Isotopic enrichment without change in diet: an ontogenetic shift in δ15N during insect metamorphosis

Abstract

Summary 1Insects are frequently sampled in food web studies and are assumed to follow the pattern of enrichment in 15N across trophic levels observed for other organisms. However, few studies have examined 15N discrimination of metamorphosing insects. 2We measured the δ15N of larvae, pupae and adults of four Lepidoptera (Bombyx mori, Galleria mellonella, Manduca sexta and Vanessa cardui), one Diptera (Sarcophaga sp.) and one Coleoptera (Tenebrio molitor) fed on artificial diets. 3The tissues of larvae were enriched in 15N relative to their diet and this enrichment was explained by the production of 15N-depleted frass. Surprisingly, the tissues of adults were enriched in 15N relative to larvae in all but one species (T. molitor). Because, we measured the tissues of adults immediately after they emerged from pupal cases, the enrichment was not due to a change in diet. Rather, it was due to the excretion of 15N depleted metabolic waste (‘meconium’) that resulted from protein metabolism during metamorphosis. 4To our knowledge, the 15N enrichment that seems to accompany metamorphosis in holometabolous insects had not been previously recognized. This enrichment must be accounted for in ecological studies that rely on stable isotopes to identify both the trophic position and diet of insects.