PhD student, Ritabrata Chowdhury, Insect Biomechanics Group, is first author of a recent paper in Biology Letters which uncovers a previously unknown interaction between plants, ants, and caterpillars in the rainforests of Borneo: kind of arms race between plants and insects.

Macaranga trees house colonies of aggressive ants, protecting them against herbivores, but specialized caterpillars of the genus Arhopala have outwitted the ants and eat the leaves. The ant-plant M. trachyphylla, however, has evolved sharp, hooked trichomes that can impale and kill Arhopala caterpillars. Arhopala amphimuta caterpillars, in turn, can walk over the sharp trichomes unharmed. These adaptations and counter-adaptations to physical defences suggest a co-evolutionary arms-race similar to that between insects and chemical plant defences.

Ritabrata describes the discovery as very serendipitous. He said, 'It was the first field trip of my PhD in Brunei, in May 2023. One day Prof Walter Federle and I were looking at different Macaranga trees, he showed me that the surface of one Macaranga feels very sandpaper-like. So we immediately collected it and saw that they had hooked trichomes. 

'Although it had been mentioned that this plant, M. trachyphylla bears hooked trichomes (the only one out of about 300 described species of Macaranga!), their functional significance was still unexplored. As we had already studied how hooked trichomes of Passionvines act as a physical defence against herbivorous Heliconiini caterpillars, this seemed like the discovery of a parallel system from a different part of the world. From there onwards, we found that these hooked trichomes of Macaranga indeed act as a physical defence against certain lycaenid caterpillars of the genus Arhopala and that one caterpillar species can walk on the trichomes unharmed.'
 

Read the paper: Ritabrata Chowdhury, T. Ulmar Grafe, Faizah Metali and Walter Federle Arms race of physical defences: hooked trichomes of Macaranga ant-plants kill lycaenid caterpillars, but one specialist has a counter-defence Biol. Lett. 21:20250005  https://doi.org/10.1098/rsbl.2025.0005

Image: Ritabrata Chowdhury in Brunei, May 2023.