Influence of environmental conditions on the breathing rhythms of the pine weevil Hylobius abietis (Coleoptera: Curculionidae)
¹Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Kreutzwaldi St. 5, EE51014 Tartu, Estonia; e-mail: email@example.com
²Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Kreutzwaldi St. 64, EE51014 Tartu, Estonia
The metabolic rate and respiratory patterns of adults of the large pine weevil, Hylobius abietis (L.) (Coleoptera: Curculionidae) was compared in hydrated and dehydrated conditions using a constant volume electrolytic respirometer-actograph combined with an infrared actograph. The weevils displayed continuous pumping movements in hydrated conditions (hydrated individuals) while the cyclic gas exchange was entirely lacking or only unclear small releases of CO2 (bursts, B) were observed. However, when a short period of quiescent state (tonic immobility) was artificially induced by a mechanical stimulus, 3–6 clear cycles of gas exchange appeared, with the frequency of 14–18 cycles per hour, i.e. the discontinuous gas exchange cycles or DGCs were displayed. These were recognized as CFO (closed, flutter, open) type of gas exchange cycles known by closed-flutter-open phases of spiracular movements without active ventilation, i.e. pumping movements, during the bursts. At 0oC, when the muscular activity was suppressed, all the hydrated weevils showed CFO cycles, nearly one burst per hour. When H. abietis adults were kept in dry conditions without food for 3 days (dehydrated state), CFV (closed, flutter, ventilation) cycles appeared with the frequency of 5–6 cycles per hour, while the bursts were associated with the pumping movements of the abdomen. Dehydration caused lengthening of the flutter period nearly two times. During the flutter, regular miniature inspirations were recorded. After being in the dehydrated state for 5 days, the adults of H. abietis displayed CFO and CFV cycles with a frequency of about one cycle per hour, while during the extremely long interbursts periods clear inspiration movements were recorded. Metabolic rate in the hydrated weevils was 0.36 +- 0.05 ml O2 g-1 h-1, but after they had been kept in dehydrated conditions without food for 5 days, metabolic rate decreased essentially, being only 0.16 +- 0.02 ml O2 g-1 h-1. The authors suggested that in hydrated H. abietisadults the lacking of clear cyclic gas exchange was due to the almost continuous pumping movements, or active ventilation, externally not observable. The active ventilation without DGC was obviously a normal mode of respiration in the hydrated weevils. However, the hydrated weevils were able to display the cyclic gas exchange during the forced quiescent periods but the artificially evoked tonic immobility was considered as a stress state. Thus our results supported the hypothesis that DGC is a water conserving mechanism in beetles.