Measurements of tritiated water (THO) efflux using a cup, ventilated with dry air, attached to the pronotum of Periplaneta, have been used to obtain unambiguous measurements of the permeability of this structure. Permeability values were 53% of those determined gravimetrically. Our results support the proposal that cockroach cuticle permeability is hormonally controlled, to the extent that compounds extracted from the brain have been shown to be capable of inducing permeability change over relatively short periods. Fresh brain homogenate injections from hydrated donors produced a 28% increase in pronotal permeability in 5 h, rising to 46% the day after. Injections of saline or of fresh brain extracts from dehydrated and normally hydrated cockroaches had no effect. However, previously frozen brain homogenates, from donors at all hydration levels, significantly increased pronotal permeability the day following injection.
The capacity of the pronotum to undergo increases in permeability over relatively short periods was also evident in other experiments. THO permeabilities of excised pronotal discs (5.16×10−10±0.31×10−10ms−1) were an order of magnitude higher than those of intact pronota (0.49×10−10±0.06×10−10 ms−1). In vivo permeability of pronota of accidentally injured cockroaches was significantly higher than that of uninjured animals, even though the pronota themselves were not damaged.
We argue that the observed changes in cuticle permeability are too small primarily to serve osmoregulatory functions. We suggest, instead, that the changes might be associated with the control of secondary processes in which the cuticle is involved and which unavoidably promote water loss.