6.6.3. Aridity

In terrestrial environments, temperature and humidity are intimately linked, and responses to high temperatures are inseparable from concomitant water stress. Although free water may be unavailable in the arid tropics for long periods, many insects are active year- round in places such as the Namib Desert, an essentially rain-free desert in southwestern Africa. This desert has provided a research environment for the study of water relations in arid-zone insects ever since the discovery of “fog basking” amongst some tenebrionid beetles. The cold oceanic current that abuts the hot Namib Desert produces daily fog that sweeps inland. This provides a source of aerial moisture that can be precipitated onto the bodies of beetles that present a head-down stance on the slip face of sand dunes, facing the fog-laden wind. The precipitated moisture then runs to the mouth of the beetle. Such atmospheric water gathering is just one from a range of insect behaviors and morphologies that allow survival under these stressful conditions. Two different strategies exemplified by different beetles can be compared and contrasted: detritivorous tenebrionids and predaceous carabids, both of which have many aridity-tolerant species.

The greatest water loss by most insects occurs via evaporation from the cuticle, with lesser amounts lost through respiratory gas exchange at the spiracles and through excretion. Some arid-zone beetles have reduced their water loss 100-fold by one or more strategies including extreme reduction in evaporative water loss through the cuticle (section 2.1), reduction in spiracular water loss, reduction in metabolism, and extreme reduction of excretory loss. In the studied arid- zone species of tenebrionids and carabids, cuticular water permeability is reduced to almost zero such that water loss is virtually a function of metabolic rate alone — i.e. loss is by the respiratory pathway, predominantly related to variation in the local humidity around the spiracles. Enclosure of the spiracles in a humid sub-elytral space is an important mechanism for reduction of such losses. Observation of unusually low levels of sodium in the hemolymph of studied tenebrionids compared with levels in arid-zone carabids (and most other insects) implies reduced sodium pump activity, reduced sodium gradient across cell membranes, a concomitantly inferred reduction in metabolic rate, and reduced respiratory water loss. Uric acid precipitation when water is reabsorbed from the rectum allows the excretion of virtually dry urine (section 3.7.2), which, with retention of free amino acids, minimizes loss of everything except the nitrogenous wastes. All these mechanisms allow the survival of a tenebrionid beetle in an arid environment with seasonal food and water shortage. In contrast, desert carabids include species that maintain a high sodium pump activity and sodium gradient across cell membranes, implying a high metabolic rate. They also excrete more dilute urine, and appear less able to conserve free amino acids. Behaviorally, carabids are active predators, needing a high metabolic rate for pursuit, which would incur greater rates of water loss. This may be compensated for by the higher water content of their prey, compared with the desiccated detritus that forms the tenebrionid diet.

To test if these distinctions are different “adaptive” strategies, or if tenebrionids differ more generally from carabids in their physiology, irrespective of any arid tolerance, will require wider sampling of taxa, and some appropriate tests to determine whether the observed physiological differences are correlated with taxonomic relationships (i.e. are preadaptive for life in low-humidity environments) or ecology of the species. Such tests have not been undertaken.




Chapter 6