The range of insect dormancy responses

Abstract

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 develop ment 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 condi tions) 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 under standing 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.

Female reproductive patterns in the millipede Polydesmus angustus (Diplopoda: Polydesmidae) and their significance for cohort-splitting

“…It is uncertain whether this type of life cycle, previously unrecorded in the Polydesmida (Snider, 1981a;Hopkin & Read, 1992), can influence the dynamics of field populations. It is unknown whether the lack of aestivation results from genetic differences, in which case it would be heritable, or from environmental influences, both of which can affect the occurrence and duration of diapause in arthropods (Tauber et al, 1986;Danks, 2002). Environmental conditions that induce aestivation in P. angustus are not yet fully elucidated (David et al, 2003) and may have been sub-optimal in our laboratory cultures.…”

Insect adaptations to cold and changing environments

“…For example, in one or another circumstance or species, certain key molecules (cryoprotectants) can have one or more effects among multiple possibilities: they appear to favour supercooling or disable nucleators, resist or protect against desiccation, protect frozen membranes or other cell constituents directly, modify the freezing process, mitigate cellular damage from ice crystallization or recrystallization while tissues are frozen or while they are thawing, and repair damage after thawing. In the same way, developmental delays can serve multiple purposes: they may conserve energy, protect against adversity, synchronize the feeding stage with food resources, optimize the timing of reproduction, synchronize individuals with one another, prevent a risky generation late in the year, or assist in further life-cycle programming by allowing the environment to be monitored for a longer period (Danks 2002a). Moreover, there is more than one way to achieve the same end: selection is insensitive to exactly how a functional requisite is met.…”

“…Finally, the core structure of life cycles depends on whether development or delay is the default condition (Danks 2001(Danks , 2002a. When the default is active, development proceeds unless delays are specifically signalled.…”

“…Passive defaults permit very conservative, slow, or complex life cycles, especially when there are multiple requirements for development to resume. Passive or active defaults can be used to govern not only diapause, but also other seasonal developmental responses such as wing morph and instar number (Danks 2002a). …”

“…It is significant that even when the reliable, frequent, and recognizable signals from photoperiod are available, most species integrate information from temperature or food to modify this photoperiodic control (e.g., Goehring and Oberhauser 2002;Luker et al 2002). Evidently, even a small contribution of additional evidence from other cues about likely future environmental changes has adaptive value (Danks 2002a).…”