The conclusions on the structure of the cuticle in Tenebrio have been summarized on p. 204.
Observations on the deposition of the cuticle are in general agreement with those made on Rhodnius.
Mitosis and chromatolysis precede the formation of the definitive epidermis. The basic layer of the epicuticle, ‘cuticulin’, is then laid down. It consists of condensed lipoproteins (subsequently tanned, it is supposed, by quinones) and its deposition is immediately preceded by the peak in the secretory cycle of the subepidermal oenocytes.
Pore canals from the epidermal cells penetrate the cuticulin layer and pour out silver-reducing material (believed to be dihydroxyphenols in insoluble form) upon its surface. This material is confined to the areas overlying the cell bodies during all but the last stages in its formation, when it fuses to give a more or less continuous layer.
During the last few hours before moulting a wax layer appears to be laid down over this polyphenol layer. By the time moulting occurs the polyphenol layer is almost covered and the insect is nearly waterproof. During the first day after moulting, while the secretion of the wax is being completed, the loss of water by transpiration is about four to six times the normal.
Very soon after moulting the dermal glands discharge the cement layer over the surface of the wax. The substance of this layer and the contents of the dermal glands reduce ammoniacal silver after extraction with boiling chloroform. It is suggested that it consists of polyphenol-containing material associated with protein and lipides.
(It is shown that in Rhodnius the cement layer is formed by the admixture of secretion from the two types of dermal gland previously described. The one produces a solution of protein, the secretion of the other agrees in properties with that here described in Tenebrio. The similarity of this arrangement to that discovered by Pryor in the colleterial glands of the cockroach is pointed out.)
In addition to these cement glands there are glands of unknown function opening into the floor of the pits in the cuticle. These are highly developed in the sternites of the male, small and inconspicuous in the female.