CORRELATION BETWEEN MUSCLE STRUCTURE AND FILTER CHARACTERISTICS OF THE MUSCLE-JOINT SYSTEM IN 3 ORTHOPTERAN INSECT SPECIES

In orthopteran insects, neural networks for joint control exhibit diff erent characteristics due to behavioural specializations. We investiga ted whether these differences are generated purely by the neuronal net works, or whether characteristics of the muscles or joint architecture (muscle-joint system) are also involved in these behavioural speciali zations. We compared the properties of the muscle system moving the fe mur-tibia joint of the middle and hindleg of three species, Carausius morosus, Cuniculina impigra and Locusta migratoria. Four aspects were analysed for the tibial extensor muscle: (i) the frequency-dependence of motoneuronal activity in response to sinusoidal stimulation of the femoral chordotonal organ (fCO), (ii) the muscle structure, (iii) the innervation pattern of the muscle and (iv) the histochemical propertie s of the muscle fibres. These aspects were compared with the filter ch aracteristics of the open-loop femur-tibia control system and of the m uscle-joint system involved. Whereas in both phasmid species (Carausiu s morosus and Cuniculina impigra) the motoneuronal activity steadily i ncreases with sinusoidal stimulation of the fCO in the frequency range 0.01-5 Hz, in Locusta migratoria there is a decrease in motoneuronal activity between 0.01 and 0.3 Hz. The muscle structure is basically si milar in all three species, as the number of singly innervated muscle fibres (supplied by the fast extensor tibiae motor neurone, FETi) decr eases from proximal to distal. The number of triply innervated fibres supplied by the FETi, the slow extensor tibiae (SETi) and the common i nhibitor 1 (CI1) is maximal in the middle of the muscle, and the numbe r of dually innervated fibres (supplied by SETi, CI1) increases from p roximal to distal. Differences between the locust and the two phasmid species exist in the distal portion of the muscle. The phasmid extenso r tibiae muscle contains a morphologically distinct bundle of muscle f ibres, not present in the locust, which is mostly dually innervated an d which is larger in Cuniculina impigra. Similar results were obtained for the histochemical characterisation of the muscle fibres as reveal ed from their staining for myofibrillar ATPase activity. The number of histochemically identified fast fibres decreased from proximal to dis tal, while the number of slow fibres increased. In Carausius morosus a nd Locusta migratoria, the percentage of slow fibres increased by up t o 60-70 % at the distal end, while this increase was to almost 100 % i n Cuniculina impigra. Apparently, the larger this distal region and th e higher the percentage of slow, dually innervated fibres in it, the l ower is the upper corner frequency (the stimulus frequency at which th e joint control system produces a movement with 70 % of its maximal re sponse amplitude) of the muscle-joint system. In summary, it appears t hat the upper corner frequency of the open-loop system in Locusta migr atoria (< 0.05 Hz) results at least in part from properties of the neu ronal joint control network, but in Carausius morosus (0.5-1.0 Hz) and Cuniculina impigra (0.1-0.2 Hz) it results from the upper corner freq uency of the muscle-joint system.