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.

The Hornet mimic Large Alder Sawfly

One of my first posts at BugBlog dealt with a strange caterpillar I was unable to ID. The caterpillar turned out to be the larvae of a rare sawfly, Cimbex connatus. The following summer I came across an adult, but it was dead and squished, so I never blogged on it. Today, just coming out of work I spotted what I thought it was a hornet on the ground. I got very excited, as I have never seen one, but my excitement turned quickly into disappointment and then into more excitement when I realised that it was in fact an adult, alive - although on its last legs - Cimbex. This is an awesome European hornet mimic, down to the abdominal pattern, size and general colouration, including the wings, and general body shape. @RichardComont at Twitter identified it as Cimbex connatus, the large alder sawfly.

Sawflies are a large and heterogeneous group in the hymenoptera, the order that includes stinging insects such as bees ants and wasps, although sawflies are harmless. The family to which Cimbex belongs has characteristic clubbed antenna. Males have enlarged hind legs and mandibles - which apparently use to fight with other males for access to females. Females have a saw-like ovipositor. They lay their eggs on specific trees or bushes and their caterpillar-like larvae feed on the leaves and come to the soil to pupate. 

I believe my specimen to be a female, as she has got an enlarged abdomen with the ovipositor sheath being visible underneath it. It measures 21 mm. I mounted it on a piece of white tack to photograph it in a more life-like position.

A side view of Cimbex connatus

A front view reveals a fierce-looking face, with large, sharp jaws and clubbed antenna.

This species is one of three large Cymbex species found in the U.K. it was regarded as extremely rare if not extinct. According to Mark Boddington:

Following a half-century hiatus to 1997, we have recent records of Cimbex connatus (Schrank) from more than ten counties in southern through eastern England, reaching as far north as the Humber. Over much of this area, C. connatus is now comfortably the most frequently encountered member of the genus, followed by C. femoratus (L.) and C. luteus (L.), the latter species in particular being infrequently seen. 

The expansion of Cimbex connatus in the UK might be linked to the increasing use of Italian alder trees, Alnus cordata, and similar species to line avenues in towns and cities.

Cimbex connatus larva found under Italian alder (23/09/2008)

A bright caterpillar for a wet day

This morning walking out of the house in the drizzle, this caterpillar was curled up in the middle of the footpath.

It was large, very bright green-yellow, with a black stripe on its back. I found a plastic bottle around - a good use for litter - and put it in it until I got back home to take a photo. It was next to a fallen lime leave and I got some fresh lime leaves as a background for the photo. The result is quite nice for the strong contrast of colors.

 I wonder if it was looking for somewhere to pupate. I have seen Lime Hawkmoth caterpillars at this time of the year as limes are a common tree in my street, but it is the first time I see this species. I have put a thread in the Wild About Britain site to see if somebody helps with its ID.

 Shockingly, and brilliantly, the 'caterpillar' turned out to be a larvae of the large alder sawfly, Cimbex connatus, thanks to the WAB site for identification. WAB is a fantastic, very rewarding place to identify bugs. Usually it is a matter of hours until a discussion arises about an unidentified bug photo is posted, often involving experts in the field.

 Sawfly larvae are remarkably similar to butterfly and moth larvae - to me a great example of convergent evolution-, but they can be told apart for having a larger number of 'false legs' and other details. They are hymenopterans that lay their eggs into the tissues of plants. It turns out that this sawfly species is quite rare in the UK, with only a few records, and that it had probably fallen out of alders, next to the lime tree. Alders do not lose their leaves until late in autumn, or even early winter, so this bug had still some eating to do.
 A great find for such a wet morning.