AJP - Regulatory, Integrative and Comparative Physiology, Vol 247, Issue 2 230-R236, Copyright © 1984 by American Physiological Society
Allometry of water vapor absorption in two species of tenebrionid beetle larvae
P. A. Coutchie and J. Machin
Water vapor absorption is compared in the two tenebrionid larvae Tenebrio
molitor and Onymacris marginipennis. Thresholds for absorption and
"absorption capacity," which depend on ion transport by Malpighian tubules,
are consistently different over the entire size range of the two species.
Because of lower thresholds, uptake rates of Onymacris are over double
those of Tenebrio in larvae of the same size at identical ambient water
activities. By contrast, passive determinants of uptake, rectal
conductance, and morphometry are similarly scaled with size in the two
species. A ventilatory mechanism of vapor entry into the rectum is proposed
for both species, since the anal canal is too long and narrow for
exclusively diffusional entry. Vapor uptake in relation to larval mass was
described by a simple diffusion model through rectal tissue, where flux
varied directly with surface area and inversely with the distance between
the lumen and Malpighian tubules. Failure of rectal conductance to increase
proportionally with body mass means that size-specific vapor uptake
declines with larval size in both species.
