One of the most venomous insects are the caterpillars of the Moth genus Lonomia. To date, no one has calculated the LD50 values of Lonomia venom; the rate of human fatality has been documented as 1.7%, compared to a rate for rattlesnakes at about 1.8%, despite the fact that the amount of venom is only a minute fraction (less than 0.001) of the amount in a snake bite. Accordingly, it seems likely that when measured, the LD50 for Lonomia venom will be among the lowest for any natural toxin known. While there are many species in the genus, the most troublesome species is Lonomia obliqua (on the photos) from Brazil, and it is this species which most of the medical research has centered upon. Other famous species is Lonomia achelous from Venezuela.
See more: Most toxic insect venom
Source: Wikipedia, AFPMB, Mundobutterfly