1. Insects Do Not Disappear in Winter: Each Species Spends It in a Set Life Stage
Insects grow from eggs to larvae, then through a pupal stage to become adults. The stage at which they spend the winter is almost fixed for each species [2]. When an insect stops growing to wait out the winter, this state is called "dormancy." Generally, a single species enters dormancy at only one specific stage in its life cycle, and this happens only once in its life [1]. However, because this is a general rule, there are exceptions.
Winter is actually a good time to look for these hidden insects. Because leaves have fallen from trees, pupae and cocoons attached to branches become much easier to find [2]. It is not a season with no insects, but a season where you can spot the ones that are hiding.
2. How Praying Mantis Eggs Survive Winter in Foamy Egg Cases
With very few exceptions, praying mantis species across Japan, from Hokkaido to Kyushu, spend the winter as eggs [4]. The adult females lay these eggs in autumn and then die. Only the next generation survives the cold.
The eggs are enclosed in a mass called an ootheca. This structure is like a foam that contains a lot of air, giving it insulating properties [5]. It does not generate heat like a heater, but it prevents the outside cold from reaching the eggs easily. One record shows that a single ootheca of the Japanese giant praying mantis can contain about 300 eggs [4]. Since the number varies by individual and source, it is safer to think of it as "several hundred."
3. Why Overwintering Praying Mantis Eggs Hatch at Different Times
You might assume that if an insect overwinters as an egg, it is in dormancy. However, it has been reported that the Japanese giant praying mantis overwinters as an egg but does not enter dormancy [6]. Eggs laid at different times reach different stages of development by winter. As a result, they hatch at different times in the spring.
This staggered hatching is explained as helping to avoid cannibalism among the newly hatched larvae and competition for prey [6]. Adult mantises adjust when they start laying eggs by reading the day length. During the long days of summer, sexual maturity is delayed. As days shorten in autumn, maturity progresses quickly. This helps avoid eggs hatching before winter arrives, while still allowing late-emerging adults to lay eggs before the cold sets in [6].
4. Where Larvae and Pupae Hide in Winter: Soil, Water, and Branches
The Japanese rhinoceros beetle spends the winter as a larva inside the soil. The adults that flew around in summer are not alive in winter [3]. Similarly, dragonfly larvae, known as "yago," spend the winter as larvae in the water [3]. However, this applies only to dragonflies that overwinter as larvae; it is not true for all dragonflies.
The Japanese swallowtail butterfly overwinters as a pupa. Those that become pupae in autumn stay that way through winter and emerge as adults in spring [3].
5. How Insects Sense Day Length and Temperature to Prepare for Winter
Insects are said to predict the arrival of seasons by sensing changes in day length (the length of bright hours in a day) and temperature. They enter dormancy before the environment actually becomes harsh, because waiting for the cold to arrive would be too late [1].
For the Japanese swallowtail, whether the pupa will overwinter depends on the day length experienced during the larval stage. The butterfly measures the length of darkness. It has a mechanism where even a brief flash of light during the dark period resets the measurement [7]. This suggests that strong artificial light at night could confuse their seasonal judgment.
To emerge in spring, pupae that have overwintered need to experience a certain amount of cold. For the Japanese swallowtail, the required number of days below 4°C (about 39°F) varies by region. It is estimated to be more than 30 days in northern Tohoku and Hokkaido, more than 60 days in southern Kanto and Kansai, and more than 90 days in Kyushu and southward [7]. A warm indoor environment is not enough to satisfy this requirement. These numbers are rough guidelines.
6. How Adult Ladybirds Gather in Groups to Survive Winter
The Asian lady beetle (Harmonia axyridis) overwinters as an adult, often in groups. They are known to gather in places like bridge railings, the bases of trees, and cracks in walls or concrete [8].
Even within the same group of insects, strategies can differ. Most grasshoppers overwinter as eggs in the soil. However, a grasshopper called tsuchi-inago (Patanga japonica) hides in dead grass and overwinters as an adult [3]. This shows that while the overwintering stage is fixed for a species, it can vary even among closely related insects.
7. Try It Yourself: Investigate How Three Insects Spend Winter
If you want to investigate further, choose three insects you like. Write down your prediction of which stage—egg, larva, pupa, or adult—they use to survive winter. Then, check your answers using field guides or the websites of facilities that offer information in Japanese. If your prediction was wrong, write a one-line hypothesis about why that insect chose that specific stage.
For official information, you can read the "Find Insects Overwintering" page from Kodomo no Kuni (Children's Country) and the "Find Praying Mantis Eggs" page from the Climate Change Organism Survey, both available in Japanese.