SUMMARY Mechanical power output and metabolic power input were measured from an asynchronous flight muscle, the basalar muscle of the beetle Cotinus mutabilis . Mechanical power output was determined using the work loop technique and metabolic power input by monitoring CO 2 production or both CO 2 production and O 2 consumption. At 35°C, and with conditions that maximized power output (60 Hz sinusoidal strain, optimal muscle length and strain amplitude, 60 Hz stimulation frequency), the peak mechanical power output during a 10 s burst was approximately 140 W kg –1 , the respiratory coefficient 0.83 and the muscle efficiency 14–16 %. The stimulus intensity used was the minimal required to achieve a maximal isometric tetanus. Increasing or decreasing the stimulus intensity from this level changed mechanical power output but not efficiency, indicating that the efficiency measurements were not contaminated by excitation of muscles adjacent to that from which the mechanical recordings were made. The CO 2 produced during an isometric tetanus was approximately half that during a bout of similar stimulation but with imposed sinusoidal strain and work output, suggesting that up to 50 % of the energy input may go to muscle activation costs. Reducing the stimulus frequency to 30 Hz from its usual value of 60 Hz reduced mechanical power output but had no significant effect on efficiency. Increasing the frequency of the sinusoidal strain from 60 to 90 Hz reduced power output but not CO 2 consumption; hence, there was a decline in efficiency. The respiratory coefficient was the same for 10 s and 30 s bursts of activity, suggesting that there was no major change in the fuel used over this time range. The mass-specific mechanical power output and the efficiency of the beetle muscle were each 2–3 times greater than values measured in previous studies, using similar techniques, from locust flight muscles, which are synchronous muscles. These results support the hypothesis that asynchronous flight muscles have evolved in several major insect taxa because they can provide greater power output and are more efficient than are synchronous muscles for operation at the high frequencies of insect flight.

The force-velocity relationships during isotonic shortening were determined for the metathoracic second tergocoxal muscle of the locust Schistocerca americana (Drury). This muscle is a synchronous flight muscle. During the plateau of a tetanic contraction, the maximum shortening velocity (V max ) determined from the force-velocity curve was 5.2 muscle lengths s −1 (25°C) and the curvature (a/P o ) was 0.62. The maximum isometric force ( P 0 ) was 36.3 N cm −2 . Early in a twitch (at times shorter than the isometric twitch rise time) the values for V max and curvature were similar to those during the tetanic plateau, but the curves at different times during the twitch intercepted the force axis at values less than P 0 . Later in the twitch, V max declined. A variable termed degree of activation (DA) is developed as a measure of the force-generating capacity of a muscle when this may be time-varying, as throughout most of a twitch. DA is determined from the shortening velocity at an intermediate load and is the predicted intercept of the force-velocity curve with the force axis relative to the tetanic intercept. In the locust muscle, DA rose to a maximum in 2–3 ms after the end of the latent period. DA reached 80° of the tetanic value during a single twitch; during the second twitch of a pair, the peak DA reached approximately the tetanic value. After a brief plateau, DA declined approximately exponentially. The time constant of DA decay was about 14 ms. Note: Present address: Department of Physiology, Emory University, Atlanta, GA30322, USA.

1. Mechanical work output was determined for an indirect flight muscle, the first dorsoventral, of the tobacco hawkmoth Manduca sexta . Work output per cycle was calculated from the area of force-position loops obtained during phasic electrical stimulation (1 stimulus cycle −1 ) and imposed sinusoidal length change. There was an optimal stimulus phase and an optimal length change (strain) that maximized work output (loop area) at constant cycle frequency and temperature. 2. When cycle frequency was increased at constant temperature, work output first increased and then decreased. It was always possible to find a frequency that maximized work output. There also always existed a higher frequency (termed the ‘optimal’ frequency in this paper) that maximized the mechanical power output, which is the product of the cycle frequency (s −1 ) and the work per cycle (J). 3. As temperature increased from 20 to 40°C, the mean maximum power output increased from about 20 to about 90 W kg −1 of muscle (Q 10 =2.09). There was a corresponding increase in optimal frequency from 12.7 to 28.3 Hz, in the work per cycle at optimal frequency from 1.6 to 3.2Jkg −1 muscle and in mean optimal strain from 5.9 to 7.9%. 4. Two electrical stimuli per cycle cannot increase power output at flight frequencies, but if frequency is reduced then power output can be increased with multiple stimulation. 5. Comparison of mechanical power output from muscle and published values of energy expenditure during free hovering flight of Manduca suggests that mechanical efficiency is about 10%. 6. In the tobacco hawkmoth there is a good correspondence between, on the one hand, the conditions of temperature (35–40°C) and cycle frequency (28–32 Hz) that produce maximal mechanical power output in the muscle preparation and, on the other hand, the thoracic temperature (35–42°C) and wing beat frequency (24–32 Hz) observed during hovering flight.

The relationships between muscle length, fractional change in length (strain) and work output during cyclic contraction were examined in scaphognathite levator muscle L2B of the green crab Carcinus maenas (L.). The muscle was subjected to sinusoidal strain at 2 Hz and to phasic stimulation in the strain cycle. At an average length and stimulus phase which are optimum for net work output, the work from muscle L2B during shortening rises to a peak or a plateau with increasing strain. The failure of shortening work to increase continuously with strain is due, in part, to the greater shortening velocity associated with greater strain, and to the consequent reduction in muscle force during shortening at higher velocity. The work required to re-lengthen a muscle following contraction is a complex function of strain, with an initial peak followed by a work minimum and then a monotonic rise in work with further increase in strain. The early work minimum is a result of shortening inactivation which reduces muscle force and thus the work which must be done to re-lengthen the muscle. Because shortening work rises to a peak or plateau with increasing strain while lengthening work, for the most part, increases with strain, there is a sharp optimum strain (about 8%) for net work output. Muscle relaxation becomes slower with increasing muscle length. As muscle length is increased, fusion of tension from cycle to cycle becomes more pronounced and shortening inactivation becomes a more important determinant of optimum strain.

The mechanical power output was measured from scaphognathite (SG = gill bailer) muscle L2B of the crab Carcinus maenas (L.). The work was determined from the area of the loop formed by plotting muscle length against force when the muscle was subjected to sinusoidal length change (strain) and phasic stimulation in the length cycle. The stimulation pattern (10 stimuli per burst, burst length = 20% of cycle length) mimicked that which has been recorded from muscle L2B in intact animals. Work output was measured at cycle frequencies ranging from 0.5 to 5 Hz. The work output at optimum strain and stimulus phase increased with increasing cycle frequency to a maximum at 2–3 Hz and declined thereafter. The maximum work per cycle was 2.7 J kg −1 (15 °C). The power output reached a maximum (8.8 W kg −1 ) at 4 Hz. Both optimum strain and optimum stimulus phase were relatively constant over the range of burst frequencies examined. Based on the fraction of the total SG musculature represented by muscle L2B (18%) and literature values for the oxygen consumption associated with ventilation in C. maenas and for the hydraulic power output from an SG, we estimate that at a beat frequency of 2 Hz the SG muscle is about 10% efficient in converting metabolic energy to muscle power, and about 19% efficient in converting muscle power to hydraulic power.

1. Contraction of scaphognathite muscle L2B of the green crab Carcinus maenas is strongly dependent on stimulus number and frequency. Single, supramaximal stimuli evoke little or no tension. When stimulated with shocks in either short bursts (10 stimuli in 0.5s or less) or long bursts (5 s of stimulation), the isometric tension from the muscle increases with increasing stimulus frequency to a maximum at about 150 Hz at 15°C, beyond which tension declines with further increase in stimulus frequency. 2. There can be facilitation of both contraction and relaxation between short bursts of stimuli. Facilitation of contraction is seen as increasing tension on successive bursts of a series, even when the interburst interval is long enough for relaxation to be completed during the interval. Interburst facilitation lasts at least 10 s. Facilitation of relaxation is seen as progressively faster relaxation from burst to burst of a series, and relaxation to lower tension levels when the interburst interval is so short that relaxation is incomplete in the interburst interval. 3. Maximum isometric tension occurs at muscle lengths slightly longer than the longest muscle length reached in vivo . Tension declines rapidly with changes in muscle length away from the optimum length. The maximum isometric tension was about 12 N cm −2 . 4. The maximum shortening velocity of a tetanically activated muscle was determined as 1.9 lengthss −1 (Ls −1 ) by extrapolation of force-velocity curves to zero force and 3.3 Ls −1 by slack test measurements. 5. The scaphognathite muscle would be classified as a slow or tonic muscle on the basis of its requirements for multiple stimulation to reach full activation, and as a moderately fast muscle on the basis of its force-velocity properties.

1. The male cicada, Okanagana vanduzeei , produces a calling song with a pulse repetition frequency of 550 Hz. This sound is produced by a pair of tymbals, each of which is buckled by a large tymbal muscle. Males sing in full sun and the operating temperature of the tymbal muscles is 40–45°C. 2. Analysis of the songs of animals with the tymbal mechanism destroyed on one side, and of the sounds produced by directly manipulating a tymbal, indicates that the two tymbals normally buckle synchronously and that only one sound pulse is produced per tymbal muscle contraction. This implies that the contraction frequency of each tymbal muscle is 550 Hz. 3. Recordings of calling songs from animals with implanted electrodes show that there is usually synchrony between left and right tymbal muscle contractions and that each tymbal muscle can operate at a frequency of about 550 Hz. The recordings also show that there is a 1:1 correlation between muscle electrical and mechanical activity, i.e. these muscles are synchronous and not asynchronous muscles. 4. The ultrastructure of the tymbal muscle is clearly that of a very fast, synchronous muscle: the myofibrils are small, the sarcoplasmic reticulum is extraordinarily well developed, and the T-tubules lie at the 1/4 and 3/4 positions along the sarcomere. 5. When set up for isometric recording, with their nerve supply severed, the tymbal muscles often show spontaneous electrical and mechanical oscillations. The frequency of these oscillations is strongly temperature dependent, and at higher temperatures approaches the normal operating frequency of the muscle. The tendency to oscillate is so strong that single twitches could only very rarely be evoked by electrical stimulation. A twitch duration of under 6 ms was observed at 30°C. 6. We conclude that the tymbal muscle of O. vanduzeei is indeed a synchronous muscle, albeit a highly modified one, capable of operating at 550 Hz. We suggest that the advantages of asynchronous muscles lie chiefly in their greater economy of structure and operation, rather than in an ability to operate at higher frequencies than synchronous muscles. Note: This paper is dedicated to the late Professor J. W. S. Pringle who was a pioneer in studies of cicada muscles and a major contributor to our understanding of insect muscle in general. We think he would have enjoyed the new perspective given by the tymbal muscles of Okanagana vanduzeei .

1. The mesothoracic wings of tettigoniid insects are used in song production and flight; the metathoracic wings in flight only. In Neoconocephalus triops the wing stroke frequency during flight is about 25 Hz; the frequency during singing about 100 Hz. 2. The twitch duration of mesothoracic, first tergocoxal (Tcxl) wing muscles is only about one-half the duration of the upstroke or downstroke portion of the wing cycle. During tethered flight the Tcxl muscles are activated on each cycle with short bursts of action potentials, each burst typically containing four action potentials. Activating the muscles with brief, tetanizing bursts increases the duration of muscle activity and the mechanical power output per wing cycle above that obtainable with single twitch contractions of the muscle. 3. The mechanical power output was determined for mesothoracic Tcxl muscles undergoing sinusoidal length change and stimulated phasically in the length cycle. At 25 Hz, the power at optimum muscle strain and optimum stimulus phase was 5 Wkg −1 at 30°C for muscles activated with single stimulus per cycle and about 33 Wkg −1 for muscles activated with bursts of stimuli in the normal pattern of flight. 4. The maximum power output at 100 Hz, the singing frequency, was 18 Wkg −1 . This was achieved with a single stimulus per wing cycle. 5. From published values of oxygen consumption by tettigoniids during singing, it is concluded that the efficiency of conversion of metabolic to mechanical power during singing is about 3%.

1. The mechanical power output of a synchronous insect muscle was determined by measuring tension as the muscle was subjected to sinusoidal length change and stimuli which occurred at selected phases of the length cycle. The area of the loop formed by plotting muscle tension against length over a full cycle is the work done on that cycle; the work done times the cycle frequency is the mechanical power output. The muscle was a flight muscle of the tettigoniid Neoconocephalus triops . The measurements were made at the normal wing-stroke frequency for flight (25 Hz) and operating temperature (30°C). 2. The power output with a single stimulus per cycle, optimal excursion amplitude, and optimal stimulus phase was 1.52 J kg −1 cycle −1 or 37W kg −1 . The maximum power output occurs at a phase such that the onset of the twitch coincides with the onset of the shortening half of the length cycle. The optimum excursion amplitude was 5.5% rest length; with greater excursion, work output declined because of decreasing muscle force associated with the more rapid shortening velocity. 3. Multiple stimulation per cycle increases the power output above that available with twitch contractions. In this muscle, the maximum mechanical power output at 25 Hz was 76 W kg −1 which was achieved with three stimuli per cycle separated by 4-ms intervals and an excursion amplitude of 6.0% rest length. 4. The maximum work output during the shortening of an isotonic twitch contraction was about the same as the work done over a full sinusoidal shortening-lengthening cycle with a single stimulus per cycle and optimum excursion amplitude and phase.

1. Isometric contraction kinetics and force-velocity relations were examined in wing muscles of two tettigoniid insects, Neoconocephalus robustus and N. triops . The muscles were first tergocoxal muscles of the mesothoracic and metathoracic segments. The metathoracic muscle is a flight muscle. The mesothoracic muscle is used in flight and in stridulation. 2. In the field, the wing stroke frequency during stridulation by N. triops is about 100 Hz; the thoracic temperature during singing is about 30 C; and the temperature gradient between the thorax and surround is about 15 C. Published data for N. robustus give the wing-stroke frequency during stridulation as about 200 Hz at a thoracic temperature of 35–40 C. The wing-stroke frequency during flight by both species is approximately 20 Hz at 25 C. 3. The twitch time course is similar in equivalent muscles of the two species. At 35 C the twitch duration (onset to 50% relaxation) is 5.5-6.5 ms for mesothoracic muscles and 11–13 ms for metathoracic ones. Twitch and tetanic tension per unit area are about twice as great in the metathoracic muscles as in the faster, mesothoracic ones. 4. Despite the differences in isometric contraction kinetics, the maximum shortening velocity (V max ) is similar in mesothoracic and metathoracic wing muscles. V max values (lengths per second, 35 C), determined by extrapolation of force-velocity curves, were 10.1 (mesothoracic) and 11.1 (metathoracic) for N. robustus ; 12.2 (mesothoracic) and 16.1 (metathoracic) for N. triops . With N. triops , V max was also determined from the time taken to re-develop tension following quick release. The values obtained were somewhat higher than from extrapolation of force-velocity curves, but again similar for mesothoracic and metathoracic muscles. 5. Twitch time course becomes more rapid and V max increases with increasing temperature. Neither twitch nor tetanic tension is greatly affected by temperature change in the range 25–35 C. 6. As for many other fast muscles, force-velocity plots for these muscles have little curvature. It is suggested that the relative straightness of these plots is a consequence of internal viscosity.

1. The mesothoracic dorsal longitudinal muscle (DLM) of the katydid Neoconocephalus robustus is used in stridulation and flight; the metathoracic DLM is used in flight only. The DLM's in the two segments have radically different maximum operating frequencies, 200 Hz for the mesothoracic muscle during stridulation and 20 Hz for the metathoracic muscle during flight. 2. Cable analysis was used to determine the passive electrical properties of mesothoracic and metathoracic DLM fibres. Fibres in the two segments are of similar diameter and have similar sarcoplasmic resistivity. The apparent membrane resistance is lower, the apparent membrane capacitance higher, and the time constant shorter in mesothoracic fibres than in the metathoracic homologues. 3. The depolarization evoked by neural stimulation in both mesothoracic and metathoracic fibres is principally an excitatory junctional potential (e.j.p.) with little or no contribution from voltage-dependent, inward current channels. At short interstimulus intervals the second e.j.p. of a pair is reduced in amplitude relative to the first e.j.p. The period of e.j.p. depression is shorter in mesothoracic than in metathoracic fibres. It is suggested that the faster recovery of e.j.p.'s in mesothoracic fibres is due to more rapid recovery of the transmitter release mechanism in their motorneurones. 4. In mesothoracic but not metathoracic fibres the voltage response to large depolarizing currents is usually oscillatory, and the recovery of e.j.p. amplitude as a function of time in paired shock experiments is sometimes oscillatory. The oscillation frequency is 250–300 Hz (35 °C) which is higher than the natural operating frequency of the muscle.

1. The tymbal muscles of the cicada Cyclochila australasiae Donovan are synchronous, those of Platypleura capitata (Oliv.) asynchronous. The tymbal structure and the individual sound pulses are similar in the two species. The sound pulse frequency during calling song is higher in P. capitata (389 Hz) than in C. australasiae (234 Hz). 2. Muscle action potentials during protest songs are simultaneous in the two tymbal muscles of P. capitata , alternating in C. australasiae . Each action potential, or direct electrical stimulus to the muscle, is followed by a single, sometimes bimodal, sound pulse in C. australasiae , and by a burst of sound pulses in P. capitata . 3. The isometric twitch duration (30 °C) is much longer in P. capitata (107 ms) than in C. australasiae (18 ms) and the tetanic fusion frequency correspondingly lower. 4. Myofibrils are slightly wider and sarcoplasmic reticulum much sparser in tymbal muscles of P. capitata than in those of C. australasiae ; otherwise the muscles are structurally similar. 5. Flight muscles in both species are synchronous with an action potential for each contraction. 6. Tymbal muscles of the two species are used to illustrate the diagnostic features of synchronous and asynchronous muscles. Note:

1. A suction electrode is described which allows long-term recording of spontaneous activity from essentially unrestrained Hydra . These electrodes were used to hold the animal while measuring column contractions and to measure the conduction velocity of the electrical potentials (contraction pulses = CPs) associated with contraction. 2. CPs are conducted in the column at 3.4-8.1cm./sec. The conduction velocity generally differs in the basal and hypostomal directions but the direction of faster conduction varies from animal to animal. 3. The latency between CPs and measurable column contraction is 0 1-0.15 sec. During isometric contractions following single CPs there is a rapid tension rise reaching a peak in about 1 sec. which is usually followed by partial relaxation and then a second, slower tension rise. Available evidence indicates that both tension components are due to contraction of longitudinal musculature and both are activated by the same conducting system.

1. Brief electrical potentials can be recorded from a suction electrode over the marginal sphincter or over a tentacle of the anemone Calliactis polypus following appropriate stimulation of the anemone. These potentials are thought to be muscle action potentials because they precede contraction by about 12 msec. (29-31° C.) and their size is smoothly graded with the size of the contraction. 2. The tentacles and sphincter are activated by a through-conducting system in the oral disk and column. As with other anemones studied, two stimuli are required to evoke sphincter contraction. The maximum interval between an effective pair of stimuli is about 600 msec, and the sphincter potential and contraction increase with decreasing intervals to a minimum interval (as short as 15 msec.) below which there is no response to the second shock. Tentacles behave similarly except that they often produce small potentials and sometimes tiny contractions to single stimuli. 3. During repetitive stimulation the muscle potentials facilitate and the individual contractions both facilitate and sum. The tentacle musculature becomes maximally active earlier in a stimulus burst than does the sphincter.

1. Boloceroides can swim orally by repeated aboral-oral flexions of its tentacles which make up over 90% of the weight of the anemone. Tentacles at different distances from the mouth beat slightly out of phase, with more marginal tentacles leading more oral ones. Isolated tentacles occasionally give aboral-oral flexions like those of attached tentacles during swimming. 2. Early in a bout of swimming the stroke frequency is about 1·2 strokes/sec. and the swimming velocity about 1·9 cm./sec. The stroke frequency is slightly higher in small animals than in large ones. The swimming velocity achieved is independent of animal size. 3. In tethered swimming the maximum forward force developed during a stroke increases with animal size approximately as the square of the diameter of the tentacle crown. About 8o % of the forward impulse produced during the effective portion of the stroke cycle is negated by rearward impulse during the recovery portion of the cycle. The average forward force for the whole cycle is only about 5% of the maximum force.

1. There are three, non-polarized conducting systems in the stalk of Tubularia . These are termed the distal opener system (DOS), the triggering system (TS), and the slow system (SS). 2. The DOS controls opening of the distal tentacles of the hydranth. Activation of the DOS produces a small electric pulse in the stalk which is conducted at about 15 cm./sec. At the base of the distal tentacles this pulse is followed by a slow potential. The slow potential but not the pulse shows antifacilitation and is quickly blocked by excess Mg 2+ , as are distal tentacle movements. This is taken to indicate that the pulse and slow potential are a result of nerve and muscle activity respectively. 3. The TS can trigger the potentials which normally appear spontaneously in the neck region of the hydranth. The TS conducts at about 17 cm./sec and has a lower threshold than the DOS. No electrical correlate of TS activity has been found. 4. Activation of the SS produces large (up to 1 mV.), slowly propagated (about 6 cm./sec.) potentials in the stalk. The SS is very labile. In a fresh animal the SS threshold is two to three times that of the DOS and the SS often fires repetitively to stimuli above its threshold. SS activity has apparently no effect on polyp behaviour or on spontaneous electrical activity in the hydranth.

1. Electrical pulses (amplitude -0.05 to -15 mV.; duration 20-120 msec.) have been recorded from the stolon of Cordylophora lacustris following stimulation. These pulses are propagated with an average velocity of 2.7 cm./sec. at 22° C. 2. Brief electric shocks of little more than threshold intensity can evoke bursts of pulses. The number of pulses in a burst increases with stimulus intensity, but the shape and size of individual pulses do not. 3. Repetitive stimulation causes facilitation of both size of single pulses and number of pulses in a burst. Refractory period, if present, is variable. The minimum interval between two pulses is about 200 msec. 4. Mechanical stimulation evokes pulses identical to those evoked by electrical stimulation. 5. The greater the number of pulses recorded in the stolon near a polyp, the greater and faster is the contraction of that polyp. 6. The number of pulses, but not their individual sizes, decreases with increasing distance from the point of stimulation. 7. It is concluded that conduction in the stolon and the electrical pulses are due to nervous activity and that the conducting system is a network having interneural junctions which sometimes require to be facilitated.

1. Conduction of excitation in response to local mechanical or electrical stimulation has been studied in various hydroid species. 2. There are systems in the coenosarc of the stems and stolons of all species which conduct excitation at rates of from 1 to 3.5 cm./sec. 3. Two physiological types of conducting systems have been found. ( a ) Through-conducting systems, showing: all-or-none response, sharp threshold, reproducibility. ( b ) Local systems, showing: spread of response dependent upon stimulus strength, no sharp threshold, responses which decline with increasing distance from the stimulated point, variability. 4. Colonial co-ordination is better developed in those species whose colonies are structurally better developed. It is effected in most species by either ( a ) or ( b ). Both types are present together in Hydractinia echinata .

1. Syncoryne mirabilis polyps rapidly bend toward the source of a mechanical disturbance in their vicinity. A small moving copepod a few millimeters from the polyp creates sufficient stimulus to evoke this response. 2. The hydroid will respond to a mechanical pulse from an electrically driven prodder as well as to single movements of the prodder either toward or away from the polyp. With the prodder 0.5 mm. from the polyp mid-line, threshold displacement of the prodder is only 2.5-3µ. Repeated pulses of increasing amplitude, initially below threshold intensity, will not evoke bending if the frequency is over 5/sec. This means that stimuli which cause no overt response can still inhibit polyp bending to normally supra-threshold subsequent stimuli. 3. A polyp will not respond to slow currents of water, but will bend towards the source of sudden jets of water directed at it. 4. The capitate tentacles are not necessary for the movement perception, but after their removal the threshold is raised and the accuracy of bending reduced. The response does not require that the polyp be attached to a fixed stalk; it also occurs in isolated polyps floating in a density gradient. 5. The hypothesis is presented that polyp bending is due to unequal stimulation of opposite sides of the polyp because of decreasing stimulus intensity with increasing distance from the source of disturbance.