Easily bleeding sawfly larvae

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

More information

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.

Another Vapourer encounter

This sunny morning I had to stay at home, as my 7 year old daughter was unwell. On our trip to the doctor I noticed a flying insect I couldn't decide if it was butterfly or moth, it looked like it wanted to settle, so we waited until it did. If finally settled on some railings, flew again and resettled on another part of the railings, and then I realised it was a male Vapourer. This most wonderful of moths! Where there is a male Vapourer settling a female can't be far. Females don't look like moths, they are flightless, their wings reduced to little more than stumps, and they have bulging bodies full of eggs. Females actually move very little in their adult lives, as they just sit on top of their cocoon - which they made as a caterpillar and later emerged - waiting for males to home in their pheromones. Males are a wonderful chestnut colour, a bright moon-shaped white dot on each wing, with feathery antennae, and they fly often during the day.
The male had actually found a female on a corner of the railing and, almost immediately, they mated. I took a few photos with my phone and we carried on.
 On our way back a little later, we checked the railing. The male was gone, but the female wasn't wasting any time, and was busy laying eggs on the remains of her hairy cocoon.