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Regulating Body Temperature - the Key to Insect Survival, February 20, 2009
This review is from: The Thermal Warriors: Strategies of Insect Survival (Paperback)
The Thermal Warriors: Strategies of Insect Survival
Author: Bernd Heinrich
221 Pages
Publication Date: 1996
Cambridge, MA: Harvard University Press
USBN 0-674-88341-1
When I run, or work outside, or bask in the sunshine at the beach, I get hot. When I can't take any more of the heat, I perspire, I take a break, and I sit in the shade. Likewise, in winter, when I wait for the bus or sit on a ski lift and a cold wind begins to blow, I shiver to warm up. Surprisingly, insects perform similar actions to regulate their body temperature. Bernd Heinrich, author of The Thermal Warriors: Strategies of Insect Survival, introduces us to this concept in the preface to his interesting and in depth work by stating, "We humans engage in endurance contests [marathons] such as these only rarely, for the sheer fun or foolishness of it, or for some symbolic trifle. For insects, however, the struggle to keep body temperature within an acceptable range is constant, and often it is a matter of life or death. Each insect is a "thermal warrior" in a constant struggle with its predators and competitors in the context of its physical environment" (viii). Heinrich, as a physiologist, ecologist, and evolutionary biologist, expertly conveys the fact that thermoregulation is an essential part of insect life.
Heinrich begins his book by explaining that thermoregulation is not a relatively new aspect of insect life. The earliest insects, arising at least 350 million years ago, had body temperatures that were a direct reflection of ambient temperature. Thermoregulation has a lot to do with flight, the operation of the insect's flight motor, and the temperature of the thorax and abdomen. Thermoregulation also encompasses the balance of heat production necessary for activity with the rate of heat that can be lost. Heinrich compares the insect flight motor to the engine of an automobile - both need to heat up to perform work and cool down so that they do not overheat and combust. The insect flight motor is biologically complicated with chemical and mechanical sensors that send signals to a reaction center that controls the motor, the wings, and all corresponding muscles. What was most interesting, in these initial chapters of the book, was the high degree of specialization that must occur in the proteins, compounds, and muscles that must work in order for insects to regulate their temperature to perform flight. This specialization, Heinrich notes, is relatively unknown and highly speculative. The degree of adaptation over the wide range of temperatures at which flight mechanisms operate is astonishing.
I have observed bees flap their wings while perching on a flower before taking flight, but I never realized that they were warming up, or for that matter, that they had to warm up, lest they couldn't fly. Heinrich gives the example of the tobacco hornworm sphinx moth that heats up in flight due to its large muscle mass. Insects, like this moth, cannot dissipate heat to equalize their body temperature with ambient temperature. These insects must operate their flight motors at high temperatures, requiring them to warm up to achieve the body temperature needed to operate the flight motor. The warm up action can be equated with shivering because muscles must contract and elongate during this phase which produces heat as a byproduct of cellular metabolism. The heat produced by these actions is absorbed by the thorax, the segment of the insect's body that houses the flight motor, thus warming up the flight motor.
Many insects also warm up by basking. There are many basking postures assumed by insects, and facts show that basking in the sun does indeed work to increase body temperature while protecting against the loss of heat. For example, the thorax of the Pachliopta aristolochine needs "approximately 8 minutes to heat to an equilibrium temperature (about 42°C) from 25°C in sunshine, while the spread of wings heat to about 46°C in just 30 seconds" (55). What is interesting about the action of basking it not only the fact that it works so efficiently (30 seconds to warm up!) but that there are so many ways to go about doing it and insects have learned how to best achieve positions to get the most warmth for the least amount of energy expenditure. The eastern blue butterfly, Everes comyntas, performs dorsal backing - spreading its wings in a "V" to reduce exposure of the thorax to minimize cooling, while allowing its wings to warm u by "reflective heating." Another butterfly, the Colias eurythene, assumes a lateral basking posture in which it tilts its upright wings to the sun, shielding its thorax from convective cooling, while exposing its lateral wing surface to the sun's warming rays. Basking its simply a way to increase the temperature of the flight motor without having to expend any energy - like the bees that shiver to warm up!
Like car engines, insect flight motors must not overheat, or they cease to work. Insects have evolved to incorporate convective heat loss into their arsenal of thermoregulatory techniques. Tiger beetles, Cincindenla hybrida, for example, can stilt above the hot desert sand to reduce temperature gain from the hot sand. These beetles can also resort to short bursts of flight, which passes air over their thorax, cooling their flight motor. Evaporative cooling also works - the larvae of the Australian sawfly, Perga dorsalis, spread rectal fluid on their abdomen when desert temperatures get too high.
The level of detail in Heinrich's first six chapters is astounding and somewhat overwhelming, but this information clearly expresses that extensive research has been conducted to study these insects and that thermoregulation is such an integral part of an insect's survival tactics. Heinrich goes on to explain in Chapter 7 that form and function determine how well an insect thermoregulates and conserves energy. The scaly wings of the Lepidopterans (butterflies and moths) are a perfect example. These wings have evolved to act as an insulating coat for the insects, reducing heat loss during cool nights. African flower scarab beetles conserve energy by seeking shade, reducing the amount of energy that they would need to expend to cool off.
Heinrich equates the performance of thermoregulation with an arms race. In Chapter 11, he writes, "the regulation of body temperature is expensive in terms of water and energy, and sometimes in terms of time and exposure to predators. In many cases, the burden of these costs, which must be borne if the insect is to be active, can be exploited to advantage by a competitor having greater resources. Over time, the escalation of resources devoted to competition may result in a spiraling arms race, leading to the ultimate exclusion of one of the competitors" (140). Thermoregulation also plays a part in insect health. What I found amazing was that comparisons can be made in the human and inspect response to sickness - we heat up! Studies have shown that insects, like the American migrating grasshopper, Melanoplus sanguinipes, when exposed to pathogens, heat up - stopping the pathogens that would ultimately kill the insect, from replicating.
Heinrich concludes his book with a discussion of the role insects play in the lives of humans - as pollinators and as pests - activities that are controlled by temperature. Integrated Pest Management, a program that decreases the use of pesticides in the environment has even incorporated heat treatments to kill insect pests - using methods of thermoregulation (that insects have perfected) to eliminate pests without harming the environment.
Bernd Heinrich's natural history book about thermoregulation in the insect world perfectly conveys the methods in which insects thermoregulate and why they perform such measures to control their body temperature. At times dense with scientific and mathematical explanation, the book is highly accessible nonetheless as Heinrich uses analogies and diagrams to express concepts that even those without a scientific background could understand. The beauty and complexity of the methods of thermoregulation is expressed in each of the fourteen chapters. Heinrich is thoroughly and extremely knowledgeable about the subject matter and incorporates studies and experiments conducted by noted physiologists and ecologists to support his own research.
Having read Heinrich's other book, Mind of the Raven, published some years after Thermal Warriors, I can say that I always finish Heinrich's books having learned something completely new. Although this is the first book that I have read on the topic of insects, I can now appreciate their complexity and adaptation as a result of Bernd Heinrich's work.
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