The flannel moths or crinkled flannel moths (scientific name Megalopygidae) are a family of insects. They occur in North America (11 species) and the New World tropics. The larvae are called puss caterpillars, and with their long hairs, resemble cotton balls. They have venomous spines that can cause a painful sting and inflammation lasting for several days. In some cases, the sting may cause headache, nausea, and shock-like symptoms. Perhaps the most notorious for stinging is the caterpillar of Megalopyge opercularis.