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Flight-motor-driven respiratory air flow in the hawkmoth Manduca sexta

Lutz T. Wasserthal^*

Institut für Zoologie I, Universität Erlangen-Nürnberg, Staudtstrasse 5, D-91058 Erlangen, Germany

View larger version (30K): [in a new window]   Fig. 1. Split-specimen chamber used for measuring CO2 emission from specified spiracles of hawkmoths. The device allows the air pressure in both chambers to be measured and adjusted. In A, the anterior chamber with the air flow from the anterior spiracles is connected to the CO2 analyser directly, and the posterior chamber with the air flow from all other spiracles is connected to the CO2-absorbing vessel. In B, the air flow passing through the posterior chamber is conveyed directly to the CO2 analyser, while the air flow from the anterior chamber passes via the CO2-absorbing vessel. The wingbeat is recorded by projecting the shadow of the wings onto a silicon photocell installed on the bottom of the transparent experimental chamber. SpI, mesothoracic spiracle; SpII, metathoracic spiracle; Spabd, abdominal spiracles.

View larger version (58K): [in a new window]   Fig. 2. Semi-schematic cross section of the region between the posterior mesothorax and anterior metathorax at the level of the mesoscutellar air sac showing the positions of the mounting rod and artificial spiracle (based on X-ray tomography and histological sections). DLM II, DLM III, dorsolongitudinal muscles of the mesothorax and metathorax, respectively; DVM III, dorsoventral muscle of the metathorax.

View larger version (60K): [in a new window]   Fig. 3. Intratracheal pressure relative to atmospheric pressure during shivering (A) and steady flight (B) in tethered Manduca sexta. During shivering, the mean pressure is approximately equal to atmospheric pressure (0Pa on this scale). During flight, the mean pressure at the anterior spiracles (SpI, horizontal broken line) is negative, whereas that at the mesoscutellar air sac (ScutII, horizontal thick line) is positive. The inset moth pictures show wing position and wingbeat amplitude. Sampling rate was 40kHz.

View larger version (82K): [in a new window]   Fig. 4. Intratracheal pressure measured at the anterior spiracles (spiraclesI) and mesoscutellar air sac during steady flight in Manduca sexta and with increased wingbeat amplitude during forced flight. At the anterior spiracles, the mean pressure is negative, and pressure pulse amplitude increases with wingbeat amplitude, with mean pressure (white line) becoming more negative. At the mesoscutellar air sac, the mean pressure (white line) is positive and is less affected by the increased wingbeat amplitude. Wingbeat amplitude was measured as the amount of shade cast by the moving wing onto a laterally arranged silicon photocell. Maximum values of shading curves (the lowest voltage produced by the photocell) correspond to highest amplitudes. Sampling rate was 40kHz.

View larger version (31K): [in a new window]   Fig. 5. Effects of wingbeat amplitude on mean intratracheal pressure during flight in Manduca sexta. (A) At the anterior spiracles (spiracles I), the wingbeat amplitude maxima coincide with the pressure minima. (B) At the mesoscutellar air sac, the wingbeat amplitude maxima coincide with the pressure maxima. Recording techniques are as described in the legend to Fig.4. Voltage values for wingbeat curves have been converted into angular degrees of wing position from 0° (no flight) to 110° (with maximum angle corresponding to maximum wing-stroke amplitude during steady flight).

View larger version (31K): [in a new window]   Fig. 6. Intratracheal pressure changes measured at the anterior spiracle (spiracleI) and at the mesoscutellar air sac during a single wingbeat in Manduca sexta. Recording techniques are as described in the legend to Fig.4. The pressure minima occur during the downstroke (110mV), the maxima during the upstroke (128mV).

View larger version (43K): [in a new window]   Fig. 7. CO2 emission during steady flight in a male Manduca sexta (mass 1.5g, 9 days old). The results are from a split-specimen chamber (see insets at the top of the figure). In A, only air from the anterior chamber (connected to the anterior spiracles) was conveyed to the CO2 analyser directly, while air from the posterior chamber first passed through a vessel containing NaOH to absorb CO2. No CO2 emission was detected from the anterior spiracles (spiracles I), although the air presssure in the anterior chamber was slightly lower than in the posterior chamber. In B, only the air flow from the posterior chamber (including the posterior thoracic, spiracles II, and all abdominal spiracles) was conveyed to the CO2 analyser without first passing through the NaOH vessel. All CO2 is expired from these posterior spiracles. Sampling rate was 400Hz.

View larger version (30K): [in a new window]   Fig. 8. Induced respiratory flow reversal causing CO2 emission from the anterior spiracles (spiracles I) during flight of a male Manduca sexta (mass 1.4g, 10 days old). Results from a split-chamber experiment (see inset at the top of the figure) with stepwise pressure increases applied to the posterior chamber. CO2 emission from the anterior spiracles (SpI) begins when the pressure difference P between the posterior chamber and the anterior chamber exceeds 25Pa. Sampling rate was 400Hz.

View larger version (44K): [in a new window]   Fig. 9. Effects of intermittent flight activity on CO2 emission from the anterior spiracles (spiracles I) in a split-chamber experiment. Application of an artificial pressure difference P of approximately 50Pa between the two chambers produced a CO2 output from the anterior spiracles (SpI) of approximately 2.5µls-1. During the pauses between flights, CO2 output rises to approximately 3.5µls-1; it then decreases again during the flight phases (stippled) with a latency of approximately 1s. Sampling rate was 400Hz.

View larger version (178K): [in a new window]   Fig. 10. Scanning electron micrographs of the anterior thoracic spiracle after descaling the prothoracic cuticle. (A) Peritrema with dense cuticular filter lamellae. (B) After removal of most of the filter lamellae, the half-opened inner valve lip (IL) and the orifice (O) are visible. (C) Outer layers of the filter meshwork; enlargement of boxed region in A. (D) Transverse section of the base of the anterior lamella revealing that approximately six rows of cuticle processes are connected by anastomoses. Enlargement of boxed region from B.

View larger version (129K): [in a new window]   Fig. 11. Details of the posterior thoracic spiracle after descaling the cuticle around the wing hinge. (A,B) Light scanning micrographs. (A) Subalar intersegmental cleft with stretched intersegmental membrane (ISM) in extreme wing-up position showing the orifice (O) of the posterior spiracle with wide open external lip (L). (B) Intersegmental cleft constricted in the wing-down position enclosing the posterior spiracle. The metathorax is adducted to the mesothorax by contraction of the dorsal longitudinal muscles along the direction of the arrow. BP, basalar pad; FW, base of forewing; Pl II, pleuron of mesothorax; Pl III, pleuron of metathorax. (C,D) Scanning electron micrographs. (C) External valve lip (L) with marginal spines and spinose perispiracular cuticle (arrowheads). (D) Detail of cuticle at the transition between smooth tracheal intima and spinose perispiracular cuticle (see arrowheads in C).

View larger version (27K): [in a new window]   Fig. 12. Schematic lateral view of the thorax of Manduca sexta showing the deformation (arrows in upper diagrams) of the thorax during the downstroke (A) and upstroke (B) of the wings and its coupling with the adduction and retraction (arrowheads) of the metathorax enclosing and exposing the posterior thoracic spiracle (SpII) in the subalar cleft. The increase in volume of the thoracic air sacs and closure of SpII during the downstroke produce a retrogradely directed air stream, with inspiration through the anterior spiracles (SpI) and expiration through the posterior thoracic spiracles (broken arrows). I, prothorax; II, mesothorax; III, metathorax.