These are on the underside of every leaf of a young Sycamore sapling. They are totally specific to Sycamore.
I investigated when it looked like predatory bugs massed among them but they are nymphs.
High power:
Olympus EM-1 (manual mode), Laowa 25mm f2.8 2.5x-5x ultra-macro at f8, twin TTL flash hand-held.
Low power:
Olympus EM-1, (aperture priority), Olympus 4/3 50mm f2 macro, f11, hand-held.
All with twin RC flash.
_5215147_DxO Sycamore Aphids Periphylus acericola Laowa 25mm f11 Crosseye 1600.jpg
_5215144_DxO Sycamore Aphid Nymphs Crop Crosseye 1600.jpg
_5215155_DxO Sycamore Aphid Drepanosiphum platanoidis 50mm 1024 .jpg