Megalopygidae is the scientific name of a group of insect species known generally as crinkled flannel moths, or simply flannel moths. 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.
A Megalopyge opercularis caterpillar on Kent Island, Maryland, in September 2014: This is a very toxic caterpillar.