Winter Moth Caterpillar Operophtera brumata

Winter Moth caterpillar
Winter Moth caterpillar
Winter Moth caterpillar
The Winter Moth is unusual in the adult being active in the coldest months. Its caterpillar is active in the spring and those which are not predated to feed young birds pupate on the ground in June.
Winter Moth caterpillar
Winter Moth caterpillar
Winter Moth caterpillar
The Winter Moth caterpillar, like many Geometrid caterpillars, is a "looper" meaning that it uses its two pairs of back prolegs as an anchor while raising its body forward then bringing them up to the three front pairs of legs in a loop to move around.
Winter Moth caterpillar
Winter Moth caterpillar
Winter Moth caterpillar
The caterpillar loops across rose aphids. The young one above right is only about 1cm long (scale shown in mm), but they will reach nearly an inch before pupating.
Winter Moth caterpillar
Winter Moth caterpillar
Winter Moth caterpillar
This small caterpillar found on my apple tree looks like an early stage of Winter Moth larva with fainter markings but could also be the similar early stage of the November Moth larva.