The pattern of the dorsal abdominal cuticle in the nymph and adult of Rhodnius is described.
If a patch of epidermis is killed by burning, the cells at the margin of the burn divide and migrate towards the centre until the defect is repaired. During this process the cells, and the daughter cells derived from them, retain the power of laying down the type of cuticle for which they were originally determined. Consequently a centripetal displacement of the pattern results at the next moult.
Cells migrating in this way in the early nymphal stages carry with them also the prospective imaginal characters. Similar derangements of the imaginal pattern therefore result; but these do not become manifest until metamorphosis.
Thus the individual cells appear to be all-important in the determination of the abdominal pattern.
The nymphal cuticle bears bristles arising from little plaques distributed regularly over the surface. At successive moults new plaques appear in the spaces between the old ones. If at a given moult the separation of the plaques is artificially increased or diminished this has no influence on the number of new plaques developed. But if the separation is increased at one moult, the number of new plaques may be nearly twice the normal at the succeeding moult.
This result is explained by supposing that each plaque exerts an inhibitory influence around it which prevents the development of new plaques; and that the radius over which this influence is exerted is determined not by the absolute distance but by the number of epidermal cells intervening. The nature of the inhibitory influence is discussed.
The number of new plaques is not predetermined at a given moult; for it may be increased if moulting is induced by exposure to the hormones of a younger nymphal stage.
If the plaques or bristles are brought close together during the moult, those of the next instar are reduced in size; if widely separated they are enlarged.
At the first moult after a burn the nymphal cuticle at the site of injury is devoid of plaques and bristles. Plaques of normal orientation and distribution are developed at subsequent moults. This regeneration is an exaggeration of the normal process of plaque formation that takes place at each moult.