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The butterflies of the Phengaris genus (also known as Maculinea) are known to be brood parasitic. During the fourth instar, the caterpillars leave their food plant and mimic ant larvae, causing the ants to take them back to their nest as if they were ant larvae that had escaped.

While in the nest, the caterpillars mimic ant larvae by means both of surface chemicals and acoustic mimickry (including, I understand it, mimicking sounds made by queens!) After pupating, the pupa continues to engage in acoustic mimickry, although I can't find any reference to whether it does (or even could!) engage in continued chemical mimickry.

But I can't find anything in the literature regarding the adult butterfly's emergence from the pupa and exit from the ant nest. A non-academic book and some web pages claim that the alcon blue (Phengaris alcon) and mountain alcon blue (Phengaris rebeli) adults are no longer engaged in any form of mimickry at this point, and may be attacked by the ants. These accounts differ as to how likely an attack is, how much danger the butterfly is in, and the level of protection afforded by the butterfly scales.

The webpages I mention belong to a University of Copenhagen researcher (Dr. David Nash) who has published work in this field. This suggests that the claim is probably correct.

That said, none of the peer-reviewed publications coauthored by him appear to mention it, and each of the two webpages creates a different impression as to the level of danger involved:

"If an ant tries to bite the butterfly it will only get a mouthful of scales." states one, suggesting that there is little the ants can do to harm or hinder the butterfly. But the other states "The adult has to get out of the ant nest quickly to prevent the ants killing it."

The book is written by someone else. It cites three papers, which do discuss the larvae/pupae and ants. But none of these have any information regarding this specific topic.

A paper by Elfferich provides some information in the case of Phengaris nausithous, the so-called "dusky large blue". The author states: "The butterflies always emerged between 00.00 and 06.00 and walked out of the nest ... During the night there was only a little ant activity and there was no real aggressive behaviour ... we may conclude that no pheromone to reduce aggression of the ants is produced by the butterfly."

Some published books (not from the academic literature) claim that the "large blue" (Phengaris arion) continues acoustical, and (according to one book) chemical mimickry. Far from being in danger, two of the books claim that they are escorted outside by ants as though they were acting as a bodyguard!

I believe that this is probably true. The reason for this is that two of the books (pub. 2008 and 2010 were describing discussions between the authors and Jeremy Thomas, an Oxford professor who has published research in this field. In fact, Professor Thomas's research identified the butterfly's dependence on ants, the reasons for its decline in the UK, and he was able to reintroduce the species after it became extinct in the UK. The other book (pub. 2014) mentioned Professor Thomas repeatedly, and described a book by him as "One of the essential wildlife books".

Even so, I don't have access to material in which Professor Thomas or any of his colleagues actually state this instead of just being mentioned by others. So it's still second hand information covering only one Phengaris species.

And to complicate matters further, in a paper that predates the three books, Thomas and other researchers stated that Phengaris arion and Phengaris teleius (scarce large blue) adults "show no interaction with ants after eclosion from pupae in the outer cells of Myrmica nests, from which they emerge while the ants are quiescent in the early morning".

Does anyone know of any peer-reviewed sources which confirm, for the various species, whether:

  • The adult butterfly continues to engage in mimickry?
  • Whether this is chemical, acoustic, or both?
  • The ants are more likely now to realise it's an intruder, and may attack it?
  • How likely this is?
  • How much danger are the butterflies in, if so?
  • Some accounts claim that the alcon/rebeli has scales which make it impossible for attacking ants to grasp them. Others refer to the scales protecting them from being bitten. If these attacks are a risk, how well is the butterfly protected and do attacks from angry ants still get through?

I've looked at too many papers to list here, but my sources include:

Non-paywalled:

Barbero F, Thomas JA, Bonelli S, Balletto E and Schönrogge K. (2009). Queen Ants Make Distinctive Sounds That Are Mimicked by a Butterfly Social Parasite. Science 06 Feb 2009: Vol. 323, Issue 5915, pp. 782-785. DOI: 10.1126/science.1163583

Thomas JA, Schönrogge K and Elmes GW. (2005). Specialization and host associations of social parasites of ants. In: Insect Evolutionary Ecology: Proceedings of the Royal Entomological Society's 22nd Symposium (Fellowes M.D.E., Holloway G.J. and Rolff J., Eds). 475–514

Als TD, Nash DR, and Boomsma JJ. (2001). Adoption of parasitic Maculinea alcon caterpillars (Lepidoptera : Lycaenidae) by three Myrmica ant species. Animal Behaviour, 62, 99-106. https://doi.org/10.1006/anbe.2001.1716

Akino T, Knapp JJ, Thomas JA and Elmes GW. (1999). Chemical mimicry and host specificity in the butterfly Maculinea rebeli, a social parasite of Myrmica ant colonies. 266 Proc. R. Soc. Lond. B. http://doi.org/10.1098/rspb.1999.0796

Elfferich NW. (1998). New facts on the life history of the dusky large blue Maculinea nausithous (Lepidoptera: Lycaenidae) obtained by breeding with Myrmica ants in plaster nests. DEINSEA 4: 97-102. ISSN 0923-9308

Paywalled:

Elmes GW, Thomas JA and Wardlaw JC. (1991). Larvae of Maculinea rebeli, a large‐blue butterfly and their Myrmica host ants: patterns of caterpillar growth and survival. Journal of Zoology, 224: 79-92. doi:10.1111/j.1469-7998.1991.tb04789.x

Elmes GW, Thomas JA and Wardlaw JC. (1991). Larvae of Maculinea rebeli, a large-blue butterfly, and their Myrmica host ants: wild adoption and behaviour in ant-nests. Journal of Zoology, 223: 447-460. doi:10.1111/j.1469-7998.1991.tb04775.x

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I've been emailing some of the various researchers who worked on the papers I've cited. Jeremy Thomas and Judith Wardlaw both took time out of their (probably very busy!) schedules to reply, and they sent me very detailed replies as well as copies of papers I didn't have access to. Thanks to their replies, I'm in a position to post an answer to my own question.

Dr. Wardlaw, in the course of her research into the Phengaris rebeli (mountain alcon blue) species, had:

"looked after hundreds of rebeli caterpillars and reared them through pupation to adulthood. I watched their emergence from the ant nests in captivity where they were trapped in chambers together without means to escape. I also encountered many nests in the wild. In natural nests the caterpillars tend to crawl to upper chambers in preparation for pupation and eventual emergence and often these are narrow chambers which do not have huge numbers of workers in attendance. The ants do respond to the squeaks etc of the pupae and attend them; when emergence is imminent - much wriggling and increase in sounds emitted - my clear recollection is that the ants are almost stupefied and disperse away from the pupae - the emerging butterflies are not molested by the ants at all although they may revisit the pupal cases to lick any residual body fluids which may be ejected at the time of eclosion. When they did encounter the emergent butterflies, I never witnessed any hostility shown to them so whether the scales act as protection or not, didn't seem to be an issue."

Professor Thomas had this to say regarding P. alcon, P. arion, and two non-Phengaris blues:

"The best and most natural film of the adult emergence in an ant nest is the Netflix film "From deserts to grasslands" in their 'Our Planet' series ... by David Attenborough (NB, some of the script they provided him with has minor inaccuracies so ignore the commentary: I was not shown the final script!). There's a sequence on Alcon large blues about 2/3 in. ... It shows the ants quite excited as the adult butterfly begins to break through the pupal case - but always benign and non-threatening - and then indifferent to it when it emerges and crawls to the surface to blow up wings, unaccompanied by ants. That's my observation of P. arion too (i.e. very different from Silver-studded blue which is tended throughout eclosion). The behaviour with Large Blues is consistent with the possession of a stridulation organ on the pupal case, which produces sounds that mimic the queen ant's, and this will undoubtedly be broadcasting 'I belong here' sounds to the worker ants as the pupal case splits, but not thereafter. We have recordings of this from the Adonis Blue which has a similar organ to the large."

(The Silver-Studded Blue and Adonis Blue have mutualistic relationships with their ant hosts. They are not parasitic. I don't have Netflix so I can't provide a link to that video.)

In the case of Phengaris nausithous, note that the published research by Elfferich (which I quoted in my question) testifies to the same thing. There is no obvious defence mechanism on the newly emerged adult, but it isn't in any sort of danger from the ants, and doesn't need one. Note also the book quote in which Prof. Thomas states that the same is true for Phengaris teleius.

In a comment on my question, I mentioned Liphyra brassolis. This butterfly species also parasitises ants, and does rely on loose scales to defend itself from the ants after emerging. If you follow the PDF link in that comment, it includes photographs of the newly emerged butterfly with these scales. It also includes one photograph of an ant that has tried to attack the butterfly, and so become covered in scales as well.

Those scales are very conspicuous, both on the butterfly and the ant, and there was certainly nothing like them in any of the videos I saw of the Phengaris emergences:

"The Large Blue Butterfly Adopted By Ants" - BBC Earth. This one focuses on P. arion.

An extract from "Life in the Undergrowth", presented by David Attenborough.

Behind-the-scenes of the filming of ants-nest interiors for the Netflix "Our Planet".

I couldn't find a suitably licensed image of the L. brassolis covered in loose scales, so I can't include one in this answer. But as I say, you can just click on the PDF link to see it. In addition, there are some spectacular photos of the butterfly's underwings at:

https://the-butterfly675.blogspot.com/2009/05/another-re-discovery-for-singapore.html

https://www.ifoundbutterflies.org/sp/1094/Liphyra-brassolis

http://www.butterflycircle.com/checklist/showbutterfly/224

http://www.samuibutterflies.com/insects/butterflies/lycaenidae/liphyrabrassolisbrassolis/

The only credible evidence that the adult Phengaris are threatened by the ants is that one, unsupported statement on David Nash's University of Copenhagen webpage. I have emailed Dr. Nash, but have not received a reply. Nevertheless, I am convinced that the claim is not correct.

By contrast, we have a great deal of evidence that the various Phengaris species are not in any danger from the ants after emerging from their pupae, and neither have nor need a defence mechanism based on loose scales. I am confident enough in this that I am posting it as my answer.

As an aside, I also looked up the Ichneumon eumerus parasitoid wasp species, which lays its eggs in the caterpillars of P. rebeli (and maybe also P. alcon, I'm not sure.) After hatching and eating its pupated host from the inside, the adult wasp eventually emerges from the butterfly's chrysalis.

According to a 1993 paper (cited below), the emerging wasp is attacked by the ants! It emits an allomone which causes most of the ants to attack each other, but those closest to the wasp will still attack it. It relies on its armour plating to keep it safe from these as it leaves the nest.

The ants will later show some form of aggressive behaviour toward the empty chrysalis - something they don't do towards the ones that unparasitized butterflies emerge from.

Reference (paywalled):

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