The patterns of folding in the epicuticle of Rhodnius result from mechanical forces directed by the epidermal cells and not from the action of physical forces arising spontaneously in the secreted substance. This conclusion is supported by observations on the cuticular folds over the abdomen of the larva and adult; on the formation of the plaques and the tactile setae arising from them; and on the taenidial folds in the tracheae.
At the time of formation of the outer epicuticle the boundaries of the definitive epidermal cells are clearly defined by adhesion zones. The total surface area of the cuticle is thus the sum of contributions determined by the individual cells.
The area of outer epicuticle deposited is greater than that of the apical plane of the cell. The cell surface is thrown into folds over which the epicuticle is laid down. But before the inner layers are deposited the cuticle is moulded by mechanical forces controlled by the epidermal cells.
Microtubules and fibre bundles appear to play an active part, notably in the outgrowth of setae, and perhaps in the raising of the initial folds in the abdominal surface and in the tracheae. The taenidial folds in the tracheae arise mainly at the sites of the folds in the preceding instar.
Cellular movements or changes in shape lead to aggregation of epidermal cells to form the ‘stellate pattern’ of the larval abdomen and the transverse ‘ripple pattern’ of the adult.
Distension of intercellular and intracellular vacuoles controls the moulding of the definitive stars in the larval cuticle, the transverse ridges in the adult, the smoothing of the cuticle over the plaques, the raising of the plaques in dome-like form above the rest of the cuticle, the expansion of the setae and the shaping of the taenidial folds in the tracheae.
The basement membrane provides a resistant substrate for these mechanical forces.