Digestion of Pollen Components by Larvae of the Flower-Specialist Bee Chelostoma florisomne (Hymenoptera: Megachilidae)

Abstract

Pollen digestion in larvae of the solitary bee Chelostoma florisomne (Megachilidae), a pollen-specialist (oligolectic) species on Ranunculus spp. (Ranunculaceae), was investigated using histological techniques. Serial sections from the anterior-, median-, and posterior-midgut/hindgut regions of the larval alimentary canal were selectively stained and examined for disappearance of lipids, proteins, and carbohydrates from ingested Ranunculus pollen grains. During digestion, pollen contents were observed to gradually extrude through the grain apertures. Most of the digestion of lipids and proteins occurred in the anterior- and median-midgut, whereas carbohydrates disappeared more slowly and mainly in the median- and posterior-midgut. In the hindgut, pollen comprised mainly empty exine shells, which were crushed and often broken, suggesting that some components of the wall are degraded during digestion, causing collapse of the outer, chemically resistant wall layer (exine). Lipids were completely digested, but small quantities of proteins and carbohydrates were often still present in grains in the posterior midgut/hindgut. The pollenkitt appeared to be completely absorbed, with none evident in the posterior midgut or in larval feces. The findings indicate that this oligolectic bee has the digestive equipment necessary to effectively remove and absorb the nutrients present in the pollen of its restricted food source. Comparison with similar studies in adult honey bees suggests that differing digestive abilities might be an underlying factor in pollen specialization in bees.