Marsh Fritillary Larvae

[Willrow]

I've now done three Marsh Fritillary larval counts at Aberbargoed Grasslands NNR Monmouthshire's (Gwent) last remaining colony for this threatened species. Last Saturday was pretty cold and a mean north-easterly wind and thick grey cloud cover had me thinking that I might be wasting my time, more than likely the caterpillars would be tucked away down in the grasses out of the cold wind, nice surprise they were not! The hollows among the tussocks provided a good 'micro climate' and although the larvae needed searching for I was amply rewarded with a count of 105 in about two hours or so.

I was intrigued with the size differences between the larvae, some had perished and were little bigger than a basmati rice grain, whether they were the shrivelled husk remains of parasitical wasp (Cotesia bignelli) infection or the result of some climatic problem, remains unaswered, but the different instar sizes can be judged by the photographs provided below - using a one pence piece for scale comparison - a one pence piece is approximately 20mm in diameter. Marsh Fritillary caterpillars are 26 to 30mm in length at final instar (5th or 6th ?) and approximately 90% of my count fell within this size, the remainder were considerably smaller - as the middle image below shows.

Another interesting observation was the absolute blackness of the smallest caterpillars compared to the larger individuals which appear to be 'peppered' with white. I've re-read Jeremy Thomas's wonderful essay on the Marsh Fritillary in The Butterflies of Britain & Ireland and realize that there is still so very much for me to learn about this fascinating fritillary. I'm now going to spend time looking for the chrysalis and know that this could prove another challenge...but hey! its great fun isn't it

Bill

[Pete Eeles]

Good work Bill! Yes, as they get older the larvae will disperse, ultimately leading a solitary existence in their final instar and prior to pupation. Good luck with finding a pupa - although it might feel like looking for a needle in a haystack it can be done!

Cheers,

- Pete

[David M]

Excellent studies, Bill. I've now tried twice to locate these larvae in areas where adults thrive and have, as yet, found none.

[Willrow]

Many thanks Pete, if I have one thing going for me its unlimited patience to the task, whether my knees are up to the pupal search though is another matter

Thanks for the kind words David. I'm pretty sure there is a 'window' in the season before dispersal when the larvae is more evident, after dispersal its as Pete mentions above, the proverbial 'needle in haystack'. I'm sure you'll just hit on that 'window' one day Well done on your Crymlyn Small Blue your certainly off to a great new season start!!!

Bill