During our last meeting we considered one aspect of living in temperate and polar regions of our planet, that is the business of cold hardiness in insects. We emphasized mechanisms of resisting and tolerating freezing. Biosynthesis of anti-freeze compounds allowed body fluids to cool far below a normal freezing point without ice crystal formation. This process is known as supercooling. In addition to supercooling, certain insects can tolerate freezing, for which the most important adaptation is that they can tolerate a remarkable level of cellular dehydration. These physiological adaptations to life in very cold conditions can be taken as rather extreme examples of a sort of biological recognition that conditions in almost every environment, aquatic, marine, terrestrial or atmospheric, are subject to regular and irregular changes. Today we want to regard some of the kinds of changes in environment, and look at some of the ways insects respond to changes.

Many insect species adapt to regular, seasonal changes in environmental conditions by manifesting various expressions of diapause. We want to appreciate what diapause is, and touch upon some aspects of physiological regulation of diapause. Tauber developed a comprehensive definition of diapause:

This is a large definition, and it is meant to convey several points: First, diapause is not a single physiological event, but rather is a syndrome of several physiological and behavioral manifestations all bearing on adapting insects to forthcoming environmental changes.

Second, the environmental signals that are translated into a diapause syndrome are called token stimuli because they do not themselves impact on insect life. Token stimuli simply herald the coming changes.

Third, the definition is biologically large because it includes dormancy and seasonal migration and seasonal polyphenism. To keep our terms straight, let us agree that dormancy is a period in a life cycle during which growth, maturation and reproduction are held in check. Dormancy can occur in any season. Polyphenism refers to seasonal changes in color or structure of an organism. There are many examples of this among insects, including increased wax secretion, variations in wing length and quality of cocoons (tighly versus loosely spun).

Fourth, the events in a dispause syndrome are not aimed directly at organismal survival during any particular set of environmental conditions. Diapause has more to do with synchronization of entire life cycles to seasonal changes in environments. It affects survival during unfavorable conditions, but it also bears a large weight upon other features of life history, including growth, development and reproduction. Hence, diapause is a unifying biological concept. Because it is under genetic control, it is an important substrate in evolution of life histories and in speciation.

Insects also cope with unpredictable environmental changes, such as droughts and overpopulation. Short-term or localized changes might include sudden temperature changes or local food shortages. Insects can adjust to these sorts of changes with aseasonal quiescence or aseasonal migration. These adaptations differ from diapause in the manner of physiological control. We will see that diapause is controlled by endocrine regulation; aseasonal adaptations are under central nervous system control.

Diapause is a genetical feature of insect life histories. All metamorphic stages in life cycles can be utilized as a diapause stage, but diapause usually occurs in a stage characteristic of a given species. Recalling from the definition that diapause is a phenomenon that anticipates future seasonal changes, we see that token stimuli must be perceived by an insect, then translated into a biological response. These token stimuli are perceived only during specific stages, and as with the diapause itself, timing of perception of token stimuli is under genetic control.

Perception of token stimuli and expression of diapause can be rather close in time, or widely separated between two generations. Here are some examples of the possibilities. Many insect species undergo diapause during the egg stage. In the cricket Teleogryllus, young eggs are sensitive to diapause-inducing stimuli, and diapause follows in eggs that are slightly further along in development. There are also cases in (for example, a cricket and two lepidopteran species) in which diapause-inducing stimuli are perceived during various stages in development of the mothers. Finally, in the aphid Megoura the grandparental generation is the sensitive one.

Going back to the idea of a diapause syndrome, we want to emphasize that diapause is not simply a reduction in metabolism and growth, but is a combined expression of several related, but different, physiological adaptations. These can include prediapause behaviors (such as preparing a hibernaculum), accumulation of glycerol for cold-hardiness, and color changes to track changes in plant colors, as well as alterations in energy metabolism. Insect endocrine systems play a central role in all of these events. At one time it was thought that diapause was mediated by a single endocrine mechanism for each life stage, but, as usual, the situation is more complicated.

It is generally agreed that diapause in adult insects attends reduced corpora allata activity, and hence, reduced hemolymph concentrations of juvenile hormone (JH). As the name suggests, we usually regard JH in terms of insect developmental physiology. However, its actions in regulating developmental events in juvenile insects is just one aspect of JH biology. JH is normally present in adult insects. JH is involved in regulating aspects of sexual maturation, such as egg development in females of some species and accessory gland development in males of some species. It follows that reduced JH titres (in endocrinology, titre refers to circulating levels of a hormone) would at least impact on reproductive aspects of diapause. In some cases adult diapause can be terminated by treating diapausing adults with JH.

Adult diapause is induced by short day lengths in the Colorado potato beetle. Diapause can be induced in beetles maintained under long day conditions by extirpating the corpora allata. In this species corpora allata activity is regulated by stimulatory neurohormones from the brain. Termination of diapause, accordingly, requires long day conditions. The sources of the stimulatory neurohormones are elements of the pars intercerebralis and corpora allata. Hence, in the Colorado potato beetle complete regulation of diapause involves neurohormones and JH. Just to give you a more detailed picture, the case is more complicated still in this beetle. Individual features of the diapause syndrome appear to be regulated by specific titres of JH, so that JH probably plays several different regulatory roles.

Diapause is under slightly different regulatory mechanisms in the bug Pyrrhocoris. Here JH and neurohormones are involved, as in the potato beetle. This bug differs, though, because there are inhibitory neurons from the brain that directly inhibit corpora allata activity, and thereby maintain low JH titre and diapause. The point here is recognizing an element of direct neurological control of diapause in this insect.

The classic work on pupal diapause was carried out on the giant silkworm, Hyalophora cecropia. When given their choice, physiologists often work on larger insects. The work on the giant silkworm was carried out by Carrol Williams and his co-workers at Harvard University. I guess you might say at the Nebraska University of the east coast. In this species pupal diapause follows cessation of brain neurosecretory activity, which stops prothoracicotropic hormone (PTTH) release. When we treat the physiology of post-embryonic development we will see PTTH stimulates prothoracic glands to release ecdysteroid, the moulting hormone. For our current interest in diapause, we note after a period in cold temperatures, "reactivated" brains release PTTH. PTTH stimulates release of moulting hormone, and this hormone signal leads to active adults.

The pupal diapause of another lepidopteran, Heliothis zea, is independent of PTTH. Low temperatures somehow act directly upon prothoracic glands, which reduces titres of moulting hormone. Pupae of flesh flies break diapause in response to moulting hormone, but the precise mechanism is slightly different from the lepidopterans just discussed. In flesh flies a sharp but brief peak of JH occurs before release of ecdysteroid. The JH is thought to positively influence, or somehow prepare, tissues to respond to ecdysteroid.

Larvae of many insects undergo diapause. In some speces, induction and maintenance of diapause depend on continuous presence of JH. In the rice stem borer, Chilo suppressalis, JH titres remain high throughout diapause, and corpora allata activity decreases as diapause draws to an end. This physiological mechanism of regulating diapause occurs in other species, too, but does not explain diapause in all insect larvae. In another mechanism, some diapausing larvae continually secrete JH from active corpora allata. In these species, diapause is induced and maintained because the JH inhibits release of PTTH.

Diapause interupts embryological development at a very early stage in the eggs of some insect species, such as the silkmoth Bombyx mori. This results from a maternal diapause hormone produced by the mother. Maternal diapause hormone is produced in the subesophageal ganglia, then transported via hemolymph circulation to eggs within the ovary. Circulating maternal diapause hormone causes diapause in eggs. A similar mechanism may occur in other insects, including the bug Adelphocoris.

Maternal diapause hormone is released from the subesophageal ganglia. The hormone release is triggered by enviornmental events. Silkworm females which experienced long days and warm temperatures during their egg and early larval periods lay eggs that enter diapause. Females which experienced short days do not produce maternal diapause hormone, and their eggs do not enter diapause. The maternal diapause hormone of Bombyx has been purified. It is a dipeptide, consisting of two different peptide chains.

Diapause hormone acts to arrest embryonic development at the stage of germ band formation. There is a tight correlation between diapause and carbohydrte metabolism. The developing ovaries accumulate large amounts of glycogen during the induction phase of diapause. When diapause begins, the glycogen is converted to polyols. At the end of diapause the polyols are converted back into glycogen, which becomes the major source of energy to carry out embryonic development. Diapause hormone is thought to influence carbohydrate metabolism in eggs.

The follicle cells of the ovaries are the targets of diapause hormone. Let us consider a model of diapause hormone action. Diapause hormone is thought to interact with specific receptors on the plasma-membrane of follicle cells. The hormone receptor translates the hormone signal into reduced intracellular levels of cGMP. The decreased cGMP levels activates increased membrane trehalase activity. This activity leads to increased trehalose hydrolysis and in-bound transport of glucose. The glucose is rapidly taken up into glycogen.

Beside its effect on carbohydrate metabolism, diapause hormone is thought to exert other actions on ovaries. The physiology, ecology and evolution of insect diapause remain contemporary areas of study. A far more complex picture of hormonal regulation of diapause will emerge in due course.