Diapause

Diapause in insects is a dynamic process consisting of several distinct phases. While diapause varies considerably from one taxon of insects to another, these phases can be characterized by particular sets of metabolic processes and responsiveness of the insect to certain environmental stimuli. For example, Sepsis cynipsea flies primarily use temperature to determine when to enter diapause. Similarly, Chrysoperla plorabunda lacewings regulate their reproductive cycle using daylight length, with adults entering reproductive diapause when there are less than 12-13 hours of daylight. Diapause can occur during any stage of development in arthropods, but each species exhibits diapause in specific phases of development. Reduced oxygen consumption is typical as is reduced movement and feeding. In Polistes exclamans, a social wasp, only the queen is said to be able to undergo diapause. Comparison of diapause periods The sensitive stage is the period when stimulus must occur to trigger diapause in the organism. Examples of sensitive stage/diapause periods in various insects: Induction The induction phase occurs at a genetically predetermined stage of life, and occurs well in advance of the environmental stress. During this phase, insects are responsive to external cues called token stimuli, which trigger the switch from direct development pathways to diapause pathways. Token stimuli can consist of changes in photoperiod, thermoperiod, or allelochemicals from food plants. These stimuli are not in themselves favourable or unfavourable to development, but they herald an impending change in environmental conditions. Diapausing puparia of the flesh fly, Sarcophaga crassipalpis, increase the amount of cuticular hydrocarbons lining the puparium, effectively reducing the ability of water to cross the cuticle. Initiation Photoperiod is the most important stimulus initiating diapause. Insects may also undergo behavioural changes and begin to aggregate, migrate, or search for suitable overwintering sites. in diapause clustering on oyamel trees. One tree is completely covered in butterflies. These butterflies were located on a preserve outside of Angangueo, Michoacán, Mexico Maintenance During the maintenance phase, insects experience lowered metabolism and developmental arrest is maintained. Termination may occur at the height of unfavourable conditions, such as in the middle of winter. Over time, depth of diapause slowly decreases until direct development can resume, if conditions are favourable. Termination can also occur in specific time frames linked to reproductive periods, such as in the beetle Colaphellus bowringi: diapause ends for spring-reproducing beetles between late February and early April and for autumn-reproducing beetles between mid August and early October. Post-diapause quiescence Diapause frequently ends prior to the end of unfavourable conditions and is followed by a state of quiescence from which the insect can arouse and begin direct development, should conditions change to become more favourable. This allows the insect to continue to withstand harsh conditions while being ready to take advantage of good conditions as soon as possible. ==Regulation==

Diapause in insects is regulated at several levels. Environmental stimuli interact with genetic pre-programming to affect neuronal signalling, endocrine pathways, and, eventually, metabolic and enzymatic changes. Environmental Environmental regulators of diapause generally display a characteristic seasonal pattern. In temperate regions, photoperiod is the most reliable cues of seasonal change. Depending on the season in which diapause occurs, either short or long days can act as token stimuli. Insects may also respond to changing day length as well as relative day length. Temperature may also act as a regulating factor, either by inducing diapause or, more commonly, by modifying the response of the insect to photoperiod. Food availability and quality may also help regulate diapause. In the desert locust, Schistocerca gregaria, a plant hormone called gibberellin stimulates reproductive development. During the dry season, when their food plants are in senescence and lacking gibberellin, the locusts remain immature and their reproductive tracts do not develop. Neuroendocrine The neuroendocrine system of insects consists primarily of neurosecretory cells in the brain, the corpora cardiaca, corpora allata and the prothoracic glands. Prothoracicotropic hormone stimulates the prothoracic glands to produce ecdysteroids that are required to promote development. Adult diapause is often associated with the absence of JH, while larval diapause is often associated with its presence. In adults, absence of JH causes degeneration of flight muscles and atrophy or cessation of development of reproductive tissues, and halts mating behaviour. The presence of JH in larvae may prevent moulting to the next larval instar, though successive stationary moults may still occur. In the corn borer, Diatraea gradiosella, JH is required for the accumulation by the fat body of a storage protein that is associated with diapause. Diapause hormone regulates embryonic diapause in the eggs of the silkworm moth, Bombyx mori. DH is released from the subesophageal ganglion of the mother and triggers trehalase production by the ovaries. This generates high levels of glycogen in the eggs, which is converted into the polyhydric alcohols glycerol and sorbitol. Sorbitol directly inhibits the development of the embryos. Glycerol and sorbitol are reconverted into glycogen at the termination of diapause. ==Tropical diapause==

Diapause in the tropics is often initiated in response to biotic rather than abiotic components. For example, food in the form of vertebrate carcasses may be more abundant following dry seasons, or oviposition sites in the form of fallen trees may be more available following rainy seasons. Also, diapause may serve to synchronize mating seasons or reduce competition, rather than to avoid unfavourable climatic conditions. Diapause in the tropics poses several challenges to insects that are not faced in temperate zones. ==See also==