Automeris sp. (Saturniidae)
This is by far the most beautiful caterpillar I have seen so far here at the ACG, but its not like this caterpillar needs a "Do Not Touch" sign - there is just something nasty about the look of those spines. Many caterpillars of this genus lay multiple eggs on the same host plant, which seems to be the habit of many butterfly and moth species with aposematic caterpillars. Unfortunately, when one of the parataxonomists was collecting this specimen she accidently touched another with her forearm. The area of her arm near where she touched the caterpillar was swollen and looked painful. The sting of some caterpillars can be extremely painful and occasionally fatal so caterpillars with spines should always be handled with care. Some caterpillars have urticating hair that can be similarly painful or irritating.
Update: I met with the parataxonomist later and she told me that it didn't hurt that much and felt hot - more like a burn. After a day the swelling went down and the mark was gone shortly after that. The pain caused by other species can be much more extreme, so generally heed the animal's warning and stay away.