Bee visits and the nectar of Echium vulgare L. and Sinapis alba L.

Abstract. 1. The time-course of anthesis of Echium vulgare is described.
2. Diel changes in the sugar concentration of the nectar, the quantity of nectar and the quantity of sugar per flower are illustrated for E.vulgare and for Sinapis alba.
3. These changes are interpreted in terms of (a) the periodicity of secretion and (b) the influence of microclimate and insect visits on post-secretory changes in the composition and volume of nectar.
4. There was hour-to-hour and day-to-day variation in the species composition and the proportion of workers taking nectar rather than nectar plus pollen among the social bees visiting E.vulgare.
5. Honeybee visits to E.vulgare were more numerous in humid weather, when there was enough nectar per flower for their relatively short tongues to reach, and in an area where the corollas grew shorter than they did elsewhere.
6. The significance of changes in the caloric content, volume, concentration, viscosity and sugar composition is discussed from the points of view of insects and ecologists.