One leaf of one sallow plant has been turned to fretwork by a host of small sawfly larvae.
Although they feed quite openly during the daytime, they are easily overlooked, the bare leaf veins being the most noticeable thing. Their habit of cocking their tails up in the air when alarmed is characteristic
They are too young to be certain of the species but I suspect it is Croesus septentrionalis. I will be able to tell when they are larger.