Using Microrespirometers to Measure O2 Consumption by Insects & Small Invertebrates

A variety of physiological studies require the measurement of oxygen consumption. Unfortunately, the techniques for measuring respiration rate often require expensive equipment or difficult procedures that are not available or suitable for use in student laboratories. In this article I describe a sensitive, simple and inexpensive microrespirator that may be used readily by students. This constant pressure microrespirometer is similar to ones described by Engelmann (1963) and Conradi-Larsen (1974). It is useful for a range of invertebrates weighing 5500 mg and for measurements at low, even subzero, temperatures (Lee & Baust 1982a,b; Bennett & Lee 1989). For smaller animals or at very low rates of oxygen consumption, several individuals may be run in a single microrespirometer. A general discussion of respirometric methods is available in Umbreit, Burris & Stauffer (1972). For every molecule of oxygen that is taken up by an organism, one molecule of carbon dioxide is released when carbohydrate is the primary energy source. In a closed system, normal respiration causes no net change in atmospheric pressure because each molecule of oxygen that is removed is replaced by one of carbon dioxide. However, in this microrespirometry technique, when carbon dioxide molecules are released they are removed from the atmosphere and taken up by a potassium hydroxide solution. This results in a decrease in pressure within the closed environment and causes the potassium hydroxide solution to move down the micropipet. Since the movement of the potassium hydroxide is directly proportional to the amount of oxygen consumed, by knowing the volume of the micropipet and the measurement interval, one can easily determine the rate of oxygen consumption.