The cuticle of the cockroachPeriplaneta americanaI. The appearance and histological structure of the cuticle of the dorsal surface of the abdomen

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Abstract
The fully developed dorsal abdominal cuticle of the cockroach is differentiated into hard tergites alternating with flexible intersegmental membranes. The tergites are coloured owing to the presence of an exocuticle, only the outer zone of which, however, is amber or brown in colour. The inner zone, although resembling the outer in being homogeneous and refractile, is colourless. At the anterior and posterior margins of the tergites the colourless zone of the exocuticle progressively increases in thickness at the expense of the coloured zone, which disappears as the intersegmental membrane is approached. Still nearer to the intersegmental membrane the colourless zone itself becomes thinner and disappears, leaving the cuticle in the soft and unmodified arthrodial membrane condition. The cuticle of the transitional zone between tergite and arthrodial membrane, although not of uniform constitution or precisely defined, is conveniently referred to as the intermediate sclerite. The epicuticle of the tergites of the fully hardened cuticle shows a well-marked polygonal surface pattern but is not readily recognized histologically. That of the arthrodial membranes, bearing only scattered and minute rounded knobs, is, however, clearly seen in sections. In the soft newly moulted insect the topographical distinctions between the regions of the cuticle are not accompanied by marked differences in composition. In the absence of an amber exocuticle the cuticle of the future sclerites resembles that of the arthrodial membranes, but differs in its staining reactions. It will be shown that the differences in staining reactions are due to the presence already of impregnating substances in the sclerite cuticle.
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