neotropical satyrine butterfly, was compared with that of its closest regional relatives. The comparison was driven by M. maculata 's unusual life history; adults live for nearly a year, undertake a seasonal altitudinal migration away from their breeding ground, and spend spending most of this period in reproductive diapause. SMR of M. maculata was about half that of its nearest relatives and much lower than for neotropical butterflies in general. Evidence from this and other studies suggest that reproductive diapause may be a mechanism for reducing energy requirements during migration. In the third study, repeatability of SMR measurements of butterflies was examined. Unless SMR measurements are repeatable, questions of its adaptive importance cannot be addressed. SMR, body mass and feeding uptake were measured daily over the adult lifetimes of Vanessa cardui adults. SMR measurements of younger butterflies allowed to feed were not repeatable. I argue from simulations and from measurements of wild-caught butterflies that wild-caught animals will usually not yield repeatable metabolic rate measurements. However, measurements of butterflies not allowed to feed, or of animals allowed to feed but retained for several days prior to measurement, can yield repeatable SMR measurements from which adaptive issues can be approached. In the fourth study, SMR was measured at temperatures from 15 to 30°C for 496 wild-caught neotropical butterflies representing 4 families and 29 species. At 25°C, the mass scaling relationship of standard metabolic rate (SMR) for individuals ranging from 0.018 to 0.924 g was SMR = 3705 M0.691, where SMR is in muW and M is body mass in grams. Analysis of covariance found significant differences between suborder taxonomic groups, yet only about half of variation lay between, rather than within, groups.