Here unknown instar at about day 14. No measurement, but a centimeter or less. Last evidence of feeding. Either willow no longer accepted or the caterpillars overwinter and quit feeding (I mostly suspect the latter). Two continued to be found daily with no clear evidence of feeding and no growth, but seemingly healthy. Then on morning of August 30 (about day 21) found no sign of caterpillars or of potential predators. Did find two small brown down-flowed patches of dried liquid on container walls and wonder if the caterpillars were consumed by some microbe. No exuviae or recognizable parts found. So maybe they somehow escaped the container after all those days.