We saw this tiny caterpillar on the Oa and watched it 'loop' its way across my rucsack. I was fascinated to read the explanation for this behaviour in wikipedia:
The name "Geometridae" ultimately derives from Latin geometra from Greek γεωμέτρης ("geometer, earth-measurer"). This refers to the means of locomotion of the larvae or caterpillars, which lack the prolegs of other Lepidopteran caterpillars in the middle portion of the body, with only two or three pairs at each end. Equipped with appendages at both ends of the body, a caterpillar will clasp with its front legs and draw up the hind end, then clasp with the hind end (prolegs) and reach out for a new front attachment - creating the impression that it is measuring its journey. The caterpillars are accordingly called loopers, spanworms, or inchworms after their characteristic looping gait.