Two monarch caterpillars eat milkweed leaves. I placed the microphone close by and was easily able to record the sound as they ate. Standing nearby I could even hear it when all else was quiet.
The caterpillars eat about one large leaf each day or more. These two are very close to going into their chrysalis stage and are probably trying to get all the nutrition they can before they transform into monarch butterflies (about ten days after they go into their chrysalis).
