The secret of how caterpillars tame ants

Two Silverline caterpillars inside the ant nest, among the ant brood, being tended by ants   | Photo Credit: Krushnamegh Kunte

Scientists from the National Centre of Biological Sciences (NCBS) in Bengaluru have got fascinating insights into the relations of butterfly caterpillars and their ant hosts using advanced X-ray MicroCT technology. The intriguing caterpillar-ant associations have been a subject of study for decades resulting in the understanding of their evolution and ecology.

Butterflies of the family Lycaenidae, popularly known as Blues and Hairstreaks, have gone in a completely unexpected direction to deal with their predators and do not avoid predatory ants at all.

Taming the enemy

Over millions of years of evolution, this butterfly group has evolved a range of adaptations that have tamed their ferocious ant predators into protectors and providers. In fact, caterpillars of this family closely associate with ants, becoming strange bedfellows. Not only do ants not eat these caterpillars and pupae, they actually care for them and aggressively protect them from other predators and parasitoids, thus creating an enemy-free space for the butterfly’s early stages.

The study using X-ray MicroCT technology describes the association between caterpillars of the Lilac Silverline Apharitis lilacinus butterfly and cocktail ants. This butterfly was rediscovered in India a few years ago after a gap of almost 100 years by a student Nitin Ravikanthachari. This species has an obligate association with a single species of cocktail ant called Crematogaster hodgsoni.

Foster parents

Scientists who have been studying the relationship between the two species, found that females of the Lilac Silverline deposit eggs at the entrance of cocktail ant nests, sometimes on sand and away from plants. Caterpillars are completely dependent on the ants after hatching from eggs, and they are constantly attended by their hosts. The caterpillars live inside the ant nests, often among the ant broods, and are cared for by the tending ants just like the rest of their own brood.

“As far as known, caterpillars exclusively feed on regurgitated food provided by ants; they have not been observed so far feeding on plant tissue, like most other caterpillars do. As expected from their close relationship, the caterpillars possess all the main ant-associated organs that have been described in other obligate ant-associates, and these organs are very well developed in this species,” Associate Professor of NCBS Krushnamegh Kunte said.

While the presence of the ant-associated organs in these caterpillars and their relative development with respect to the nature of ant associations is well known, the functional morphology of these organs has been poorly understood. This was because the organs inside the body and their relative positions with connecting muscles and nerves get destroyed in traditional methods of dissection and staining.

Dipendra Nath Basu, a PhD student under Dr. Kunte found the problem peculiar and challenging and turned to X-ray microtomography, or MicroCT (CT-scanning is used widely in hospitals) to shed light on the functional morphology of the ant-associated organs.

Reward on demand

The organs of caterpillars primarily studied by the scientists are called dew patches in some species and nectar glands in others. These organs produce carbohydrate-rich secretions to attract and reward the tending ants. For the most part, these sugary secretions, which ants drink readily, keeps the ants interested in tending and protecting the caterpillars.

“The MicroCT scan revealed how surrounding muscles may contract and relax, enabling the caterpillar to control the release and re-absorption of secretory droplets from dew patches and nectar glands that lure and sometimes deceive ants. Dew patches, for example, operate on a ‘lasso bag’ control mechanism using surrounding muscles,” Mr. Basu said

The detailed insights into the functional morphology of the ant-associated organs and the mechanisms of gland operations indicate how caterpillars may be able to have a fine control over when they reward ants, and how much, thus optimizing their investments and returns, the researcher said.

MicroCT scans also revealed additional adaptations of these obligate ant-associates. The caterpillars have a thick skin or ‘dermis’ with chitinous thoracic and abdominal plates, which shield their front and rear ends from ants should they become momentarily aggressive, he added.

The details of the study authored by Mr. Basu and Dr. Kunte have recently been published in a paper titled “Tools of the trade: MicroCT reveals native structure and functional morphology of organs that drive caterpillar–ant interactions” in the Nature group’s journal, Scientific Reports.

“While caterpillar-ant interactions have been explored as fine examples of multi-partner interactions, i.e. between caterpillars, ants and potential predators, what is important and novel about this study is the study of the mechanisms that facilitate these fascinating interactions,” said Renee Borges of the Centre for Ecological Sciences, Indian Institute of Science, an expert on plant-animal and other inter-species interactions.

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Printable version | Jun 16, 2021 2:09:40 AM |

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