Physiological energetics of a midge, Chironomus riparius Meigen (Insecta, Diptera): normoxic heat output over the whole life cycle and response of larva to hypoxia and anoxia.

The rate of metabolism of laboratory reared Chironomus riparius was monitored by direct calorimetry over the entire life cycle from egg to adult stage. The metabolic response of the fourth instar larva to decreasing oxygen concentrations and anoxia was also measured. Normoxic measurements were carried out at 20°C and the hypoxic-anoxic experiments at 10°C. In larvae with body sizes ranging from 0.0028 to 0.645 mg ash-free dry mass (afdm), the rate of heat dissipation was related to body mass by a power function, with a mass exponent of 0.71±0.02 corresponding to an exponent of -0.29 for the relationship between mass-specific metabolic rate and body mass. However, the allometric equations applicable to larvae would not predict the metabolic rates of eggs, pupae and adults. Single egg batches used in the experiments consisted of 354±90 eggs, the individual egg with a mass of 0.99±0.01 μg (mean±SD). The mass-specific rate of heat dissipation of the egg (13.7±1.8 μ W mg-1 afdm) was considerably lower than that of the first and second instar larvae (44-53 μ W mg-1) but equal to that of fourth instar larvae (13.1±3.9 μ W mg-1). Heat dissipation by a pupa shortly before adult emergence was high (14.8±1.8 μ W mg-1), probably due to high metabolism during metamorphosis. Emergence of the adult in the calorimeter was indicated by a short but intense burst of heat. The newly emerged imago had a ca. 20-35% higher metabolic rate than the pupa. In response to reduced O2 partial pressure the fourth instar larva of C. riparius displayed metabolic regulation. In continuously declining oxygen partial pressure, the fourth instar larva maintained its aerobic energy metabolism (4.2 μ W mg-1) with only a small decrease down to 0.8 kPa, corresponding to an oxygen concentration of 0.42 mg O2l-1 H2O. Below this critical oxygen concentration (Pc), the rate of heat dissipation decreased rapidly down to the anoxic level which was only 14-17% of the normoxic level. The high relative reduction of metabolic rate under anoxia gives a wrong impression of short-term tolerance of C. riparius to anoxia. The absolute energetic costs of C. riparius associated with anaerobic energy metabolism (0.64±0.11 μ W mg-1) are almost 6 times higher than those of more anoxia tolerant invertebrates such as sphaeriid bivalves.