Isotopic fractionation with morphological change and sexual specificity in the lappet moth Euthrix potatoria

The delta N-15 values of adult holometabolous insects exceed those of larvae, but otherwise little information on terrestrial invertebrates has been obtained in food-web analyses using stable isotope ratios (delta N-15, delta C-13). Changes in delta C-13 during metamorphosis and differences between males and females have not been examined. We collected the larvae and cocoons of Euthrix potatoria (L.) (Lepidoptera: Lasiocampidae) in the field and used them to assess the species' isotopic fractionation. Each emerged moth was divided into five body parts. We conducted stable N and C isotope analyses for each body part, as well as for cocoons and exuviae, and also compared stable isotope ratios between sexes. We confirmed delta N-15 enrichment through metamorphosis and estimated that delta N-15 enrichment is accomplished by the relative concentration of N-15 due to the excretion of copious meconium, which contains abundant N-14. We also observed changes in delta C-13 values through metamorphosis. Both isotope values tended to change more in males than in females. The proportion of the whole-adult weight represented by meconium was higher inmales than in females, suggesting that high meconium secretion in males contributes to the sexual difference in delta N-15. These phenomena may be common in Holometabola, which require a pupal stage. For more accurate food-web assessments, it is important to consider stable isotope changes during different life cycles, as well as sexual differences.