Can you identify these tiny structured objects on the hairs of a fly leg?

We have a genuine mystery this month for readers to help us with. Does anyone know what the structured white objects are in the image below? The original query we received at Micscape is presented, together with a few pointers. The image left sets the scene with an image of the flies hairy 'knees'!

Query received from Frank Placido, physicist, University of Paisley, Scotland, UK.

'Hi, I am a physicist with a Hitachi field emission microscope which we use from time to time to show visitors and school kids. The best objects for this are invariably biological since they have a scale which people can relate to. Never having time for preparation, we tend to pick up dead flies from the windowsill etc. Nothing unusual in that, but we keep seeing these unexplained "spherical objects" which no-one seems to be able to identify. I attach a couple of images to see if you or your colleagues can help. One is of a fly's knee to set the scene and the other is a blow-up of a hair on the knee showing the objects that I mean. The diameter of these objects is 500 nanometres. Somebody once said that they are bronchiosomes (or something like that) but I have not been able to find any references to check this out. I hope this is of interest to you!

Cheers, Frank Placido'

Frank later commented.

The "things" have been found on several flies from various internal window-sills. These windows are hardly ever cleaned! Did I say that the diameter of the spheres is 500 nanometres? The regular structure would seem to suggest a biological origin. They seem to be too small to be fungal spores, pollen or even diatoms - all of which we have looked at many times (we provide a service to the biology department here). On the other hand they are too big to be viruses. None of our biologists have a clue.

Some comments by the Micscape crew:
'The word "bronchiosomes" suggest something to do with breathing - certainly insects obtain their supply of air through a narrow system of hollow tubes called tracheoles which connect the outer air supply to internal organs through their exoskeleton. However the objects in your images look like something attached, rather that the openings of tubes.'

'Those spheres are really interesting. They're either eggs or fungal spores - can't decide which but my money is on the former.
A further thought. There may be a chance of them being ISCOMs. These are man-made structures that are being used experimentally as synthetic vectors in vaccinations (ISCOM = Immuno Stimulating COMplex). Some work has been done in using them as vectors of pesticides but is still very early days. Possibly the fly has come into contact with these. Just a thought.'

Editor's notes: So this is as far as we can get, can you solve the mystery for Frank (and the Micscape crew!) or provide further pointers? If they are of biological origin e.g. eggs or spores do you know from what genus/ species, or if man-made their likely origin. We'd be delighted to hear from you if you can help. Update: mystery solved, read article in March issue.


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