This afternoon my son found this larvae on a tree trunk as he was getting out of the car. The fat, bright green larvae of Cimbex connatus, a hornet-mimic sawfly as adult, looks remarkably like a caterpillar. My daughter wanted to pick it up, but I suggested that she just touched it instead and we left it alone. My son remarked how hard its skin felt, I guess he was expecting the soft, velvety skin of a caterpillar. As she did so she winced and shouted that the 'caterpillar had squirted something onto her'. Intrigued, I touched it too, felt the toughness of its skin and then the caterpillar sprayed my hand, which was covered on green-blue droplets. Curious, I did some research, which revealed that many sawfly larvae use a similar 'reflex bleeding' to ladybirds'. When disturbed, usually by predators such as ants or wasps, they squirt hemolymph out of cracks that open in their skin through the slight mechanical damage. This is called 'easy bleeding' as it's different from a normal wound. The skin is very flagile when touched, but the blood dropplets remaining on the body are quickly reabsorbed and the skin heals within minutes. This hemolymph, which is the name of arthropod blood, contains chemical compounds that the larvae store from their food plants which are distasteful to wasps or ants. Jean-Luc Boevé and Urs Schaffner measured both the skin resistance to mechanical damage and the distastefulness to red ant, Myrmica rubra workers in 43 species of sawfly larvae. They found that both traits varied a lot across species: the more fragile the skin of the sawfly larvae species the most deterrent to ant workers they were, showing that both traits are part of the same chemical defence strategy. In addition, those species fed on plants which are known to contain chemicals distasteful to vertebrates or invertebrates. In another set of experiments, Caroline Müller and Paul Brakefield showed that small white butterfly larvae were rapidly predated by arthropods, while most sawfly larvae survived unscathed. However, when they spread the small white caterpillars with the blood of the sawfly larvae, they became immune to the wasps attacks, most likely as the compounds made the caterpillars distasteful. This 'easy bleeding-distastefulness' chemical defence strategy might have evolve to deter invertebrate predators, as the species showing it tend not to show warning colouration, and indeed are cryptically coloured.
The larvae contracts it's head upon being touched
Boevé, Jean-Luc, and Urs Schaffner. Why does the larval integument of some sawfly species disrupt so easily? The harmful hemolymph hypothesis. Oecologia 134.1 (2003): 104-111.
Müller, Caroline, and Paul M. Brakefield. Analysis of a chemical defense in sawfly larvae: easy bleeding targets predatory wasps in late summer. Journal of chemical ecology 29.12 (2003): 2683-2694.