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First published online December 22, 2003
Journal of Experimental Biology 207, 545-552 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00762

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Metabolic activity and water vapour absorption in the mealworm Tenebrio molitor L. (Coleoptera, Tenebrionidae): real-time measurements by two-channel microcalorimetry

Lars L. Hansen, Hans Ramløv^* and Peter Westh^*

Department of Life sciences and Chemistry, Roskilde University, DK-4000 Roskilde, Denmark

^* Authors for correspondence (e-mail: hr{at}ruc.dk, pwesth{at}ruc.dk)

Accepted 24 October 2003

This work describes a new calorimetric method in which the metabolic heat production and water exchange rates of an insect larva are measured simultaneously and in real time. The experimental set-up is based on two independent calorimetric cells, which are perfused by a stream of air at controlled relative humidity (RH). The resolution for metabolic heat flow and water flux is 1 µW and 5 µg h^-1, respectively. The method was used to investigate water vapour absorption (WVA) in drought-stressed larvae of the common mealworm Tenebrio molitor. It was found that during exposure to a linear increment in RH of 3% per hour, the larvae initiated WVA upon passing a threshold value of 92.7±0.6%RH. The rate of water absorption subsequently increased to reach a maximal level of 86±6 µg h^-1, 10-15 h after passing the threshold value. Concomitantly, the RH in the calorimetric cell was reduced to 88.6±0.5%.

The metabolic heat production of the larvae was 5-6 J h^-1 g^-1 wet mass in the initial part of the experiment. However, this value doubled 2-3 h prior to the onset of WVA, when the RH had reached 88%. This increase in metabolic heat production gradually tapered off over the following 24 h of WVA, during which time WVA remained high. Animals exposed to RH protocols that did not induce WVA showed no such anomalies in metabolic heat flow. This may suggest that the increased metabolism reflects the preparation of the WVA apparatus. Finally, the method was used to quantify water losses in the microgram range associated with wriggling and tracheal ventilation.

Key words: water exchange, specific corrected metabolic heat flux, controlled relative humidity, water vapour absorption threshold, mealworm, Tenebrio molitor

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