Thursday, 29 March 2012

A mystery

I was peeling a stem of Soft Rush (as one does) yesterday, and spotted what looked like a caterpillar with an orange head and a green tail inside the pith:

Juncus stem and 'caterpillar'
(For scale, the Juncus rush is about 3mm. in diameter.)

But when I examined 'it' under a lens, I found I had a collection of separate 'things' all lying adjacent to each other:

Juncus stem with 15 separate 'things'
I immediately thought of the micromoth Coleophora alticolella, which feeds on Juncus flowers, and thought that these might be the overwintering eggs, so I fired the picture off to a couple of experts (isn't the internet wonderful?) for their opinion. The consensus is that these aren't micromoth eggs, but the overwintering eonymphs of Juncus-mining sawflies.

I have mentioned before that sawflies have an 'interesting' development lifecycle and these are a prime example of that. Whereas butterflies and moths go through a strict egg-larva-pupa-adult lifecycle, sawflies can exist in a range of states including some intermediate nymph stages like we see here.

Now we notice a few things about the photograph:

1) we have three different colours: orange, white and green (very patriotic, for an Irish specimen)

2) the white ones have a brown dot on them

3) the brown dots appear in different places, but overall they describe a smooth(ish) arc.

Until something emerges (or a range of somethings emerge) from these cocoons, we can only speculate about what we actually have here:

  • It could be that the coloured specimens are about to emerge, or that they have failed and are dead.
  • The arc of brown dots could be the eyes of the eonymphs, or they could be the eggs of a parasitic wasp.

I have fired the images off to a specialist group who are experts on sawflies and I am currently waiting to see what they have to say. I could separate some of the cocoons and put them under a microscope, but I'm very keen to leave them intact to see what emerges, so I have put the sample into a sealed tube and hope to be able to identify the adults after they emerge.

I have searched the internet and can find no images of the eonymphs of likely species, so this might well be the first image of its kind.

It's only March and already we have mysteries.


Gill said...

Fascinating. My first reaction (on realising it wasn't a caterpillar) was eggs, with the orange ones being more mature than the white ones (cf. orange tip) - and laid with the oldest at the top. My explanation for the out-of-order green one at the bottom was "parasitised" or "dead".

Maybe nymphs have similar colour changes. Do they (all?) move when poked?

Stuart said...

Gill, I'm keeping the sample intact to see what emerges, so I'm not planning on disturbing them in any way. Nice thought, though.

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Rebecca Nason said...

Just came across your website after searching for Owl Midge Sp on google - what a great site, full of great photos and interesting and unusual subjects! Well done! Will visit again! Rebecca Nason

Stuart said...

Rebecca, welcome to the blog, and thanks for your kind comments. I certainly try to find the interest in the things that surround us, but are virtually ignored.