Firstly a White-marked Tussock Moth (Orgyia leucostigma), the caterpillars of which may be found feeding on an extremely wide variety of trees, both deciduous and coniferous, including apple, birch, black locust, cherry, elm, fir, hackberry, hemlock, hickory, larch, oak, rose, spruce, chestnut, and willow. Defoliating outbreaks are occasionally reported especially on Manitoba maple and elm in urban areas. Outbreaks are usually ended by viral disease.
And then, nearby and, of course, on some Milkweed, there was the not terribly dissimilar caterpillar of a Milkweed Tussock Moth (Euchaetes egle) … more orange and fewer tufts. It acquires from its host milkweed plant cardiac glycosides which are retained once it becomes adult and seemingly deters predators from feeding on them.
These little guys are quite eye-catching...they look like they are from the 1970's - orange shag carpet comes to mind!
What magnificent looking caterpillars!