When it’s a Sawfly.
Did you know that there is a primitive group of the hymenoptera with free-living larvae that look very much like the caterpillars of the lepidoptera? Known as the Sawfly, there are morphological distinctions between them and the caterpillars and they can also be distinguished by this defensive posture adapted by the Sawfly larvae (Arge sp.) pictured here.
A Thousand Acres of Silphiums 