Swallowtail Diary | Rye Harbour Nature Reserve

Swallowtail Diary

Sunday, 30th May 2021

Swallowtail Diary
photo by Chris Bentley

The Swallowtail butterfly is a very rare native of Britain and only found in a few wetland sites in Norfolk where it only feeds on Milk Parsley. In some years the continental sub-species does cross the channel and there is a good summary of the last good year for them in 2013 - click here.  With recent hot weather and winds from the south there have been a few records this year along the south coast, but the Rye Harbour one was the first recorded for the nature reserve. An adult was seen feeding on Hemlock at Dungeness on 7th June and another was seen in a garden in Sevenoaks. An adult was on Fennel in a garden near Hastings Country Park on the 30th July and on 16th August a caterpillar was seen on the same plant.

This is the ongoing account of the continental Swallowtail butterfly and its offspring at Rye Harbour Nature Reserve in 2020.

3rd August - a Swallowtail butterfly was seen briefly by our warden Chris Bentley on a Buddleia bush in my garden next to the nature reserve. Despite a couple of hours of searching we could not relocate it.  However, my neighbour, Tina photographed it with her phone in the afternoon. On close examination of the photo that evening, it looked like she may have been egg laying. Tina said " I was honoured to see this beautiful butterfly in my garden and to photograph her laying her eggs... without realising it."

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4th August - early next day Tina and I searched the Fennel and found seven eggs. These small Fennel plants were due to be weeded, so we carefully saved the plants and put them in pots on a light, but not sunny windowsill. The yellow eggs were just 1mm diameter. I doubted whether they would hatch, because it would need the butterfly to have successfully mated in France before it launched out across the sea...

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7th August - the eggs started turning brown, they were either developing, or rotting...

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9th August - overnight the eggs had hatched into 2mm long black and spiky caterpillars
and there were now nine !!!

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This was their first home, a well ventilated container with lots of Fennel plants on a windowsill that gets no direct sunlight.

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10th August - the caterpillars that hatched yesterday have already grown to just over 4mm long and are feeding greedily on the Fennel plants. We offered them Wild Carrot, but they only fed on the Fennel plants that their egg was laid on and they wandered very little on the plants.


12th August - the largest caterpillar at now 9mm long, had moulted and had orange spots

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13th August - the largest caterpillar had moulted again overnight, it was now more colourful and with white "feet". In the photo below the new look is on the right and its old larval skin is on the left... it then ate the cast skin.

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15th August - the 7 day caterpillars go through periods of rest and then frantic eating of the leaves and stalks of Fennel. They are now about 12mm long.


The caterpillars fight it our for the best place on the Fennel plants - Gladiator style.


16th August - a freshly moulted caterpillar eats its larval skin - an hour in 11 seconds.


18th August - This is the largest of the 9 caterpillars - it is 19mm long and becoming more colourful with every moult.

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19th August - Chrissy Reidy from BBC Southeast News came to see and report on the Swallowtail story.


20th August - as they get larger they are tackling the thicker stems of the Fennel


21st August - the caterpillars home where they rest - eat - repeat.  2 hours in 14 seconds


22nd August - In just 2 weeks the Swallowtails have gone from 1mm eggs to 35mm long multi-coloured Fennel eating machines - nature is amazing if you take the time to observe it closely.

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23rd August - eating the Fennel leaves is a bit like eating a branched breadstick, but look at the dexterity in lining it up with all six legs.


24th August -  the caterpillars, now 40mm long, were  transferred into a larger overwintering cage

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28th August - the most advanced caterpillar crawled to a top corner of the cage and changed into the chrysalis stage in which it will overwinter - you can just see the little silk belt that it is attached with.

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This is the next caterpillar preparing to undergo the change to the chrysalis. It is attached by silk at its rear end and by a belt, but it is still holding on with its prolegs - at 9am

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By 1pm it had let go with its legs and prolegs and gone into a stage that looks like it's resting, but it's preparing its body to moult the final larval skin and create the chrysalis stage.

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29th August - it stayed at this stage above all day.

30th August - at 3am it became very active and wriggled out of its larval skin. This happened very quickly, perhaps just 10 minutes - the skin split down the back and the wriggling moved the skin down to drop off, but keeping the silk belt in place - with photos taken every 15 minutes this was the only image of it between stages. It is blurred because of the movement. 

Imagine being inside a sleeping bag with a belt on the outside and getting out of the bag but leaving the belt on! 

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The fresh chrysalis is green, and you can see the wings and eyes of the adult, but it will go brown over the next day. It should stay in this stage until next May!

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Beneath the chrysalis lies the old larval skin with the large stripey head capsule and the hard, black legs..

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Here is a timelapse of the 45 hours in 20 seconds


31st August - only 1 mobile caterpillar left, 2 are in the suspend stage and 6 are chrysalis. Before going into the suspend stage the caterpillar spins a belt of silk that holds it to the stem throughout the next 2 stages. Here is a clear photo of the belt holding the chrysalis, which has already become browner.

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1st September - so after more than 40 hours in the suspend stage these 4 images were each taken 5 minutes apart overnight..

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2nd September - the last of the nine caterpillars has started to turn into a chrysalis and here are three images of it making its belt for the suspend stage. The belt is made of many strands of silk produced by spinerettes near the mouth, it is anchored to the stem of the Fennel on both sides and the caterpillare moves its head from side to side and it looks like it's formed between the first and second pair of legs. What was missed (the camera batteries were drained!) was how it then gets its head under the belt to hang as seen above on 28th.

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4th September - at 3.30pm the final moult occurred quickly, so like all the others we missed the opportunity to video the change - it takes just 10 minutes. So that's all 9 now in the chrysalis stage and they have been put in in their mesh cage into a cool damp shed, out of the reach of mice, to wait for 8 months until they emerge as adults....

That's the end of our Swallowtail caterpillars, but here is what their tail end looks like... is it a little face to attract predators away from the real head end???

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12th September - a video of a large Swallowtail caterpillar on Fennel in a Battle garden was posted on Facebook today!

25th September - most of the chrysalises have turned brown, but two have stayed bright green like this one.


To be continued....

I have a memory of the shape and colour of the adult because it was illustrated on the cover of book that my uncle gave me when I was about 9 - but I had not seen one in this country until...

29th May 2021 - The first chrysalis has emerged and WOW

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2nd June 2021 -  8 of the 9 chrysalis have "hatched", but there was only one female among them! The adult emergence is a very rapid process and the compressed wings take about 10 minutes to expand as fluid is pumped along the network of veins - this 1 minute video shows 6 minutes of expansion. The last adult will emerge soon, the colours of the wings are now showing through the chrysalis.. We will release the female in the next few days after she has mated.




This post is also available on Sussex Wildlife Trust website

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