Abstract

The honeybee, Apis mellifera L. (Hymenoptera: Apidae), has recently become a model for studying brain
asymmetry among invertebrates. A strong lateralization favouring the right antenna was discovered in
odour learning and short-term memory recall experiments, and a lateral shift favouring the left antenna
for long-term memory recall. Corresponding morphological asymmetries have been found in the distribution
of olfactory sensilla between the antennae and confirmed by electrophysiological odour response
measurements in isolated right and left antennae. The aim of this study was to investigate whether a
morphological asymmetry can be observed in the volume of the primary olfactory centres of the central
nervous system, the antennal lobes (ALs). Precise volume measurements of a subset of their functional
units, the glomeruli, were performed in both sides of the brain, exploiting the advantages of two-photon
microscopy. This novel method allowed minimal invasive acquisition of volume images of the ALs, avoiding
artefacts from brain extraction and dehydration. The study was completed by a series of behavioural
experiments in which response asymmetry in odour recall following proboscis extension reflex conditioning
was assessed for odours, chosen to stimulate strong activity in the same glomeruli as in the
morphological study. The volumetric measurements found no evidence of lateralization in the investigated
glomeruli within the experimental limits. Instead, in the behavioural experiments, a striking
odour dependence of the lateralization was observed. The results are discussed on the basis of recent
neurophysiological and ethological experiments in A. mellifera.