Programmed cell death specifically eliminates one part of a locust pleuroaxillary muscle after the imaginal moult

In the hemimetabolous insect Locusta migratoria, fundamental restructuring occurs at the transition from flightless nymph to flight-capable adult. This transition involves all components of the flight circuit, which is present but not used for flight in nymphs. The meso- and metathoracic pleuroaxillary muscles, M85 and M114 respectively, constitute one component of this circuit. In the adult locust, these are flight-steering muscles, but their function in the nymph is as yet unknown. Our study reveals that adult and nymphal metathoracic pleuroaxillary muscles M114 differ profoundly. The nymphal muscle contains the distinct part M114c in addition to parts M114a and M114b characteristic of the adult. The contractions of M114c are slow and long-lasting, corresponding to its long sarcomeres and slow form of ATPase, and contrast with the adult muscle parts M114a and M114b in all of these features. We demonstrate a hormone-dependent degeneration of M114c after the adult moult. This degeneration can be blocked by actinomycin D and cycloheximide. It may thus be termed genetically programmed cell death, triggered after the adult moult and, as demonstrated here, functioning via the ATP-dependent ubiquitin pathway. Given the defined onset of degeneration after the adult moult, it is possible that M114c may fulfil a specific function in nymphs, during or shortly after moulting.

Octopaminergic innervation and modulation of a locust flight steering muscle

We demonstrate that the meso- and metathoracic pleuroaxillary flight steering muscle (M85 mesothorax, M114 metathorax) of the migratory locust are each innervated by a single dorsal unpaired median neurone (DUM3,4,5a). The soma of this neurone can be localized by retrograde staining of the motor nerve with Neurobiotin, but not with cobalt salts. The primary neurite projects in the superficial DUM cell tract, and the axons run in nerve roots 3, 4 and 5 and in all their secondary branches. Other muscle targets include the second tergal remotor coxa (M120) and the posterior rotator coxae (M122, M123, M124), but not the first tergal remotor coxa (M119) and subalar (M129) flight muscles. Octopamine-like immunoreactive varicosities occur on the pleuroaxillary muscles. Stimulation of DUM3,4,5a and octopamine (10(-6) mol l-1) superfusion increased the amplitude and the relaxation velocity of neurally evoked twitch contractions of this muscle. Octopamine also significantly reduced the tonic tension that this muscle develops when stimulated at flight frequency (20 Hz), while increasing the amplitude of each phasic twitch. A catch-like tension is also reduced in the presence of octopamine. Simulations of the motor pattern experienced by the pleuroaxillary muscles during roll manoeuvres suggest that transient changes in tension underlying corrective steering could be doubled in the presence of octopamine.

Optomotor control of course and altitude in Drosophila melanogaster is correlated with distinct activities of at least three pairs of flight steering muscles.

Flight control in the fruitfly Drosophila melanogaster is achieved by minute sets of muscles on either side of the thorax. Control responses of wings and muscles were elicited during fixed flight by moving a striped pattern in front of the eyes. For example, pattern motion from the lower right to the upper left signals to the test fly a rotatory course deviation to the right and simultaneously a translatory altitude displacement downwards. The counteracting response to the displacement of the retinal image is an increase in thrust and lift on the right, accomplished mainly by increasing the wingbeat amplitude (WBA) on that side. A comparison of such responses with the simultaneously recorded action potentials in the prominent basalar muscles M.b1 and M.b2 and axillary muscles M.I1 and M.III1 on either side suggests that three of these muscles act on the WBA more or less independently and contribute to the optomotor control of course and altitude. During flight, M.b1 is almost continuously active with a frequency equal to or slightly below 1 spike per wingbeat cycle. The spikes occur within a narrow phase interval of this cycle, normally at the beginning of the transition from upstroke to downstroke. However, the visual stimulus described above causes a substantial phase lead in M.b1 on the right; the spikes occur shortly before the end of the upstroke. Such phase shifts are accompanied by comparatively smooth 'tonic' responses of the WBA. The activities of M.b2 and M.I1 are normally very low. However, the stimulus described above activates M.b2 on the right in a phase interval approximately two-thirds into the upstroke and M.I1 on the left in a phase interval at the beginning of the downstroke. The spikes tend to occur in bursts. These bursts are correlated with WBA-increasing 'hitches' (rapid changes in amplitude) on the right and WBA-decreasing hitches on the left. As fast 'phasic' responses, the burst-induced hitches are likely to account for the course-controlling 'body saccades' observed during free flight. For unknown reasons, M.I1 is activated by pattern motion but cannot conceivably assist the other muscles in altitude control. Unlike its homologues in larger flies (Musca domestica, Calliphora erythrocephala), M.III1 does not participate in optomotor flight control. Its activation seems to support the termination of flight and wing retraction at rest. The essential properties of the three pairs of muscles M.b1, M.b2 and M.I1 resemble those found in larger flies; the muscles are controlled by motion detectors with muscle-specific 'preferred directions' in the hexagonal array of retinal elements. Optomotor control of the three pairs of muscles in Drosophila melanogaster could explain most, but not all, of the WBA responses recorded so far.

Neural control of hindleg steering in flight in the locust

Corrective flight steering with the hindlegs was investigated in intact tethered flying locusts inside a wind tunnel as well as in animals dissected for intracellular recording and showing fictive flight activity. In intact tethered flying animals, activity in the second coxal abductor muscle (M126) was highly correlated with hindleg steering and was coupled to the elevator phase of the flight cycle. Fictive flight and steering could also be elicited in animals dissected for intracellular recording of motoneurones innervating M126. During fictive flight activity, motoneurones 126 were rhythmically excited in the elevator phase, presumably from central elements of the neuronal oscillator generating the flight motor pattern, as is the case for motoneurones innervating wing muscles. During fictive straight flight, this input was subthreshold, and it could be demonstrated that simulated deviation from the flight course resulted in recruitment of motoneurones 126. Statistical analysis of the latencies of fast muscle spikes in M126 and in one wing elevator muscle showed that both received common input during flight steering. One source of this common input was identified as the sensory information from the lateral ocelli, which play an important role in the detection of course deviation. The experiments demonstrated that processing in the sensory-motor system for hindleg steering is probably organized in a very similar way to that responsible for steering with the wings.