False Assumptions

A Matter of Degrees

Nessus sphinx moth

Bernd Heinrich

It looked like a small hummingbird, but it was a Nessus sphinx moth (Amphion floridensis), one of forty-five species of North American sphinx moths. What caught my attention more than anything else were the two brilliant wasp-like yellow bands across the dark back of its abdomen contrasting with the delicate patters of chocolate brown and cinnamon. The Nessus has converged to look uncannily like a hummingbird. The Nessus is not only hummingbird-like in the proportions of its short plump body and small propeller-like wings, but it has a most bird-like short tail at the tip of its abdomen, and, unlike most sphinx moths, it is day-active. The most startling feature of the sphinx moth is its “tongue.”

The long rigid bill of a hummingbird has evolved to access nectar in generally long tubular flowers. The bill is formed, as it is in all birds, by the upper and lower jaw (or maxilla and mandible). In moths, the equivalent to a bird’s bill is the proboscis, derived from a pair of maxillae, which in most insects serve as the laterally-moving biting/chewing mouthparts. But in the case of moths, unlike in bees and hummingbirds, there is no tongue housed between the two maxillae. Instead, the maxillae have been modified to abut permanently against one another to form an interlocking tube that has become a sucking straw. Sphinx moths that do not feed as adults lack this structure, while in others that feed from flowers in the same way that hummingbirds do, it can extend as much as three and a half times their body length.

A long tongue enables some moths to reach nectar that is even beyond the reach of hummingbirds. For hummingbirds the cost of reaching nectar with a bill sticking up to a foot or more ahead of them outweighs the benefits. It would likely be too costly for moths too, except they have the ability to curl the whole feeding mechanism into a tight mass, tuck it under their “chin” until needed, extend it in an instant, and manipulate it with pin-point accuracy. This highly evolved mechanism is extremely unlikely ever to be matched by hummingbirds, because both species are too far along with their own specializations. Selective pressure can only act on what already exists. Each specialization can be improved upon but not changed to a new design. In the case of sphinx moths, hummingbirds and flowers, the three have been engaged in a co-evolutionary arms race for a very long time.

In 1862, while Charles Darwin was studying orchids, he received specimens of Angraecum sesquipedale, an orchid that had been discovered in Madagascar in 1798. He was impressed by its very long nectary—the tube leading from the flower entrance to the nectar—and predicted a sphinx moth must pollinate the plant. He was ridiculed at the time, but in 1903 he was vindicated when one fitting his description was discovered in Madagascar by naturalists Lionel Walter Rothschild (Lord Baron) and his associate Karl Jordan of the Tring Museum in London. That species of large sphinx moth was named Xanthopan morganii.

Darwin’s insight into what pollinated the orchid was based on the morphology of the plant and that of sphinx moths. The underlying energy exchange driving the pollination—the energetics or biological thermodynamics—was not yet understood. The plant needed to invest in the production of fuel in the form of sugary nectar to attract and reward pollinators. Sugar, however, is a desired food by all sorts of animals that could eagerly raid the nectar, not pollinate, and leave so little for others that potential pollinators could not afford to compete. The plant, therefore, needed to invest also in a nectary tube that was long enough to exclude nectar thieves and reward pollinators.

The longer the nectar tube got, the more exclusive became the pollinator clientele. It was crucial that the flower reliably provided what its corolla’s shape, color, and/or scent advertised; otherwise, the pollinator would learn to ignore the signs. The longer the nectar tube, the more food energy was available exclusively for the pollinator, and the more available nectar created a more energy-dependent pollinator. On the other hand, more energy enabled sphinx moths to be selected for larger size and longer tongues, and it facilitated both hovering and long-distance flight. Pollination of widely-scattered plants became energetically feasible for long-distance flying moths, even though their very long tongues put sphinx moths at a disadvantage in competing for the small amount of nectar in flowers with short corollas that are ideally-suited for shorter-tongued insects.

An added unknown in Darwin’s time were the roles of physiology, heat production, and body temperature regulation that are also important variables in understanding the pollinator-plant equation. The activities of insects—as with all animals—depend on muscle contractions. And muscle contractions produce heat. Insects have been commonly categorized as “cold-blooded,” even though some have been found to elevate their body temperature. Early in my career, during a research study in 1969, I was astounded when I measured the muscle temperature of white-lined sphinx moths, Celerio (now Hyles) lineata, that were hovering at flowers on a cold evening in the Anza Borrego desert of California. The first individual that I captured and stuck with a thermocouple registered 44° C (or 111.4° F), certainly not cold-blooded by our human standard of a normal body temperature of around 37° C. 

White-lined sphinx moths, Celerio (now Hyles) lineata, hovering on Beloperone californica

Bernd Heinrich

A high body temperature in insects, as in vertebrate animals, is both an immediate consequence of their activity, and an ultimate necessity for it. Muscles heat up due to the heat produced as a by-product of contracting, and they must be adapted to operate near the temperature that they have been subjected to during activity over the time course of their evolutionary history. Thus, prior to activity, large moths and other large insects that inevitably generate a high muscle temperature in flight must first shiver to reach appropriate flight temperature.

A large-bodied animal passively loses heat more slowly in comparison to a small one. Furthermore, at a given body size—all other things such as insulation being equal—the cooling rate is in direct proportion to the temperature difference between the object and the environment. Sphinx moths are relatively large. They contract their massive flight muscles 30-60 times per second, they are active in the summer, and they are most common in tropical climes. They are therefore more hot-blooded from their own metabolism than almost any other animal on earth and they share with us some common thermally-related problems and traits.

We, too, are of large size and evolved in a hot environment where we were burdened with internal heat production from vigorous sustained activity during the chase for food. To exploit the problems of other animals overheating, however, we evolved a superb mechanism of dumping excess body heat by sweating. Some insects have also evolved the ability to regulate their body temperature by mechanisms to get rid of excess body heat, instead of simply reducing their rate of heat production, i.e. energy output.

 Some desert cicadas exude liquid from glands on their body that cools them when they engage in their highly muscular activity of singing. The liquid for this “sweating” comes indirectly from the juices of plants that get water through their deep underground roots. Honeybees have a different twist: they regurgitate nectar and let it evaporate from their tongue like a dog, and/or spread it with their forelegs over their thorax if it gets overheated. Sphinx moths can do neither. However, they have evolved anatomy and physiology that lets them shunt the excess heat generated by their working flight muscles to their abdomen, using blood as the heat carrier. The process acts like the radiator of a car receiving excess heat transferred from the engine to keep it from overheating.

Some species of mostly night-flying noctuids, or owlet moths, are active in early winter and spring, presumably in order to avoid predation by birds. These moths have the opposite thermal problem from sphinx moths. They are much smaller and they fly when air temperatures are sometimes near the freezing point of water. They are able to retain most of their heat in the thorax where the flight muscles reside. They have fluffy insulation on their thorax, as well as two counter-current heat exchangers: one that reduces heat loss to the head, and the other that largely prevents heat loss to the abdomen.

Some species of even smaller winter-flying moths of the Geometridae family have evolved an entirely different strategy to solve the same problem. They fly with a body temperature almost the same as the air temperature, even down to the freezing point of water. Their muscles are also adapted to the temperature experienced, but they are limited in energy output because they have large wings that act in part as sails. Their very light bodies require minimal work to stay airborne. These moths do not generate much heat, nor could they retain much of it if they did. Only the males fly. The females lack wings and are grub-like in form, rotund from their loads of eggs.

I mention these innovations of insects because, given their generally small body size, the enormous exercise that they are capable of, and the harsh environments of heat and cold where they can be active, it seems more impressive to me that many regulate a higher body temperature than our own, and do it incredibly well compared to our own evolution as warm-blooded animals. This ability of insects does not mean that most are not typically cold-blooded. Their matching of body temperature to their environment is common. But such relaxation of body temperature, when practiced by most vertebrate animals, is considered an amazing adaptation for energy economy.

The impressive thing about insects is not what most of them do most of the time. It is what they can do. Failure by observers to recognize the full range of insect behavior is like assuming humans can only walk, and not crawl or run, because statistically—on any city street and at almost any time—that is our method of locomotion. In all animals, variables of size and rate of energy expenditure determine the best strategy, at any given time.  


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