Help with 'frons dusting' in hoverflies pls

[Dillybythesea]

Looking at pics of this hoverfly today, I can see markings in the frons that come and go, presumably as shadows, but in that case, does this female have 'dusting' that would be indicative, or does more of the background have to be dark before 'dusting' comes into play? If you see what I mean?! Also, any hope of an ID?

[Matt Smith]

Hoverfly "dusting" is in fact a cover of tiny scales on the surface of the body. The angle at which these are viewed can make a difference, from some angles it can appear that dusting is absent, as can be seen from your pics. Under a microscope dusting is more obvious because you can see a change in the "texture" of the surface from smooth (no dusting) to rough (dusted).

In this case I would say the hover is dusted over much of the face - the body colour suggests Platycheirus albimanus.

[Dillybythesea]

Thanks again Matt. I'm trying to follow, although failing rather! Is it that, in the areas where they occur, the scales create the impression of 'lightness' at times/angles when otherwise the whole surface would appear dark, even though it's actually just smooth and pale? Thus in this picture, the red patches would have the scales? Sorry to be so slow!

[JRsbugs]

Originally Posted by Dillybythesea

Thanks again Matt. I'm trying to follow, although failing rather! Is it that, in the areas where they occur, the scales create the impression of 'lightness' at times/angles when otherwise the whole surface would appear dark, even though it's actually just smooth and pale? Thus in this picture, the red patches would have the scales? Sorry to be so slow!

I'm afraid that photo has some serious light reflection or other problem. Look at the last photo you posted, you will see the dusting on the frons is only along the edges of the eyes and with a small triangular protruding patch which is typical of P. albimanus. The rest of the frons is black and shiny. This varies from species to species, and is one of the key points used for ID. The colour of antennae, shape and colour of spots, shape of abdomen and colour of legs are also used. Keys can help a great deal.

Which editing programme are you using? I only have the programme which comes with Vista, but I downloaded Windows Live which is much better. It gives options to brighten, darken or lighten with contrast, increase or decrease shadows and to reduce or increase highlights. I usually find I need to increase brightness, then increase contrast a little to compensate then reduce shadows, last step to reduce highlights. These steps can improve how you see your photos no end. There are other features such as sharpen which is good compared to many programmes but I only use it lightly, colour adjustments which I never use and crop.

Windows Live will integrate with the Pictures programme, but is also available in the Start menu where you can select photos from a certain date then you can resize from there which is an option not carried over to Vista from XP and much missed by many!

[Dillybythesea]

Hi Janet - I'm always grateful that you take the time to discuss all said issues! (I have 'Corel Paintshop', which is what I use to download my pics, and draw red splots where needbe!, but tend to do little, and can certainly try to do what you say to improve brightness, etc.)

But re the dusting on this hover... this was the queer thing that got me going in the first place. These sequential shots show how the marking appears to dwindle as I/ or it change angle...

and then to this... where there's hardly any dark apparent, except the smudge over one antenna

which then repeated itself when I was head-on, where it appears to go from just a bit of dark above the right antenna (as shown in initial head-on shots above), spreading across as the angle changes, till forming the perfect impression for P. albimanus, as demonstrated by your perfect example. All shots treated, or rather untreated, the same.

[JRsbugs]

Now I think I can see what has happened there!

It appears as though something is reflecting in the flatter planes on the frons. This shows in this shot where you can see other refelctions on the shiny part at the top of the head..

[Dillybythesea]

Well I guess you must be right - but there's no flash, and it was a dull day, and the same effect when it was facing me....

Anyway, I'm glad it's not just me missing a chapter, and I shall have to experiment with getting various head angles of other individuals, if I can, and see what it does to the 'dusting'!