The dark-branded bushbrown butterfly (Mycalesis mineus) is one of the most common species in South and Southeast Asia and is found throughout the year in India. These butterflies are known to produce two different coloured pupae – brown and green.
Scientists at the Indian Institute of Science Education and Research, Thiruvananthapuram have found relative humidity was one of the deciding factors which caused the change of colour. Brown pupae are more common in drier conditions and develop faster than green pupae. The results were recently published in the journal PLOS ONE.
Sixteen female butterflies were collected from the IISER Thiruvananthapuram campus and reared in lab conditions. They were released in cages with maize, wheat and ragi plants to lay eggs. Eggs were collected every two days along with the leaf blades and kept in plastic boxes to hatch out. The hatched caterpillars (larvae) were released on maize plants in insect growth chamber. In 20-25 days, the larvae transformed into a pupa.
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The green pupae were formed mainly under the maize leaves, whereas the brown were almost exclusively found away from the leaves on substrates such as soil. “The pupal stage is the stationary phase and they are more vulnerable to predation. So it is important to camouflage. Merging with the background avoids detection and maybe an adaptive strategy in pupae,” explains Harshad Vijay Mayekar, at IISER and first author of the paper. The insect growth chambers were maintained at a humidity of 85% corresponding to wet season in nature and 60% humidity for dry season. Brown pupae were more common at lower humidity, that is, drier conditions. “When the moisture is low, it assumes that it is dry season and prefers substrates like soil. Similarly high moisture means wet season when there is more green foliage. So the larva chooses to pupate on the leaves. It is unclear whether the colour is decided based on the substrate choice [leaf or soil]; or the colour is pre-decided and then the larva picks the suitable substrate. More studies need to be carried out to understand this colour selection system,” says Dr. Ullasa Kodandaramaiah, scientist at IISER Thiruvanantha-puram and an author of the paper.
The brown pupae (18-20 days) were found to develop faster than the green ones (22-24 days). “The green colour is the result of the plant material the larva has chewed on and the colour is seen through the sheath. But it takes time and energy to produce the brown pigment. The fact that they developed faster was an interesting observation. We are further investigating this finding,” adds Dr. Kodandaramaiah .