I have just observed that some 1st instar larvae move outside of the petridish / not live on food plant during studying the life cycle of butterflies in laboratory conditions. What is the reason behind this? Maybe they did not find a suitable environment inside of the petri dish? Just a guess.
Unless this is a 'social' species, many/most caterpillar species will not feed if crowded. And don't forget: even artificial diet changes over time as it dries out -as do cut leaves - and some plant material may respond to feeding. Those factors can lower food quality and stimulate movement away.
There are many reasons for such behavior. In containment as some one written crowd and micro environment matters much. Some should be reared in isolated tubes/vials due to cannibalism.Check these point with your species of study.
I agree with the above answers and add that certain "wandering" of the 1st instar caterpillars is a perfectly natural phenomenon. In nature, females sometimes do not lay their eggs directly on food and thus, freshly hatched larvae must first look for their food.
An additional possibility is migration of the larvae at this stage to disperse, such as for example ballooning larvae of the spruce budworm. This behavior can temporarily bring them far away form their host plant, to find other hosts or parts of current host (a tree) that are suitable for colonisation.
It is very commonly noticed that first instar caterpillars of many moths, just after hatching, 'wander', may be in search of younger or tender portions of leaves, or sometimes they end up at the tips of leaves where the hardness is less.
It happens because most butterflies larvae feed on the phloem sap and at early stages of their growth, they are unable to feed on hard leaves so they scatter to neighbouring young leaves. on the other hand, sap in new tender leaves is not that much to support the newly hatched population of larvae, so they keep jumping from one feeding site to another.
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