Quantitative studies on fibril orientation in beetle endocuticle

Abstract

We present the first quantitative analysis of chitin microfibril orientation in an arthropod skeleton, using Oryctes rhinoceros and five other species of beetles. The precision of orientation of each specific endocuticle layer at a given location is shown by the low variation (± 3·3° for 10 measurements) between individuals. In beetle endocuticle there is an apparent successive clockwise rotation (in a direction away from the observer) of the thick unidirectionally oriented layers, due to thin intervening helicoidal layers, each of which rotates anti-clockwise (determined by electron microscopy) through less than 90°. The direction of the first (earliest) endocuticle layer is often parallel to the relevant anatomical axis, and to the calculated mean fibril direction. This gives a bilaterally symmetrical pattern, often matching the pattern of hair orientation, indicating direct cellular control of specific microfibril direction. We suggest that the cell polarity gradient may determine the orientation in the first endocuticle layer. This initial bilateral symmetry is lost in the second and following layers because each intervening helicoidal layer rotates through the same angle (which varies specifically with the layer number and which is less than 90°) in an anti-clockwise sense on both sides of the body. These subsequent directions could be controlled by the secretion of a predetermined amount of helicoid between each pair of layers. It is knowsn that juvenile hormone (JH) can control this. In further support of the suggestion that JH is involved is the fact that the same total amount of helicoid is secreted over a period of days despite the presence or absence of polarity effects in specific areas. We illustrate pairs of helical disinclinations associated with cuticular ducts in Dynastes. These appear identical to the χ translation dislocations of cholesteric liquid crystals, previously noted in crustacean cuticle by Bouligand.