Although winter in Dhaka is yet to hit in full swing, there is a light nip in the air, the sun’s rays do not feel so harsh and brown-grey butterflies are flittering all around.

From the staircase to the washrooms and bedrooms, these grey-brown butterflies have become common fixtures this winter and many have questioned their sudden increase.

Common Evening Brown, the species of butterfly infiltrating our homes are specific to the winter season and will be around until February-March.

Butterflies are season specific, so if a particular butterfly species is seen in winter, it will not be seen during summer, which is the case with Common Evening Brown. They maintain a very low population during summer season considering abiotic factors such as light condition, temperature, humidity, rainfall, day length, says Dr Monwar Hossain, chairman of Department of Zoology at Jahangirnagar University.

The Common Evening Brown is very active in short day length, explaining their abundance in winter.

They are low-light (shady) and low-temperature (winter) functioning butterflies, says Dr Monwar Hossain.

Why are they in our homes?

Unlike most other butterflies whose visual system functions best at daytime, Common Evening Brown’s photon or light capturing ability works best in low-light hence they are seen mostly in shady areas, such as our homes, according to Dr Monwar.

In this photo captured on November 22, 2016, several butterflies are seen sitting on a wall inside a house at Karwan Bazar area of Dhaka. Photo: Md Sezanur Rahman Trisun

Their diet comprises of organic debris—rotting vegetable peels, dirt, dead insects, sewerage waste—which is abundant around our homes.

Why during low-temperature, winter?

A butterfly has four stages: Eggs-Caterpillar-Pupa-Butterfly. Enzymes of most butterflies are activated in high temperature helping their transformation from caterpillar to pupa and then to butterfly.

However, Common Evening Brown is able to activate its enzyme in low temperature allowing their transformation into butterfly, which happens during winter.

They also get some essential minerals from the dead insects inside our homes which are abundant in winter, says Dr Monwar.

More plants, more waste, Climate Change?

Whether climate change and the changing weather patterns have any impact on the population of the butterfly, will require a long term study, says Dr Monwar.

He also believes that the growing number of small-scale rooftop gardens, indoor plants, growing initiatives of the government and NGOs to plant trees is providing butterflies with a habitat to lay eggs and also provides the caterpillars with food such as leaves.

The growing number of Common Evening Brown butterflies could also indicate that there is more organic debris—sewerage, open drainage systems, rotting organic waste—which provides food for them, says Dr Monwar.