Saturday, 14 May 2011

Double delay

After a wonderful April, it has rained every day in May so far, sometimes very heavily. Blogger has also been offline for a few days, so I have a bit of catching up to do.

Worker Bumblebees are very numerous now, although most of the current ones are from the queen's first batch of eggs and are therefore very small. The ones gathering pollen from Raspberry flowers are scarcely larger than a pea:

Worker Bombus lucorum agg.
Hawthorns are in full flower now, and in keeping with the trend this year are bearing many more flowers than usual. Some trees are almost entirely white (and some are almost entirely pink!). The fresh flowers have pink interiors, but that very quickly disappears, leaving a much duller flower:

Hawthorn blossom - "May"
Hawthorn blossom is known as 'May', and is the origin of the northern saying: "Ne'er cast a clout e'er the May is out". Which translates to "Don't divest yourself of warming clothing until the Hawthorn has flowered".

The hoverfly Cheilosia grossa feeds on Thistles as a larva. The larvae can be detected by their effect on the host plant: any thistle that branches from ground level and appears dwarfed can be considered to be potentially 'occupied':

Male Cheilosia grossa

I already showed the first of the Orange Tip butterfly eggs here. The female lays a single egg behind the flower, waiting for the seedpod to develop. She lays only one egg because the larvae are cannibals: any young larva will be consumed by an older one. In the early season, I only ever find a single egg per plant: females detect the presence of an existing egg and will move on to other suitable plants to lay their eggs. At the end of the season, however, all the suitable plants already have eggs and I find what I call the "Let's dump eggs on all the plants in case one might just actually make it" syndrome. This photo shows at least 14 eggs on one plant:

Orange Tip eggs and larva
And top of the class to anyone who spotted the first Orange Tip larva on the seedpod at the front of the picture.

The important thing to realise here is that synchronisation is crucial: if you emerge before the host plant is ready, you will have nowhere to lay your eggs. If you emerge too late, then all the suitable (southwest to south-facing plants on an embankment to catch the sun) plants will already be occupied and your late larvae will have to hope that they don't encounter one of the early occupiers. The window of opportunity is perhaps 14 days. This theme of critical synchronisation occurs time and time again in our wildlife.

The next shot illustrates one of the most stunning aspects of our natural history:

Female Dungfly infected with Entomophthora muscae fungus
The image shows a female Dungfly that has been killed by a fungal infection. The fungus Entomophthora muscae enters the digestive tract and progresses to the abdomen where it multiplies and expands. As it expands, the pink mass begins to break through the abdominal segments as shown above. But now we come to the crucial bit of the exercise: before the fungus kills the fly, it compels the fly to climb towards a high point and then to open her wings. Then it kills her. This combination of high position and open wings affords the maximum opportunity for wind-borne spore distribution for the fungus. So the fungus gains control of the movement of the fly in order to maximise the opportunity for dispersal of its spores.


Emma Springfield said...

It is so amazing to me how the natural world synchronizes everything. It is truly a wonder.

Gill said...

Entomophthora muscae fascinates me. (It seems to be much commoner in Donegal than here - maybe something to do with the damper air?) Does it "enter the digestive tract" as part of something the fly eats?

It only seems to infect female flies - is there a known reason for that?

That's a particularly good photo of it, Stuart (even by your high standards).

Stuart said...

Gill, the spores of E. muscae are sticky. I showed them on in 2008.

That post explains the process, but I can summarise it here: The wind-blown spores adhere to any surface they land on and sooner or later a fly will accidentally ingest one. I have only ever found these in females, and I suppose the egg-bearing abdomen is more suited for holding a large volume of fungal material. I suspect that the spores need high humidity to remain sticky, so that might explain why we have more of them than you do.

The bit that fascinates me most is the ability of the fungus to control the movement and configuration of the fly. Even more amazing is the fact that other Entomophthora species affect other orders such as lepidoptera: the 'Summit Disease' can force caterpillars to move to the top of plants as well. So it's not just one type of creature that they can control, and the movement mechanisms for caterpillars are very different from those in flies. I often muse about how this controlling mechanism works, and I think it might be something quite simple, such as making the fly dislike darkness, forcing it to try to find the lightest place: the top of a plant. It's fascinating anyway.

Gill said...

[2008] Hmm - I see there you say it invades the reproductive tract....

<< I often muse about how this controlling mechanism works, and I think it might be something quite simple, such as making the fly dislike darkness, forcing it to try to find the lightest place>> interesting thought - I'd always assumed it was chemical in some way - a lot of insect behaviour seems to be driven by chemicals / "smells". It would be an interesting piece of research to try and fathom what does drive it.

Stuart said...

Gill, it's a matter of timing. Ingestion first, then progression into the abdomen. Dunno how it makes that jump. Maybe it just grows and fills the abdominal space. Perhaps it needs specific hormones/chemicals to initiate the growth (some fungi are known to be responsive to chemical stimuli, leading to one fungus being triggered by the fruiting of another).