Why do we see so many spiders and insects inside our homes in the fall?

Analysis: insects and arthropods such as spiders are looking for a safe place to hibernate during the winter.

By Elizabeth Duncan and Thomas Dally, University of Leeds

The signs that autumn is here are clear to us; the days are getting shorter and the temperature is decreasing. We take this as a sign to take off our winter woolens and think about turning on the radiators. But how do insects know that winter is coming? And what do they do to prepare?

Myths have suggested over the years that insects and other invertebrates can predict the weather and, for example, we may start seeing giant spider webs if the weather is going to be cold. Evidence of those large webs before bad weather is weak. But there are definitely changes in the way insects behave this time of year, and it shows that they are predicting the changes they need to make as the climate changes.

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From RTE Radio 1’s Drivetime, Sean Dukes reports on the decline of insect numbers and species around the world.

Insects are very sensitive to environmental changes and changes in light are what insects use to signal seasonal changes. Insects, like humans, see light with their eyes. But unlike humans, insects have more than two eyes. In addition to the two large compound eyes at the front of their heads most insects also have three small eyes on top of their heads called ocelli. Light information is transmitted from their eyes to the brain and interpreted by an internal clock to detect seasonal changes, which may be important to humans and other primates.

Response to seasonal changes is very important for insects as they are cold blooded and cannot regulate their body temperature. But insects have different ways of coping with the coming cold weather. Others, like the painted lady butterfly, leave the UK in the autumn for north Africa where they breed. Some produce their own “antifreeze” proteins that allow them to cope with winter temperatures. However, most insects go through a slow process called diapause, where they hibernate during the winter.

At this time of year, these insects, along with other arthropods such as spiders, are looking for a safe place to spend the winter – that’s why we see so many spiders in our homes this time of year. You may notice groups of insects, such as beetles, stuck on windowsills and under rocks. These insects will remain safe for the winter and re-emerge in the spring.

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From RTÉ 2fm’s Louise McSharry Show, Collie Ennis from TCD on why spiders are taking our gaffs this autumn.

Due to migration, depression and natural death of some insects at the end of summer, you can generally feel that there are fewer insects around just a few weeks ago. But there are new ones emerging that we didn’t see at the beginning of the summer. Below you can see a beautiful ivy bee that only works when the ivy is blooming. Autumn also brings the appearance of masses of crane flies, which, although they look like mosquitoes, are harmless.

You may notice many wasps visiting flowers to drink the sugary nectar – at this time of year they are very hungry for sugar. Wasps don’t really care where they get their sugar fix and are likely to try to drink our sugary drinks as we enjoy the last of summer in our gardens or enjoy our walks. Their craving for sugar is actually useful as their visits from flower to flower, trying to satisfy their insatiable desire, lead them to pollinate flowers like bees.

For me the most amazing sign of autumn is the appearance of many large beetles visiting the last flowers of the season. These large bees are next year’s queens. After they are born in late summer they breed and conserve energy before finding a place to hibernate for the winter. When we see them again it will be a sign that spring is on the way when the queens come out of hibernation and make new nests.

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From RTÉ Brainstorm, everything you ever wanted to know about spiders

This response to the changing seasons and the fact that insects have been shown to change their behavior, such as mating, in response to the coming rain leads to the assumption that insects can tell the weather, predicting when rain is coming. However, it is possible that they are simply reacting to changes in the weather, like people who take an umbrella when the sky is gray. But there is great variation in how insects react to the weather and I, for one, will stick to the Met Office for my weather forecast.

Elizabeth Duncan is Associate Professor of Zoology at the University of Leeds. Thomas Dally is a Postdoctoral Fellow at the University of Leeds. This article was first published by The Conversation.

The views expressed here are those of the author and do not represent or reflect the views of RTÉ