Wild animals have various methods of overwintering. How do invertebrates spend the cold winter months? – that is what naturalist Ilmārs Tīrmanis talks about this time.
Almost all animals whose body temperature depends on the ambient temperature (poikilothermic animals) are noticeably active in our latitudes only during the warm season, while they hibernate in winter.
Some perish, others look for safe places
The vast majority of poikilothermic animal species belong to the invertebrate phylum. For most of these creatures, the development process consists of several stages. Nature has usually determined a specific option for which developmental stage each invertebrate must overwinter in. For example, insects and some other arthropods have two main overwintering options.
In the first option, the adult animal dies before the cold period, having left behind offspring in either the egg, larva, or pupa stage. In the second option, the adult individual finds a safe overwintering place for itself, where the temperature should be favourable during the winter – moderately low and steady – and waits there for the next warm season.
Admittedly, there are some insect species whose representatives overwinter in both the larval and adult stages, such as the mole cricket and the cockchafer, and there are also insects that can spend the winter in the form of an egg, larva, or pupa, for example, the housefly. Meanwhile, spiders whose habitat is linked to human dwellings can exist throughout the year, including winter, in both the adult and egg stages.
In the warmth under the ground
Terrestrial mites endure the winter at various stages of development and usually at a medium depth underground. These arachnids seem to sense that frost rarely penetrates more than half a metre into the soil in our region, especially if the ground is covered by a thick layer of fallen leaves and a generous blanket of snow.
Earthworms overwinter in the soil below the frost line. Terrestrial snails, both slugs and shelled snails, also try to crawl somewhere underground. Moreover, shelled snails prepare quite diligently – they create a winter seal out of a layer of hardened mucus in front of the opening of their shells.
By the way, some insects escape winter in a way similar to migratory birds – certain species of dragonflies and butterflies set off on a journey to warmer lands.
Life processes change
The bodies of most insects that have remained to overwinter here, along with those of many other invertebrates, prepare physiologically in a special way for some time before overwintering – they accumulate free glycerol and certain other substances that inhibit tissue rupture, preventing the formation of ice crystals within them.
When invertebrates are exposed to low temperatures, their vital processes slow down significantly or almost stop completely. When this happens, it can be said that a so-called anabiotic state, or suspended animation, has set in, also known as "diapause". Diapause lasts until the surrounding environment becomes favourable for the organism's activity, under normal conditions – until spring. The anabiotic state is often interrupted by winter thaws, which, after the frost returns, cause the death of more than one creature that has been "awakened" and activated before its time.