The three-dimensional structure of the Z disc in insect supercontracting muscles

Two type of Z disc structure have been reported in insect supercontracting muscle fibres: (i) a perforated Z disc where Z material forms a reticulum and (ii) a fragmented Z disc composed of separate, discrete Z bodies. The use of thick (1 micron) sections in conjunction with high voltage electron microscopy can distinguish between these two types while conventional thin sections may lead to misinterpretation of structure. It is shown that in one insect, the crane-fly Tipula, the larval body-wall muscles, for which a fragmented Z disc has been proposed, do in fact have a perforated disc. In the wax moth Galleria, homologous muscle fibres have a similar type of Z disc, a finding which indicates the need for re-examination of other lepidopteran muscles claimed to have fragmented discs. A redefinition of supercontraction is proposed which includes reference to the perforated type of Z disc.