Recent data suggest that oxygen limitation may induce moulting in larval insects. This oxygen-dependent induction of moulting (ODIM) hypothesis stems from the fact that the tracheal respiratory system of insects grows primarily at moults, whereas tissue mass increases massively between moults. This may result in a mismatch between oxygen supply and demand at the end of each larval instar because oxygen demand of growing tissues exceeds the relatively fixed supply capacity of the respiratory system. The ODIM hypothesis predicts that, within larval instars, respiration and metabolic rates of an individual larva first increase with increasing body mass but eventually level off once the supply capacity of the tracheal system starts to constrain metabolism. Here, we provide the first individual-level test of this key prediction of the ODIM hypothesis. We use a novel methodology where we repeatedly measure respiration and metabolic rates throughout the penultimate- and final-instar larvae in the butterfly Pieris napi. In the penultimate instar, respiration and metabolic rates gradually decelerated along with growth, supporting the ODIM hypothesis. However, respiration and metabolic rates increased linearly during growth in the final instar, contradicting the prediction. Moreover, our data suggest considerable variation among individuals in the association between respiration rate and mass in the final instar. Overall, the results provide partial support for the ODIM hypothesis and suggest that oxygen limitation may emerge gradually within a larval instar. The results also suggest that there may be different moult induction mechanisms in larva-to-larva moults compared with the final metamorphic moult.
The authors declare no competing or financial interests.
S.M.K., P.L. and K.G. designed the study, S.M.K. and P.L. conducted the experiment, S.M.K. and P.L. analyzed the data, and S.M.K., P.L. and K.G. wrote the paper.
This study was financed by the international fellowship program at Stockholm University (to S.M.K.), the Finnish Cultural Foundation (Suomen Kulttuurirahasto; to S.M.K.), the strategic research programme EkoKlim at Stockholm University (to K.G.), the Knut and Alice Wallenberg Foundation [Knut och Alice Wallenbergs Stiftelse; grant no. 2012.00558 to K.G.] and the Swedish Research Council [Vetenskapsrådet; grant no. 2012-3715, K.G. co-applicant].
Data are available from the Dryad Digital Repository http://dx.doi.org/10.5061/dryad.dr559
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.140442.supplemental
- Received March 14, 2016.
- Accepted July 19, 2016.
- © 2016. Published by The Company of Biologists Ltd