6.6.2. Heat

The hottest inhabited places on Earth occur in the ocean, where suboceanic thermal vents support a unique assemblage of organisms based on thermophilous bacteria, and insects are absent. In contrast, in a terrestrial equivalent, vents in thermally active areas support a few specialist insects. The hottest waters in thermal springs of Yellowstone National Park are too hot to touch, but by selection of slightly cooler micro-habitats amongst the cyanobacteria/blue-green algal mats, a brine fly, Ephydra bruesi (Ephydridae), can survive at 43°C. At least some other species of ephydrids, stratiomyiids, and chironomid larvae (all Diptera) tolerate nearly 50°C in Iceland, New Zealand, South America, and perhaps other sites where volcanism provides hot-water springs. The other aquatic temperature-tolerant taxa are found principally amongst the Odonata and Coleoptera.

High temperatures tend to kill cells by denaturing proteins, altering membrane and enzyme structures and properties, and by loss of water (dehydration). Inherently, the stability of non-covalent bonds that determine the complex structure of proteins determines the upper limits, but below this threshold there are many different but interrelated temperature-dependent biochemical reactions. Exactly how insects tolerant of high temperature cope biochemically is little known. Acclimation, in which a gradual exposure to increasing (or decreasing) temperatures takes place, certainly provides a greater disposition to survival at extreme temperatures compared with instantaneous exposure. When comparisons of effects of temperature are made, acclimation conditioning should be considered.

Options of dealing with high air temperatures include behaviors such as use of a burrow during the hottest times. This activity takes advantage of the buffering of soils, including desert sands, against temperature extremes so that near-stable temperatures occur within a few centimeters of the fluctuations of the exposed surface. Overwintering pupation of temperate insects frequently takes place in a burrow made by a late-instar larva, and in hot, arid areas night-active insects such as predatory carabid beetles may pass the extremes of the day in burrows. Arid-zone ants, including Saharan Cataglyphis, Australian Melophorus, and Namibian Ocymyrmex, show several behavioral features to maximize their ability to use some of the hottest places on Earth. Long legs hold the body in cooler air above the substrate, they can run as fast as 1 m s-1, and are good navigators to allow rapid return to the burrow. Tolerance of high temperature is an advantage to Cataglyphis because they scavenge upon insects that have died from heat stress. However, Cataglyphis bombycina suffers predation from a lizard that also has a high temperature tolerance, and predator avoidance restricts the above-ground activity of Cataglyphis to a very narrow temperature band, between that at which the lizard ceases activity and its own upper lethal thermal threshold. Cataglyphis minimizes exposure to high temperatures using the strategies outlined above, and adds thermal respite activity — climbing and pausing on grass stems above the desert substrate, which may exceed 46°C. Physiologically, Cataglyphis may be amongst the most thermally tolerant land animals because they can accumulate high levels of “heat-shock proteins” in advance of their departure to forage from their (cool) burrow to the ambient external heat. The few minutes duration of the foraging frenzy is too short for synthesis of these protective proteins after exposure to the heat.

The proteins once termed “heat-shock proteins” (abbreviated as “hsp”) may be best termed stress- induced proteins when involved in temperature-related activities, as at least some of the suite can be induced also by desiccation and cold. Their function at higher temperatures appears to be to act as molecular chaper-ones assisting in protein folding. In cold conditions, protein folding is not the problem, but rather it is loss of membrane fluidity, which can be restored by fatty acid changes and by denaturing of membrane phospholipids, perhaps also under some control of stress proteins.

The most remarkable specialization involves a larval chironomid midge, Polypedilum vanderplanki, which lives in West Africa on granite outcrops in temporary pools, such as those that form in depressions made by native people when grinding grain. The larvae do not form cocoons when the pools dry, but their bodies lose water until they are almost completely dehydrated. In this condition of cryptobiosis (alive but with all metabolism ceased), the larvae can tolerate temperature extremes, including artificially imposed temperatures in dry air from more than 100°C down to —27°C. On wetting, the larvae revive rapidly, feed and continue development until the onset of another cycle of desiccation or until pupation and emergence.




Chapter 6