Amino acids are universally present, in amounts up to about 4 mg/ml. We found a preference for proline concentrations in the order of magnitude of 1 to 10 mM, with a peak at about 6 mM. Preference decreased for both lower and higher proline concentrations. Interestingly, the concentration range preferred by honeybees is that found in several nectars, for example Lantana camara Alm et al. (1990), ornamental tobacco, and wild soybean species (this study). The nonlinear preference function for amino acid concentrations in nectar found by honeybees in this study and also for glycine. Proline (and hydroxyproline) are unique among the naturally occurring amino acids in that they can stimulate the salt cell of labellar chemosensory cells on the proboscis of some insects.

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