1. Application of octopamine to the metathoracic dorsal longitudinal muscle (DLM) of the cricket Teleogryllus oceanicus produced dose-dependent increases twitch amplitude, contraction rate and relaxation rate. The threshold for octopamine effects was between 10 −8 and 10 −7 moll −1 , while maximal effects were seen at approximately 10 −5 moll −1 2. The octopamine receptors were classified as octopamine 2 receptors on the basis of the differential responsiveness of the muscle to the octopamine agonists naphazoline, tolazoline, clonidine and the octopamine antagonists metoclopramide and chlorpromazine. It was not possible to distinguish between octopamine 2A or octopamine 2B receptors in this preparation. 3. Octopamine had both presynaptic and postsynaptic effects, since it increased both miniature end-plate potential (mEPP) frequency and muscle relaxation rate 4. At a calcium concentration of 11 mmoll −1 , octopamine did not affect muscle membrane potential, input resistance or EJP amplitude, but the EJP duration at half amplitude ( T ½ ) was slightly increased. In low-calcium saline (1.8 mmol −1 ), octopamine did not affect membrane potential or T ½ , but EJP amplitude was increased 5. Stimulation of the octopaminergic dorsal unpaired median neuron (DUMDL), which innervates the metathoracic DLM, increased twitch amplitude about 25% of the preparations. Failure in the other preparations wasapparently due to spike conduction failure within the metathoracic ganglion.6. These results show that octopamine can be an important modulator of metathoracic DLM tension production

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 ion-selectivity of the extrajunctional glutamate-gated ion channel in locust extensor tibiae muscle was studied using the patch-clamp technique. The alkali metal ions Li + , Na + , K + , Rb + and Cs + were all highly permeant, with reversal potentials close to 0mV. Both complete and partial replacement of Na + (180 mmoll −1 in standard saline) showed that conductance (γ) increased in the order Li + <Na + < Cs + < Rb + (approx. 70–125 pS), γK being close to γ Cs . The channel was impermeable to the large organic monovalent ions tetramethyl-ammonium, guanidinium and choline, and permeable to the smaller ammonium ion. Divalent cations (Ca 2+ and Mg 2+ ) did not contribute measurably to the ionic current. Indications were obtained that high concentrations of Mg 2+ or Ca 2+ block the channel. The results suggest that the glutamate-gated channel combines a high conductance with a restricted ion-selectivity, based on ion charge and size, the conductance being dependent on the dehydration energy of the ionic species.

1. The actions of the phenyliminoimidazolidine derivatives, NC7 and NC5 on octopamine receptors in the extensor-tibiae neuromuscular preparation of the locust are described. 2. The derivatives activate both OCTOPAMINE 1 and OCTOPAMINE 2 receptors in this preparation. They are not selective agonists of OCTOPAMINE 2 receptors that activate adenylate cyclase. 3. The relative potencies of NC5 and NC7 on OCTOPAMINE 1 and OCTOPAMINE 2 receptors differ, suggesting that they may be useful in the development of selective agonists that can distinguish between the different pharmacological subclasses of octopamine receptor. 4. The octopamine-mediated increases in cyclic AMP levels in this preparation can be separated into high- and low-affinity components. NC5 and NC7 appear to activate the high-affinity component of this response selectively.

1. The modulatory actions of FMRFamide and related peptides on tension generated in the extensor-tibiae muscle of the locust hindleg by stimulation of the slow excitatory motor neurone (SETi) depend upon the frequency of stimulation of SETi. They have no effect on the tension induced by the fast motor neurone (FETi) or upon the myogenic rhythm present in this muscle. 2. At low frequencies of SETi stimulation (1Hz and below) the predominant modulatory effects are increases in the amplitude, contraction rates and relaxation rates of twitch tension. At higher frequencies, where twitches summate but tetanus is incomplete (up to 20 Hz) these effects are superimposed upon an increase of maintained tension. 3. FMRFamide increases the amplitude and relaxation rate of slow twitch tension by different amounts in different regions of the extensor muscle. It is likely that the effects of FMRFamide are restricted to slow muscle fibres that are innervated by SETi but not FETi. 4. The modulatory actions of FMRFamide on SETi-induced tension are additive to, but do not potentiate, the modulatory actions of octopamine and proctolin in this muscle. The actions of FMRFamide show some similarities with the modulatory actions of octopamine in this preparation but they are mediated by an independent receptor system that does not change cyclic nucleotide levels. Other actions of FMRFamide are similar to the actions of proctolin.

Insect skeletal muscles are frequently innervated by small numbers of motor neurones, all of which can be uniquely identified physiologically. They therefore present excellent model systems in which to study the basic principles of neuromuscular transmission and the modulation of these effects by biogenic amines and peptides. The extensor-tibiae muscle of the hind leg of the locust is a much studied, large muscle that is innervated by three identified motor neurones and one identified modulatory neurone. Much attention has recently been focused on the modulation of neuromuscular transmission and muscular contraction in this muscle by biogenic amines and peptides. One proximal bundle of muscle fibres in the extensor-tibiae muscle exhibits a myogenic rhythm of contraction and relaxation. The rhythm is stimulated by a variety of peptides including proctolin, the AKH-related peptides M 1 and M 2 , and by small cardioactive peptide (SCP B ). In addition, it is activated by 5-hydroxytryptamine and by oneclass of adenosine analogues. The rhythm is inhibited by octopamine and by a second class of adenosine analogues. The actions of these various modulatory compounds will be discussed in terms of the likely numbers of pharmacologically distinct receptors in this preparation and their modes of action. Neuromuscular transmission and muscular contraction in the extensor-tibiae muscle is modulated by the biogenic amine octopamine and by the peptides, proctolin and FMRFamide. The actions of these modulators are discussed in relation differences in the responsiveness of various regions of the muscle, to the frequency dependence of their effects on motor neurone activityand to their modes of action. The cellular locations and mode of transmission to the muscle of some of these modulators will be considered. Octopamine and proctolin are contained within neurones which innervate the muscle, whilst FMRFamide- and SCP B -like peptides appear to be released into the locust haemolymph as neurohormones.

1. Functional changes following denervation have been studied by intracellular recording at the neuromuscular junction of the cockroach coxal muscle. 2. Spontaneous subthreshold activity disappeared together with nerve-muscle transmission in about 2 days after nerve section at 26°C. The onset of the failure was mainly dependent on the temperature and also on the length of the transected distal stump. After complete cessation of the miniature excitatory postsynaptic potentials (MEPSPs) for about 3 weeks at 26°C, the miniature potentials resumed at a slower rate. Regeneration occurred faster when axotomy was performed by crushing the nerve rather than by sectioning. 3. Resumption of the MEPSPs was accompanied by response to nerve stimulation. However, hypertonic and hypotonic saline, and excess potassium, failed to increase the frequency of the resumed MEPSPs recorded between 15 and 45 days following denervation. 4. It is suggested that the resumed spontaneous release of transmitter may be derived from regenerating nerve terminals.

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.