Silver-spotted Skipper: ova

millerd
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Silver-spotted Skipper: ova

Postby millerd » Wed Oct 10, 2012 9:03 pm

As suggested by Vince, here is a close up of a SSS egg (which was laid a matter of seconds earlier). You can clearly see three strands of something which seemed to be attaching the egg to the grass stem adjacent to the one on which the egg is sitting. There is in fact a second egg in the background, laid at the same time.

Dave
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SSS egg 070810 cu.jpg
Silver-spotted Skipper ovum: Box Hill 7th August 2010

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Padfield
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Re: Silver-spotted Skipper: ova

Postby Padfield » Thu Oct 11, 2012 7:43 pm

Thanks for posting that, Dave. Another superb picture and a tantalising insight into the world of silver-spotted skipper egg-laying! I say 'tantalising' because it looks as if something very interesting is happening here but we haven't got enough evidence yet to be sure exactly what!

Guy
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Nick Broomer
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Re: Silver-spotted Skipper: ova

Postby Nick Broomer » Thu Oct 11, 2012 9:17 pm

Denbies 12th and 18th August

As requested Vince

After watching a female Silver-Spotted Skipper ovipositing i found she had layed two eggs. One had been layed very close to the ground, the other having been stuck to a grass stem about 75mm in length with a gluey substance.
12.8.2012 Denbies 019 silver-spotted skipper`s egg-1.jpg
I returned a week later and before i left decided to check on the two S.S.S. eggs just out of curiosity, and when i arrived at the correct spot i found an additional egg had been layed probably by another female. The two eggs i had found the previous week having turned a light brown in colour.
18.8.2012 Denbies 032 silver-spotted skipper, egg after one week.jpg
The freshly laid egg was attached to two grass stems about 50mm in length and slightly thinner than the two grass stems the other eggs were attached to. But what was unusual in this case was that the two grass stems were adhered together by the same gluey substance as the female S.S.S. used to fix her egg to the stem of grass in the first photo.
18.8.2012 Denbies 077 silver-spotted skipper egg.jpg
Now she has deliberatly adhered the two grass stems together to give the egg a more secure hold or, are they two failed attempts at egg laying.

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Neil Hulme
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Re: Silver-spotted Skipper: ova

Postby Neil Hulme » Fri Oct 12, 2012 9:40 am

Some of these images illustrate very well just how tenuous the attachment of SSSk eggs to fine blades of fescue appears to be, via thin threads of 'glue'. This contrasts markedly with, for instance, the firmly cemented nature of Brown Hairstreak ova, where the entire base is in contact with a Prunus stem or thorn base; these eggs are designed to stay put. I do not have a wealth of experience with SSSk ova, but Crispin Holloway (who does) advises me that these threads afford quite a firm attachment, at least when dry, soon after oviposition. The attachment does however become significantly less firm after wetting, and with repeated rain showers throughout the long August - March life of the egg, these strands may become much weaker.

Although my experience is quite limited I have, nevertheless, come across single and small groups of SSSk eggs lying in bare earth depressions, having become detached from the fescue blades. I have little doubt that these will have a reasonable chance of hatching successfully, although there may be an increased susceptibility to other hazards or predators, above those suffered by remaining attached.

It therefore seems possible that this method of attachment might be an adaptation designed to reduce losses by incidental grazing. If a sheep or rabbit were to start nibbling at the clump of fescue, this might dislodge the egg and give it a statistically reduced chance of being ingested. I would imagine that a significant proportion of SSSk eggs are eaten by grazing animals, but even a small increase in survivability would be important for the species.

Neil

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Crispin
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Re: Silver-spotted Skipper: ova

Postby Crispin » Fri Oct 12, 2012 9:05 pm

Hello Dave and all,

Great photos, I suspect theses strands and the adhered grass stems are as a result of wet conditions(?)
At the time of the egg being laid, was the grass wet, had it recently rained?
Looking at rainfall radar for that day it looks like you may have had a shower in the afternoon on that day:
http://meteox.com/hist.aspx?URL

When I did several egg counts and egg collections (whilst working for Exeter Uni), I noticed eggs would be quite firmly attached to the Festuca blades when dry but when wet they would just slide off the leaf blade leaving behind a mucus like substance – the wet glue.
So, if there had been a recent shower just before the egg you photographed was laid and the grass was a bit wet then, I guess those strands are the wet mucus like glue and the egg was almost dropping off.
I may be wrong.
I suspect the adhered grass stems could be as a result of moisture (rain remaining on leaf) spreading the glue or it running off.

When collecting the eggs in 2010, some were very well attached to the leaf blade and I collected them with the blade attached. But some dry ones just dropped off the leaf blades – the glue was brittle. After a bit of rain or dew on the grass they slipped off.

I think the eggs are designed to drop off the grass blades after a period of time. Either they dry off and drop or they get wet and fall off.

Eggs falling to the ground could reduce the likelihood of predation by grazing livestock and rabbits, as Neil suggests.
But then, if the egg is designed to fall to the ground, why do females lay them on the grass blade in the first place? Something I hadn't thought much about, before now.
I think the same applies to many species and guess it is probably a way of ensuring the egg will be near to the host plant when it hatches in the spring, after the previous years growth dies back and to reduce predation?

What do you all think?

Crispin

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Re: Silver-spotted Skipper: ova

Postby millerd » Fri Oct 12, 2012 9:38 pm

To be honest, I really can't remember whether there had been recent rain or not. Here is a photo of the butterfly in the act of laying the eggs in the picture above - I can't see any indication that the grass was wet.

Dave
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SSS egg-laying resized.jpg

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Re: Silver-spotted Skipper: ova

Postby Nick Broomer » Sat Oct 13, 2012 10:05 pm

To answer a couple of the questions raised here. The weather conditions were very hot and dry when the first two eggs were laid, and witnessed by myself in the afternoon of the 12th August. The egg laid on 18th August with the two grass stems adhered together was found late morning also in very hot and dry conditions.
Crispin, you wrote, why do females lay them [the eggs] on the grass stems in the first place? Well, maybe the eggs are laid on the grass stems, slightly elevated to catch the sun in order to dry and harden for a period of time [depending on the weather on how long that period is,3 or 4 days, a week?] so as the egg is drying and hardening so is the glue, and as the glue dries it loses its value coinciding with the readiness of the egg to fall to the ground where they would seem to be a lot safer during the winter months from grazing animals. So maybe in some rare cases where a single grass stem is too thin to hold an egg, the female S.S.S. will adhere two stems together or use a thin thread to bind the egg to another stem in order to give her egg a better hold and a far better chance of drying and hardening correctly, and so a better survival rate.


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