Sexual dimorphism in the proventriculus of the buff-tailed bumblebee Bombus terrestris (L. 1758) (Hymenoptera: Apidae)

Research on eusocial bee species like Bombus terrestris is primarily focused on the worker caste, which is why their morphology and anatomy are already well described. This includes the alimentary tract, which is adapted for feeding on nectar and pollen. Located at the transition between crop and ventriculus is a highly specialised compartment, the proventriculus. In female workers of B. terrestris, the proventriculus is surrounded by muscles and consists of four anterior lips. A detailed description, however, is only provided for B. terrestis worker bees while studies on the proventriculus of the male reproductive caste are absent. Here, we provide a detailed analysis of the differences between the proventriculus of the B. terrestris males and females through morphometrics, histology and scanning electron microscopy imaging, and unravel a distinct sexual dimorphism. The male proventriculus is wider resulting in a greater volume than the female proventriculus. Histological analysis revealed 4 distinctive chambers of the male proventriculus, which are completely covered with hairs on the inside. In contrast, those chambers in the proventriculus of female B. terrestris, are only rudimentarily present forming only small pouches with hairs in the junctions between the proventricular folds inside the proventriculus. The morphological differences in the proventriculus may be based on different modi vivendi, as males do not return to the colony and fly longer distances. This and the synthesis of sperm and mating plug might require higher energy reserves, leading to the necessity of higher food storage capacities.