Some species like it warmer, others show their intolerance by lying on their backs - dead. Even closely related species can have different temperature requirements. Always place something in the container for the larva to cling to. Newsprint paper seems adequately sterile. Use the lens effect of the curve of the tube.
Moults almost glitter as they pass a light-dark boundary.
Repeat the inspection daily after this until the animal moults or dies. If the animal is moulting you are on your way. If it is dead you can stop feeding it. Keep some of the larvae in vials to check individual progress. If all the vialed animals die still check the bulk container as a bit of cannibalism or scavenging on a dead or dying sibling may have brought an animal or two through.