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How can shorter daylength trigger diapause in insects?

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Illustration showing a variety of adult insects, each labeled with its common name and order, including walkingstick, earwig, stonefly, tree cricket, dragonfly, female scale, cat flea, thrips, sucking louse, thysanuran, ant, scarab beetle, stinkbug, mosquito, and cabbage butterfly, highlighting their diverse shapes and sizes. — insect An illustration of insect diversity.

In insects, a decrease in daylength (photoperiod) serves as an environmental cue that triggers diapause by influencing the insect's endocrine system.12

As summer wanes, the shortening daylength acts upon the insect's brain, which functions as a regulatory center for development. Specifically, these environmental changes cause the brain to cease the synthesis and secretion of brain hormone. Because this hormone is required to stimulate the prothoracic glands, its absence leads to a halt in the production of ecdysone, the hormone responsible for growth and molting.32

When concentrations of these essential growth and molting hormones decrease, the insect enters a state of arrested development characterized by reduced metabolic activity and, in some cases, the inactivation of the corpora allata. This physiological shift allows the insect to survive adverse conditions, such as winter, by entering a state of developmental standstill until favorable environmental conditions return.1342