Diapause

Some bugs take a long nap to stay safe.

This helps when it is too cold. It helps when food is gone.

Many bugs stop growing for a while. They move less and eat less. This helps them save their energy.

Some bugs stay in one spot. They might stay as eggs or pupae. Other bugs still fly. Monarch butterflies are very active even while they rest.

Bugs watch for signs of change. They look at how much light there is. They also feel the heat or cold.

When things get better, they wake up. They can then grow and fly again.

Many animals have a special way to survive hard times. This is called diapause. It is a way to delay growth. It helps animals survive when the weather is bad. It helps when it is too cold or too dry. It also helps when food is hard to find.

Insects use diapause in many ways. Some stay very still, like eggs or pupae. Others stay active. Monarch butterflies are active, but they do not grow or mate during this time.

Diapause happens in steps. First, the animal feels a sign of change. These signs are called token stimuli. An insect might feel the days getting shorter. It might feel the temperature change. These signs tell the insect that bad weather is coming.

Next, the insect prepares. It stores fats and proteins for power. Some insects even change their skin to keep water inside. Then, the insect enters diapause. It stops growing and uses less oxygen. Finally, the insect waits. It stays in this state until the environment is good again. Some insects wake up when they feel warmth or light. This allows them to grow as soon as life is good.

Diapause is a special way that animals delay their growth. It helps them survive when the environment becomes hard to live in. This might happen during extreme cold, a long drought, or when food is hard to find.

Many living things use this trick, especially arthropods like insects. Some insects stay completely still, such as eggs or pupae. Other insects stay active but change how they live. For example, the adult monarch butterfly, Danaus plexippus, stays active but stops feeding and mating. This state is important because it helps animals wait for better times.

This way of living happens in several clear steps. First is the induction phase. This happens at a specific time in an animal's life. The animal notices a "token stimulus," which is a sign of change. These signs are not bad themselves, but they signal that bad weather is coming. Common signs include changes in daylight or temperature. Next is the preparation phase. During this time, the insect stores energy like fats and proteins. Some insects even change their outer skin to stop water from escaping.

After preparation comes the initiation phase. This is when the actual growth stops. An insect might change color or molt into a new stage. Some insects, like the fire bug, Pyrrhocoris apterus, even change their internal chemistry. They make special molecules to stop them from freezing. Then comes the maintenance phase. The insect uses very little oxygen and stays in a state of arrested development. It becomes very sensitive to the environment to make sure it does not wake up too early. Finally, the termination phase ends the diapause.

Scientists have studied many different insects to see how they do this. For example, the Sepsis cynipsea fly uses temperature to decide when to start. The Chrysoperla plorabunda lacewing uses the length of daylight. If there are less than 12 or 13 hours of light, they enter a reproductive pause. We can also look at the southwestern corn borer, Diatraea grandiosella. This insect enters diapause during its larval stages. Different species have different rules for when they start and stop. The Arctic woolly bear moth, Gynaephora groenlandica, uses diapause during its mid-larval stage.

Diapause is controlled by the animal's body and its genes. The brain and special glands send out hormones to control the process. One important hormone is called juvenile hormone, or JH. In many adult insects, the absence of JH stops them from mating. In the silkworm moth, Bombyx mori, a different hormone called diapause hormone helps the eggs. This hormone helps create high levels of sugar in the eggs. These sugars act as a way to stop the embryo from growing too soon. This complex system ensures the insect wakes up only when it is safe to grow.

Diapause is a physiological state of dormancy used by animals to survive unfavorable environmental conditions. This delay in development allows organisms to endure predictable periods of stress, such as extreme temperatures, droughts, or limited food supplies. While it is most commonly observed in arthropods, particularly insects, diapause is a vital survival mechanism across many life forms. It can occur in completely immobile stages, like eggs or pupae, or in active adults. For instance, the adult monarch butterfly, Danaus plexippus, remains active but reduces feeding and halts reproductive development during diapause.

The process follows a specific sequence of biological phases. It begins with the induction phase, which occurs at a genetically predetermined stage of life. During this time, the organism responds to "token stimuli." These are environmental cues, such as changes in photoperiod (daylight length) or temperature, that signal an impending change in the environment. These stimuli do not cause the stress themselves, but they act as early warning signs. Following induction, the preparation phase allows the insect to accumulate essential molecules like lipids, proteins, and carbohydrates. These reserves provide the fuel necessary to maintain the body and eventually restart development.

In some species, the preparation phase also includes physical changes to prevent desiccation, or drying out. The flesh fly, Sarcophaga crassipalpis, increases the cuticular hydrocarbons in its puparium to reduce water loss. Once the insect is ready, it enters the initiation phase. This is when morphological development officially ceases. This phase may involve a visible change, such as a molt or a change in color. Some insects also undergo enzymatic changes to prepare for cold. The fire bug, Pyrrhocoris apterus, develops specific enzymes that allow it to accumulate polyhydric alcohols. These molecules lower the insect's freezing point, preventing it from freezing solid.

During the maintenance phase, the insect stays in a state of developmental arrest. Metabolism is lowered, and oxygen consumption is typically reduced. The organism becomes highly sensitive to specific environmental stimuli that might accidentally trigger the end of diapause. This sensitivity ensures the insect does not wake up during a temporary warm spell in a harsh season. Eventually, the process reaches the termination phase. In obligate diapausing species, termination may happen spontaneously. However, in facultative diapausers, specific cues like chilling, freezing, or contact with water are required to end the state. This prevents the insect from terminating diapause too early in the season.

Termination can also be linked to specific reproductive timeframes. For example, the beetle Colaphellus bowringi ends diapause between late February and early April for spring reproduction. It ends diapause again between mid-August and early October for autumn reproduction. Sometimes, diapause ends while unfavorable conditions still exist. This leads to a period called post-diapause quiescence. During quiescence, the insect is ready to begin direct development immediately if conditions become favorable, but it can still withstand harsh weather.

Different species use different cues to regulate these stages. The Sepsis cynipsea fly primarily uses temperature to determine when to enter diapause. In contrast, the Chrysoperla plorabunda lacewing uses daylight length. These lacewings enter reproductive diapause when there are fewer than 12 to 13 hours of daylight. Other species show different sensitive stages. The southwestern corn borer, Diatraea grandiosella, enters diapause during its late larval stage. The Arctic woolly bear moth, Gynaephora groenlandica, enters diapause during its mid-larval stage. These variations show how diverse the survival strategies of insects can be.

On a biological level, diapause is regulated by the neuroendocrine system. This system involves the brain and several key glands, including the corpora allata and the prothoracic glands. Hormones play a central role in this regulation. Juvenile hormone (JH) is critical; in many adult insects, the absence of JH causes reproductive tissues to stop developing. In the silkworm moth, Bombyx mori, a specific diapause hormone (DH) regulates embryonic diapause. The mother releases DH, which triggers the production of trehalase in the ovaries. This creates high levels of glycogen, which is then converted into glycerol and sorbitol. These polyhydric alcohols inhibit embryo development and protect the eggs during their dormant period.