Physiology of heartbeat reversal in diapausing pupae of the tobacco hornworm, Manduca sexta (Lepidoptera: Sphingidae)

Abstract

Pulsations of dorsal vessel were monitored by the noninvasive techniques of contact thermography on the dorsal cuticle and by strain gauge detection of abdominal elongation movements. Diapausing pupae exhibited periods of forward-oriented, or anterograde pulsations (average duration of each pulsation 5-8 min, frequency of individual systolic strokes 10-15 per min) alternating with somewhat slower, backward-oriented or retrograde cardiac pulsations (average duration of each pulsation 6-10 min, frequency of systolic strokes 7-12 per min). The highest rate of hemolymph flow was associated with the anterograde pulsations. We studied cardiac functions in diapausing pupae because of the almost complete absence of extracardiac hemocoelic pulsations, which are much stronger and could interfere with the recordings of heartbeat in all other developing stages. The movement of abdomen associated with the heartbeat was extremely small, only some 0.14 to 0.9 µm (i.e. from one 428000th to one 66000th of the body length) and thus was not practical for routine recordings of heartbeat.Simultaneous recordings from multiple thermographic sensors revealed the complete absence of retrograde cardiac pulsations in the head region. There are some indications that the retrograde pulsations were also lacking in the thoracic region of the aorta. The retrograde peristalsis appeared to be used for circulatory functions in the abdomen alone. By contrast, the anterograde cardiac pulsations underwent a profound amplification in the anterior part of the abdomen, entering thoracic aorta with considerable strength before reaching the final destination in the head region. The amplification of anterograde peristalsis was manifested by enhanced hemolymph flow towards the head associated with a two-fold increase in frequency of anterograde heartbeat before reaching the head region. The sensors distributed along the dorsal vessel revealed that the rate of the backward-oriented, retrograde cardiac flow of the hemolymph was also location specific. The rate of flow was lowest at the front of the abdomen, medium in the middle and highest close to the end of the abdomen. The finding of lowest hemolymph circulation at the beginning of the cardiac peristaltic waves suggested that the physiological raison d' etre for heartbeat reversal was a need for differential enhancement of hemolymph flow towards the extremities of the immobile pupal body. The switchovers from the retrograde to anterograde cardiac pulsations were usually immediate, while the reciprocal, antero- to retro-switchovers were mostly associated with a brief cardiac arrest. Increasing temperature gradients (in 5°C steps) progressively diminished duration of both reciprocal heartbeat periods. The amplitudes of the cardiac systolic strokes also decreased with increasing temperature while the frequencies were substantially elevated.