Despite my previous posting on the sycamore having been 'live' for a full three hours, I have been amazed to receive not one comment from my legions of avid readers (hem,hem) pointing out the obvious evidence for the second life form in photo 2 of that posting. Photo 1 (left, click on it to enlarge) gives you a second close-up opportunity. Of course, on second viewing you will immediately kick yourselves for not having noted the orange leaf galls, obviously (!) evincing the presence of the microscopic mite, Aceria macrohynchus.
In truth, I myself didn't have a clue what the cause of my leaf's little orange pimples was, and I spent some time looking through literature on rusts and smuts (little fungal entities that also commonly attack leaves). Eventually however I came across the site written by the good people of Hainault Forest, and later, a site on the fauna of Cornwall in the U.K. and the RX Wildlife site, and from these arrived at the (presumed) identity of my mite. From the RX Wildlife site I also learn that my mite also goes under the pseudonyms Eriophyes macrorhycus and Aceria cephaloneus.
I have two rather basic questions about my mite that I have little hope of being able to answer myself on a reasonable timescale, and am therefore hoping someone out there may help with:
i) Have I got the identification correct (A. macrorhynchus), or are there perhaps other mite species that cause orange pimples on sycamore leaves?
ii) What purpose do the galls serve? Do they act as living quarters for my mite, or do they have another purpose?
Some 45,000 species of mite have apparently been named and it is believed that many more remain undiscovered. Given many are sub-millimetre I imagine it must be an enormous challenge, even for the experts, to know whether the mite you have under your lens represents a new discovery or simply one the 45,000 that people have seen before. I learnt recently that in the case of the related, but distinct, sub-class of arthropods, the spiders, the sure-fire route to identification is microscopic examination of the palps or epigyne. Are there similarly diagnostic features for mites or are other methods needed?
Finally, I would love to see a photo of my mite, but have entirely failed to find one on the 'web. I did however come across the excellent mite site of the US Dept. of Agriculture. I did not find a picture of A. macrorynchus, but did find not only a photo but amazingly also a video of the Canadian Thistle Rust Mite Aceria anthocoptes. Since I understand that the images from the US DA are in the public domain, I'm reproducing it here (unless someone can tell me I shouldn't?). Since the Thistle mite above and my sycamore mite are in the same genus (Aceria) I'd like to believe they're of similar appearance. I'd be delighted if someone can point me to a photo of macrorynchus however.