End of July i found this looper between some dead plant stems of Garlic mustard, its the caterpillar of the Willow beauty (Peribatodes rhomboidaria).
Caterpillars in the Geometridae family are known to mimic sticks and twigs, using their body shape with knobs and extensions and posture. This specimen is rather usual in its shape, only the third segment counted from the head has a knob on the side.
Loopers can take peculiar postures because they have less prolegs. The Willow beauty has only one pair prolegs. Read more about prolegs here.