Project Update

The Survey’s project on seasonal adaptations has continued since the last newsletter update (Newsletter of the Biological Survey of Canada (Terrestrial Arthropods) 19 (1): 13-14). This project focusses on how insects meet the need to cope with the cold winters, short seasons and other aspects of northern climates characteristic of Canada. This need is reflected too, of course, in the composition of the Canadian fauna, in which groups of northern affinity and buffered habitats tend to predominate.

The project has continued to emphasize the theme that dormancy is not a static alternative to continuous development, but rather a dynamic process with many possible elements. These elements include variations in the type of developmental delay and its control, varying sensitivity to conditions, and complex life-cycle pathways that are built up from multiple integrated components.

Recent reviews have highlighted several aspects of insect dormancy that have commonly been overlooked. For example, dormancies have multiple purposes, helping to ensure both that individuals survive adverse seasons and that their activity and development take place at favourable times of year. Dormancy and related responses can be either "active" (development continues unless signalled otherwise) or "passive" (development stops unless signalled otherwise), a difference that has led to much confusion about the nature of so-called "obligate" diapause. Much ecological theory about life cycles relies on the existence of trade-offs (e.g. size versus time), but trade-offs are not forced when certain resources are surplus, and also resources cannot be traded off if they are in very short supply. Programmed life-cycle components occur frequently even at the highest latitudes, where flexibility in response to unpredictable temperatures often has been claimed instead.

In addition, recent papers have examined wider adaptations that are linked with dormancy to a greater or lesser degree, including water balance, cold hardiness, and direct modifications of the habitat by such means as burrows, shelters or galls, and by parental actions. Recommendations have also been made as to how information about life cycles should be recorded, because much of the recent literature on insect development is unusable for the wider purpose of understanding life cycles. These difficulties stem chiefly from flawed or singular modes of design, analysis or presentation.

Titles and abstracts of recent papers

(For some earlier titles see BSC Newsletter 19: 13-14)

Modification of adverse conditions by insects. Danks, H.V. 2002. Oikos in press.

Abstract: Many insects modify their environments directly, rather than merely choosing available sites that are already favourable. The modifications are carried out by making excavations in soil and other substrates, constructing feeding or resting shelters, inducing plant responses such as galls, aggregating, building colonial nests, and through parental actions. Such environmental modifications are briefly reviewed and related to the conditions that they modify. Some of the modifications offset physical factors such as dryness or flooding and cool or freezing temperatures. Others reduce the effects of natural enemies or enhance food resources. These effects have seldom been quantified and much of the evidence is anecdotal, but preliminary generalizations are made from existing information. Although potential roles often overlap, excavations and shelters protect especially against physical factors, while aggregations, colonies and parental actions more often influence the acquisition of resources. How modifications affect the impact of natural enemies differs among different kinds of enemies and is especially difficult to test. In any event, adaptive local modifications of the environment by insects are shown to be widely distributed and important. However, their specific roles have often been assumed rather than tested, or have been overlooked along with the potential interdependence of different effects. Therefore, environmental modifications should be considered explicitly and examined with greater rigour during the study of insect life cycles.

The range of insect dormancy responses. Danks, H.V. 2002. European Journal of Entomology 99(2): 127-142. [An extended abstract (same title and author) is in Kipyatkov, V.E. (ed.), IVth European Workshop of Invertebrate Physiology, St. Petersburg, Russia, 9-15 September 2001. Abstracts: 24-27]

Abstract: Insect dormancy responses, in the broad sense of modifications of development, are examined from a general perspective. The range of responses is extraordinarily wide because environments are diverse, different taxa have different evolutionary histories, adaptations are needed for both seasonal timing and resistance to adversity, and not only development but also many other aspects of the life-cycle must be coordinated. Developmental options are illustrated by examining the wide range of ways in which development can be modified, the fact that each individual response consists of several components, and the different potential durations of the life-cycle. The concepts of alternative life-cycle pathways (chosen according to current and likely future environmental conditions) and of active and passive default responses are treated. Also introduced are aspects of variation and trade-offs. Some general conclusions that help in understanding dormancy responses emerge from such an examination. Many options are available (cf. Table 1). The nature of the habitat, especially its predictability, determines the potential effectiveness of many of the developmental options. Any particular set of responses reflects evolutionary history and hence depends on past as well as current environments. It is not necessarily obvious what kinds of selection, especially requirements for timing versus resistance to adversity, explain a particular life cycle. Life-cycle pathways have multiple components, so that components cannot be analyzed in isolation. A given feature, such as delayed development, can have multiple roles. Default responses can be either active (development continues unless signalled otherwise) or passive (development stops unless signalled otherwise), making necessary a broad approach to understanding the action of environmental cues. Even relatively minor effects that fine-tune dormancy responses enhance survival, but may be difficult to detect or measure. Trade-offs are not inevitable, not only when certain resources are surplus, but also because resources in very short supply (constraints) cannot be traded off. Life-cycle components are widely, but not universally, coordinated. These conclusions confirm that the range of dormancy responses is wider, more complex and more integrated than has often been recognized.

The nature of dormancy responses in insects. Danks, H.V. 2001. Acta Societatis Zoologicae Bohemicae 65(3): 169-179.

Abstract: This paper takes a deliberately broad view of the ultimate purposes, the structures and the controls of dormancy responses in insects. Dormancies have multiple purposes (Table 1); in particular, they serve not only to survive adverse seasons but also to ensure that activity and development take place at favourable times of year. Many elements can contribute to the structure of the responses (Table 3) including different types and different extents of delay, control, sensitivity, default conditions, components, pathways and variability. Control options come from a range of internal and external factors (Table 4). The multiplicity, complexity and integration of the various facets of dormancy confirm that responses are dynamic and hence are by no means equivalent to simple on-off devices. Consequently, dormancy responses are best understood by considering whole life cycles in the context of whole environments, normally requiring studies that go beyond the simple approaches that are still prevalent.

Measuring and reporting life-cycle duration in insects and arachnids. Danks, H.V. 2000. European Journal of Entomology 97(3): 285-303.

Abstract: Some previous work on arthropod development is insufficiently detailed or incompletely reported. Much of the published information in this area is of limited use for the general analysis of life cycles. These difficulties arise primarily because many experiments do not control fully for the strain of the material (and even its specific identity) nor for rearing conditions, do not adequately take account of the complexity of life cycles and their stages, or are restricted to only part of the life cycle. For example, such factors as variable numbers of instars, sexual differences, abbreviated or hidden stages and dormancies may mean that the "average durations" reported apply to an unknown mixture of developmental types. Nor are experiments always designed or results reported and analysed in a logical and transparent manner. Undefined terms may obscure what actual developmental intervals were measured. Highly derived developmental or demographic measures may obscure core data. Statistical information may be inadequate. Such pitfalls are reviewed here, suggesting ways to ensure that results on the duration of development are both valid for specific studies and more widely useful. General experimental difficulties, recommended background information that should be provided, recommended life-cycle intervals and their terminology, and recommended ways to report numerical and statistical information are briefly summarized in tabular form.

Insect cold hardiness: A Canadian perspective. Danks, H.V. 2000. CryoLetters 21(5): 297-308.

Abstract: The cold climates and diverse environments of Canada have allowed key studies of insect cold hardiness that developed and widened the understanding of this subject. For example, freezing tolerance, chilling tolerance, freezing resistance, supercooling, cryoprotectants and other features can be combined in many different ways, reflecting a wide range of adaptations. Many other factors interact with and influence cold hardiness, such as habitats and their selection, and water and energy balances. These findings suggest several topics that would be especially fruitful for further study in northern Canada.

Dehydration in dormant insects. Danks, H.V. 2000. Journal of Insect Physiology 46(6): 837-852.

Abstract: Many of the mechanisms used by active insects to maintain water balance are not available to dormant individuals. Physiological and biochemical mechanisms of dehydration tolerance and resistance in dormant insects and some other invertebrates are reviewed, as well as linkages of dehydration with energy use and metabolism, with cold hardiness, and with diapause. Many dormant insects combine several striking adaptations to maintain water balance that - in addition to habitat choice - may include especially reduction of body water content, decreased cuticular permeability, absorption of water vapour, and tolerance of low body water levels. Many such features require energy and hence that metabolism, albeit much reduced, continues during dormancy. Four types of progressively dehydrated states are recognized: water is managed internally by solute or ion transport; relatively high concentrations of solutes modify the behaviour of water in solutions; still higher concentrations of certain carbohydrates lead to plasticized rubbers or glasses with very slow molecular kinetics; and anhydrobiosis eliminates metabolism.

The diversity and evolution of insect life cycles. Danks, H.V. 1999. Entomological Science 2(4): 651-660.

Abstract: Insect life cycles and their control are extremely diverse and complex. They can be studied in several ways, including the analysis of ecological correlations (according to habitat, region and species), trade-offs and modelling. Many existing studies of all of these types are available, and a number of generalizations about how life-cycle adaptations evolve can be drawn from them: similar environmental challenges can be solved in many different ways; similar responses can evolve independently; responses evolve in combination; a single response may contribute to many functions; the same response may serve different functions; each species has a unique set of responses; life cycles structure different resources; expected trade-offs are not inevitable; selection is a long-term process; and the nature of environments is the key to understanding life cycles. These generalizations show especially the great extent of complexity, parallel evolution and overlap in the responses of different species, as well as the great importance of environmental resources and conditions in structuring the responses. Although some such generalizations ought to be self-evident, they have often been overlooked. Enumerating them helps to demonstrate that great care is necessary for planning relevant studies. In particular, the generalizations suggest that future work on insect life cycles will be most fruitful if it is done in a broader context than most previous studies: by analyzing genetic and environmental components and their interactions at the same time; by assessing how life cycles structure the resources of time, space and energy; by measuring natural environmental conditions and their variation in more detail and in relation to specific life cycles; by conducting comprehensive work on individual species; and by developing long-term multifaceted studies rather than doing further elementary experiments.

Life cycles in polar arthropods - flexible or programmed? Danks, H.V. 1999. European Journal of Entomology 96(2): 83-102.

Abstract: Climate features that influence life cycles, notably severity, seasonality, unpredictability and variability, are summarized for different polar zones. The zones differ widely in these factors and how they are combined. For example, seasonality is markedly reduced by oceanic influences in the Subantarctic. Information about the life cycles of Arctic and Antarctic arthropods is reviewed to assess the relative contributions of flexibility and programming to life cycles in polar regions. A wide range of life cycles occurs in polar arthropods and, when whole life cycles are considered, fixed or programmed elements are well represented, in contrast to some earlier opinions that emphasized the prevalence of flexible or opportunistic responses. Programmed responses are especially common for controlling the appearance of stages that are sensitive to adverse conditions, such as the reproductive adult. The relative contribution of flexibility and programming to different life cycles is correlated with taxonomic affinity (which establishes the general life-cycle framework for a species), and with climatic zone, the habitats of immature and adult stages, and food.

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Newsletter of the Biological Survey of Canada (Terrestrial Arthropods)

Volume 21 No. 2, Fall 2002