Histology of the ileum in bees (Hymenoptera, Apoidea)

ABSTRACT

In most hymenopterans, the ileum extends as a long tube from the midgut to the rectum, and shows nomarked anatomical specialization. The function of the ileum is not fully understood, although the presenceof an epithelial layer of cuboidal or columnar cells with apical and basal plasma membrane infoldingssuggests that this organ may be involved in water and nutrient absorption. In this work, we investigatedthe ileal morphology of 47 species of bees in the Andrenidae, Apidae (Apini, Meliponini, Xylocopinae,Centridini, Bombini, Eucerini, Euglossini, Tapinotaspidini, Exomalopsini), Halictidae and Megachilidae,as part of a study to understand the physiological and phylogenetic importance of this organ. In all cases,the ileum consisted of an epithelium containing cuboidal or columnar cells that usually had basal nuclei andapical plasma membrane infoldings, although there were variations in cell shape, position of the nucleus,degree of chromatin condensation and cuticle thickness. The epithelial cells were covered with a cuticle andtransverse sections revealed the presence of 4-6 folds projecting into the lumen. The cell cytoplasm belowthe apical plasma membrane infoldings contained numerous vacuoles of different sizes. A single layer ofcircular muscle was located beneath the epithelium. The histological organization of the ileum suggesteda role in the absorption of luminal solutes as a mechanism for regulating the hemolymph and the generalosmotic balance of these insects. However, there was no relationship between the structural organization ofthe ileum and the degree of social development and/or phylogeny of the bees.