Thermoregulation

Before the extracardiac pulsations were reported and before the tidal flow of hemolymph had been described in insects, Bernd Heinrich wrote about the use of the hemolymph in thermoregulation of flying insects. The optimum temperature for flight muscle contraction in many insects, such as the tobacco hornworm, Manduca sexta, is surprisingly high, up to 45°C. Before this moth can fly, it must warm the thorax to near this temperature, which it accomplishes by means of a series of simultaneous isometric contractions of the antagonistic pairs of flight muscles that appear to the casual observer as "shivering," or vibrations of the wings (Fig. 2).

A "thermometer" in the thoracic ganglia detects the proper temperature. When the thoracic temperature is below

FIGURE 2 Control of thoracic temperature by central nervous control of dorsal vessel contractions during external heating of the thorax (Heat). At the optimum temperature, hemolymph is pumped at maximum frequency and amplitude through the dorsal vessel to conduct heat from the thorax to the abdomen, where is it dissipated. [From Heinrich, B. (1970). Nervous control of the heart during thoracic temperature regulation in a sphinx moth. Science 169, 606-607, Copyright 1970 American Association for the Advancement of Science.]

FIGURE 2 Control of thoracic temperature by central nervous control of dorsal vessel contractions during external heating of the thorax (Heat). At the optimum temperature, hemolymph is pumped at maximum frequency and amplitude through the dorsal vessel to conduct heat from the thorax to the abdomen, where is it dissipated. [From Heinrich, B. (1970). Nervous control of the heart during thoracic temperature regulation in a sphinx moth. Science 169, 606-607, Copyright 1970 American Association for the Advancement of Science.]

optimum, the central nervous system signals the dorsal vessel to circulate hemolymph slowly. When the thoracic temperature rises above optimum, the central nervous system brings about maximal amplitude and rate of heartbeat to drive hemolymph through the thoracic muscles. The increased hemolymph flow pulls heat away from the flight muscles in the thorax and eventually delivers hot hemolymph to the abdomen, where the heat is dissipated. Then relatively cool hemolymph is redelivered to the thoracic muscles by the dorsal vessel, completing the thermoregulation cycle.

The warm hemolymph is then delivered to the head and percolates back past the ventral ganglia in the thorax to the abdomen, where the heat is dissipated. The cooler hemolymph is then delivered again to the thorax. The dorsal vessel and the very strong ventral diaphragm in the tobacco hornworm act together to move hemolymph in this analogy to an automobile radiator. When the thorax is too warm, both the amplitude and the frequency of heartbeat contractions are increased, and the rate of delivery of hemolymph increases. When the thorax is too cool, amplitude and frequency of contraction of the dorsal vessel are decreased. The activity of the ventral diaphragm acts in concert with that of the dorsal vessel.

Thermoregulation of the flight muscles of the tobacco hornworm implies a sophisticated nervous control. The overall nervous control can be easily demonstrated by severing the ventral nerve cord between the thorax and abdomen. When this is done, the moth can no longer thermoregulate because the feedback loop of temperature detection by the thoracic ganglia has been destroyed, and control over ventral diaphragm and dorsal vessel contractions has been lost. An extreme form of modified circulatory system to accommodate thermoregulation is shown in Fig. 3.

0 0

Post a comment