1. The anatomy and innervation of the coxal and femoral muscles of the pro- and metathoracic legs have been described. The pattern of nerves and muscles is similar in both legs. 2. Histological examination has shown that the motor nerve endings on the flexor tibialis muscle fibres of the prothoracic leg are of the "Doyere-cone" type, and are spaced at intervals of approximately 60 u along the length of the fibres. 3. The electrical responses of the fibres of the flexor tibialis muscle have been examined with the aid of glass capillary intracellular microelectrodes. Records have also been made of some of the mechanical responses of this muscle. 4. Two types of electrical responses are presents "fast", non-facilitating responses resembling the action potentials of vertebrate muscles, and associated with quick twitch-type contractions of the muscle fibres; and "slow", readily facilitating responses resembling small end-plate potentials, associated with slow smooth movements and maintenance of the tonus of the muscle. 5. The two responses are produced by two separate types of motor axons innervating the muscle. 6. The muscle is capable of developing a tetanus p tension of 800 g./cm2./unit cross-sectional area of individual fibres. The tetanus/twitch ratio is high. 7. Pharmacological substances which alter vertebrate neuromuscular transmission do not affect the fast response. 8. Progressively lowered temperatures lengthen the time course and to a slight extent the size of the "fast" response, and an inflexion appears in the rising phase which was taken to indicate that the "fast" response consists of two components, a junctional potential and an active membrane response. 9. Refractoriness occurs in the decay phase of the "fast" response. A pair of stimuli sufficiently close together will produce effects which summate to give an enhanced junctional potential. A similar effect was not observed in the case of the active membrane response. 10. Analyses of its blood ionic composition have confirmed that Carausius is a typical herbivorous insect, with blood sodium/potassium and calcium/magnesium ratios of less than unity. 11. The effects of changing the sodium, potassium, calcium and magnesium concentrations of the bathing fluid on the "fast" response have been studied. 12. Lowering the sodium concentration results in a decrease in the magnitudes of the resting and action potentials, but an active membrane response is present in zero sodium. The rate of rise of the action potential increases as the sodium concentration is raised. 13. When tetramethylammonium ions are substituted for sodium ions in the bathing fluid, variations in the magnitudes of the resting and action potentials similar to those seen in the corresponding sodium salines are observed; but the rate of rise decreases as the TMA level is raised. 14. It is suggested that sodium ions are not essential for the production of an active membrane response, but that they may affect the excitability of the muscle fibre membrane. 15. Calcium ions are necessary for the development of the junctional potential. It is suggested that they influence the liberation of a neuromuscular transmitter substance. They may also affect the sodium permeability of the membrane. 16. When the magnesium concentration is raised above about 150 m. equiv. per litre, the action potential declines in size, and neuromuscular block eventually results. This effect is not antagonized by calcium ions. 17. The action potential also declines in size when the magnesium concentration is lowered below about 100 m. equiv. per litre. The resting potential is not affected by magnesium ions. 18. The resting potential is directly proportional to the log. of the potassium concentration except at low concentrations. 19. These results are discussed in the light of similar work on vertebrates and other arthropods. In particular, it is considered that the fast response is produced by a process similar to vertebrate and locust neuromuscular transmission, but that the sodium and magnesium results represent adaptations to a herbivorous habitat; and the possibility that a selective ion barrier is present around the muscle fibres of Carausius is considered and rejected, with reservations in the case of sodium ions
