Overheated or dried out

What determines the upper thermal tolerance of small insects water loss, temperature or energy balance?

By virtue of their large surface to volume ratio small insects are potentially prone to rapid and critical waterloss. Moreover small insect do often have high metabolic rates related to their small size and thus they may therefore suffer from energy depletion sooner than their larger conspecifics. Both waterloss and energy depletion will be attenuated by high temperature and recent discussions in the literature have suggested that assessment of upper critical temperature limits are often confounded by dehydration or starvation.

This question is very interesting from both a methodological, a physiological and a ecological perspective and we are currently planning a number of experiments to study this problem. The figure below illustrates a typical scenario for testing of heat tolerance of a small insect which often serves as a starting point for assessment of heat tolerance.

Figure 1: An example of the experimental conditions typically used during a ramping experiment. A small insect (here exemplified by a D. melanogaster female with a body-mass of 1 mg) is placed in a small closed container (5 ml). The amount of oxygen, carbon dioxide and water vapour are estimated using the assumption that the fly is loaded into the container under conditions of normal barometric pressure (760 mmHg), a room temperature of 20 °C and a relative humidity of 50%. The total water, lipid and glycogen content are calculated on the basis of the empirical observations in this paper. It is important to consider how thermal testing can potentially affect the energy and water resources of animals and this is largely determined by the fluxes of water and energy in the animal and obviously also by the conditions under which the animals are tested. In this scenario the internal conditions in the container will start to change immediately after the container is closed as a consequence of the animal’s respiration and transpiration and

An example of the experimental conditions typically used during a ramping experiment. A small insect (here exemplified by a D. melanogaster female with a body-mass of 1 mg) is placed in a small closed container (5 ml). The amount of oxygen, carbon dioxide and water vapour are estimated using the assumption that the fly is loaded into the container under conditions of normal barometric pressure (760 mmHg), a room temperature of 20 °C and a relative humidity of 50%. The total water, lipid and glycogen content are calculated on the basis of the empirical observations in this paper. As highlighted by recent studies (Rezende et al. 2011; Santos et al. 2011; Terblanche et al. 2011) it is important to consider how thermal testing can potentially affect the energy and water resources of animals and this is largely determined by the fluxes of water and energy in the animal and obviously also by the conditions under which the animals are tested. Immediately after the container is closed the internal conditions in the container will start to change as a consequence of the animal’s respiration and transpiration and as a consequence of the change in temperature. Thus, the animal will produce CO2 and water and use O2 while catabolizing its metabolic energy reserves. The animal will also exchange water over the respiratory and cuticular surfaces in a manner that is proportional to the product of whole animal “conductance” and the driving force for this water loss (the difference in partial pressure of water between the animal and its surroundings).

 

Involved people:

Johannes Overgaard

James O’Sullivan