Vince Massimo wrote:Hi Pagurus, and welcome to UK Butterflies.
This looks like a classic case of parasitism by a tachinid fly. The fly grub has been developing within the body of the caterpillar and has now emerged through the wall of the chrysalis. It has used a silk "rope" to get to the ground and crawled away to pupate in a sheltered spot. The adult fly should emerge in about 12 days time.
More information can be found at the end of this article
http://www.ukbutterflies.co.uk/reports_ ... 8tyg7Hb4_w, but there is more detailed research available on-line.
Vince
EDIT: I see Pete has already responded
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Thank you very much for the welcome, Vince
It’s lovely to have an answer, but I’m still a bit mystified as I thought there would just be a very visible hole, but it looks more like a blob of brown glue. Unless the hole in question is the big split where I thought the butterfly was going to emerge from?. I somehow imagined it boring a hole straight out of the side. (Must read a lot more about parasitism)
Did it leave a blob of silk as a rope attachment? And the silk rope didn’t reach the ground (about 2’ away), but was attached at an angle to the wall just a short distance away, and stretched very tightly.
I keep trying to visualise how (or why) the tachnid fly larvae could have got out and sealed the hole behind it. And also how it could have attached the ‘rope’ to the wall (unless it did a spider thing and just let it drift until it stuck to something, the rope tightening up later as it dried?).
It’s all very fascinating.
I know the other pupa I've found is far too big to be a fly pupa, but I'm hoping to find the other one too. Do fly pupae show any changes before the emergence?
Thank you so much for the marvelous link. In every way it’s the best butterfly ID page I’ve ever seen. If you ever decide to write a book in that format, I will definitely buy it
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. I must apologise for not locating it before. If I had seen your photo (Small Tortoiseshell (parasitised pupa) Caterham, Surrey) that would have given me my answer
You and Dave got me thinking about whether I’d seen that kind of tachnid fly before. I went back through my files and found these two flies from three years ago (before I learned never to photograph shiny things in bright sunshine
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).
I tried to ID them, but the patterns seem very variable, so I’m not sure. Would these two be the kind to prey on butterflies?
The one that was hanging around a bunch of peacock larvae looks very guilty
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(A better shot of him is below).
À la perchoine
Perfection is reached, not when there is nothing more to add, but when there is nothing more to take away. - Antoine de Saint-Exupéry