Summer World_ A Season of Bounty - Bernd Heinrich [77]
Insects are generally renowned for their ability to retain water. Where other animals die of thirst, they can stay hydrated, mostly by avoiding heat, by their ability to conserve water because their watertight exoskelton is covered with layers of waterproofing lipids and waxes, and by their excretion of nitrogen wastes as uric acid that requires negligible water to excrete. The Apache cicada has, literally, prominent holes in it, and it has glands that excrete water from these holes. Not knowing more, one might suppose that this insect is active at the wrong time and also physiologically unsuited to life in extreme summer. Its design seems inefficient and backward.
It took two biologists, James E. Heath from the University of Illinois and Eric C. Toolson from Arizona State University, to unravel the cicada story, which is one of exquisitely elegant desert adaptation. Heath deduced from his studies that the cicada’s seemingly anomalous active time is the time when potential predators—both birds and wasps—have fled the field because they can’t stand the heat. Toolson found out that cicadas can stand high temperatures because they have glands that function like sweat glands to provide evaporative cooling in emergencies, as when the males are exerting themselves by calling deafeningly to attract females for sex.
The cicadas’ ability to defy the summer extremes, and thereby to escape enemies, would not be possible without a constant, reliable source of water. And as members of Homoptera—the aphids and their relatives—cicadas are preadapted to get that water. Near Tucson in the summer, the Apache cicadas perch all day in the shade of a paloverde branch in an arroyo and tap water from deep down in the soil. The means of access to that water are the deep roots of the bush, which grow as much as sixty feet down to the water level. The water is piped to the twigs that the cicadas tap into with their sucking mouthparts.
Thermal wars are also waged directly, in combat between one insect and another. The Asian honeybee Apis cerana japonica faces a serious predator, the giant hornet Vespa mandarinia japonica. Hornet scouts invade beehives and if successful recruit their nest mates to come in force and devastate a beehive. The hornet is far too big and heavily armored to be killed forcibly by the much smaller bees. However, these honeybees have evolved a strategy that compensates for their size deficit. They pin a hornet down by clustering in hundreds around it to form a ball, and then they shiver and produce enough heat to raise the temperature at the center of the ball, where the hornet is, to 118°F. That temperature kills the hornet but is still a degree or two below the upper tolerance of the bees (Ono et al. 1995).
A slightly different story is played out by thermal warriors during the summer close to my home in Vermont and the woods in Maine. In this case the white-faced hornets, Dolichovespula maculate, whose summer colony strategies I discussed previously, are the beneficiaries of the thermal strategy. It is often hot in the daytime during our summers, but in early and late summer nighttime temperatures commonly dip to 38°F or lower, and they may remain there early in the morning. Such temperatures are so low that many small insects hunted by these hornets cannot fly. The hornets hunt by cruising over the foliage and pouncing on any contrasting object that may be a fly or some other unsuspecting insect. They have their best chance of success when their prey has slower reaction times or cannot fly off, or both; and that is in the early morning, when it is still cool. This is when the hornets, with a muscle temperature near that of our own, leave their warm insulated paper nests in force to hunt. They are larger than their