Take-off gives bugs the shiver

The Thermal Warriors
November 1, 1996

A principal concern of nature conservationists worried about global warming, is that the predicted 2xC rise in mean temperatures, will reduce biological diversity. Many of the predicted extinctions will be due to wholesale habitat destruction, as the human population adjusts to its changing environment. Pro rata, insect species will bear the brunt of extinctions, for over 75 per cent of the 1.2 million species of animals so far described by scientists, are insects. More staggering is the estimated 9-30 million as yet uncatalogued insect species.

Despite the best efforts of British conservationists, there will be national extinctions. Understanding exactly why some species might benefit from increased temperatures while others will suffer, involves a complex integration of ideas from animal physiology, behaviour and ecology. Such an integration has been achieved by Bernd Heinrich in his new book The Thermal Warriors. Heinrich takes an evolutionist's standpoint and tries to explain how basic physiological adaptation and thermal regulation are important to life history strategies of insects. He does not directly consider the impact of global warming on insects. Nevertheless his book both provides excellent background reading for those interested in global warming, and an easily read account of one of the driving forces behind the highly evolved intricacies of insect behaviour.

All animals thermoregulate. The task is much easier for marine animals because the sea provides an extra blanket to that of the atmosphere, ensuring high-temperature stability compared with the terrestrial environment. Biologists are agreed that life began in the sea a very long time ago and that insects were among the first terrestrial life forms to evolve. In the first chapter, Heinrich quickly takes us through their early evolution, explaining how slow-moving primitive aquatic insects might have evolved wings and the associated high metabolic activity shown by most modern flying insects. It is almost incomprehensible that much of this evolution had been achieved 300 million years ago.

The simple physics of thermoregulation is outlined in chapter two. Humans and most other mammals are relatively large and the small surface-volume ratio means that we lose heat much more slowly than smaller bodied animals. In evolutionary terms, it has paid larger animals to evolve homeostatic temperature control, that is internal metabolic regulation of temperature within a narrow range. The metabolic costs of doing this are greater for small animals, and for animals such as insects, the costs are too high. Heinrich sets out to explain how evolution has produced alternative ways of generating the high temperatures required for flight and other rapid movements.

In essence, insects achieve this in three ways. First, there are individual physiological adaptations. Heinrich shows how anatomy, both internal and external, has been modified to reduce heat loss and concentrate heat in the regions of the body, usually the thoracic muscles, where it is most needed. When cold before flight, many insects shiver generating metabolic heat until thoracic temperatures approach the 36-40xC required for efficient flight. The author uses the nice analogy of the difficulties involved in starting and running a car engine in cold weather, to explain some of the problems insects face when starting flight in a cold ambient temperature.

Second, there are individual behaviours: both heat seeking and avoidance behaviours are illustrated. Basking is the most well known of these. But, basking has costs; the basking individual is often particularly vulnerable to predators. Such costs and benefits associated with temperature regulation have led to a sort of thermal arms race among insects. For example, if one insect can evolve a mechanism, either physiological or behavioural, for hunting its prey in hotter or colder conditions, then the consequences for both its prey and competitor-hunter species are enormous. If these are to survive, they must either evolve the same trait or some other counter-measure. It is this idea that led Heinrich to title the book The Thermal Warriors. Such integrated evolutionary ideas are notoriously difficult to explain clearly and a strength of this book is that Heinrich manages to make complex interactions seem obvious and simple.

Third, insects have evolved group or social behaviours for thermoregulation. Chief among these are the social bees. This is the group on which Heinrich cut his biological teeth, and his fondness for them is evident. Bees are remarkable, they maintain a group temperature of about 37xC. A colony behaves corporately like a homeostatic animal, even though individual bees have the poikilothermic (variable-temperature) attributes of other flying insects. The thermal war continues; the oriental honeybee, Apis cerana, can kill predatory hornets, many times their size, by clustering around them so tightly that they soon generate temperatures at the cluster centre that are lethal to the enveloped hornet.

The clustering behaviour of the oriental honeybees provides one of the eight pages of colour plates at the centre of the book. Six of these pages are thermo-image photographs of insects; while at first sight not as pretty as the other two photographs, they prove to be fascinating when examined closely. The rest of the book is illustrated by line drawings. There is one on almost every page, about half are informative and the remainder are mainly decorative. However, this is not a criticism because they do help with the informal, easy-to-read feel of the book. Heinrich achieves his aim of producing a primer for college students and naturalists alike, by explaining complex issues simply and not labouring them with too much scientific justification. I recommend it to all students studying environmental science, school-teachers and anyone else who wishes to gain a first insight into the integrated importance of physiology, anatomy and behaviour in the adaptation of insects to their environment.

Graham W. Elmes is senior principal scientific officer, Natural Environment Research Council, Institute of Terrestrial Ecology, Wareham.

The Thermal Warriors: Strategies of Insect Survival

Author - Bernd Heinrich
ISBN - 0 674 88340 3
Publisher - Harvard University Press
Price - £17.95
Pages - 217

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