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.

Never hold a Black Clock in your mouth

I found a squished ground beetle on the pavement today, Pterostichus madidus, and I thought I would share with you some photos from last year of a very alive individual of this species . Curiously this carabid beetle has a curious English common name: the Black Clock. According to the book Bugs Britannica, by Marren and Mabey 'clock' was a word in widespread use to mean any big buzzing insect. As many carabids, however, this is a fast, non-flying nocturnal predatory beetle which runs to hide quickly when uncovered. This one was found under a grass tussock in the garden. Look at his sharp jaws!
 Ground beetles have glands at the end of the abdomen that produce defensive, foul tasting and caustic secretions. I must quote here an incident involving ground beetles described by Charles Darwin

I must tell you what happened to me on the banks of the Cam in my early entomological days; under a piece of bark I found two carabi (I forget which) & caught one in each hand, when lo & behold I saw a sacred Panagæus crux major; I could not bear to give up either of my Carabi, & to lose Panagæus was out of the question, so that in despair I gently seized one of the carabi between my teeth, when to my unspeakable disgust & pain the little inconsiderate beast squirted his acid down my throat & I lost both Carabi & Panagæus!

The pains beetle collectors went through to capture priced ground beetles! You can admire the handsome Crucifix Ground Beetle (Panagaeus cruxmajor), also known as 'Darwin's lost beetle' here. Unfortunately, due to collecting and habitat degradation, this species, formerly widespread, is now just know from three populations in the UK.
But back to my beetle, I managed to take some shots of the individual scurrying off for cover on the soil, and then I decided to capture it and give it a session in the white bowl. I think the following two photos show the pros and cons of the natural versus the white background.

Pterostichus madidus, 12 May 2012

More information

Charles Darwin letter to Leonard Jenyns in the Darwin Correspondence project.

Crucifix ground beetle (Panagaeus cruxmajor) UKBAP plan.

Six-spot burnets: chemical weapons as nuptial gifts

Six-Spot Burnets, Zygaena filipendulae, are spectacular day flying moths. The contrast between their colour and the green meadows where they live makes them very obvious. Their body and forewing background are black, with a metallic green-blue sheen. Their forewings have six crimson-red spots. In addition they are large and heavy, and females like to perch conspicuously atop flower heads. You cannot miss a sitting burnet, but a flying one is even harder to miss: when they fly, a slow, buzzing, heavy flight, their crimson rear wings with a narrow black border become visible. As you could predict, this bright, black-with red spots contrasting colouration is a warning sign. Burnet moths are toxic, when they are injured, they release cyanide, a highly poisonous chemical. Cyanide compounds are found at some level in every life stage from egg to adult. The larvae sequester and store cyanogenic compounds from their food plants - Bird's Foot Trefoil, Lotus corniculatus - and all life stages are able to synthesize these chemicals themselves. However, production of the chemicals is costly, as larvae grow much more slowly when reared on varieties of Bird's Foot Trefoil lacking cyanogenic chemicals, as they have to synthesize them all themselves. There is a sudden loss of cyanogenic compounds from the last larval stage to the adult, and there also appears to be large variation in the amount of toxic chemicals in the adults. This could partly be explained by volatile hydrogen cyanide emissions by larvae - possibly as an antipredator strategy. Also, males convert some of their cyanogenic compounds into a pheromone; upon approaching a female, they release it. Females are able to determine how much pheromone a male is producing, the larger the quantity, the more likely the female is to accept him. But things get even more interesting. Mika Zagrobelny and co-workers, from the University of Copenhagen, collected Six-Spot Burnet larvae, pupa and adult from a local fallow field and made detailed measurements of the levels of cyanogenic compounds in the tissues of the different life stages, and also their toxic emissions. They found that females, as the larvae, emit hydrogen cyanide. Males are attracted to these chemical plumes, which form part of a pheromone cocktail produced by the female.

Average total cyanogenic compounds content in virgin and mated Z. filipendulae adults as well as in discarded males (which females would not mate with). Error bars are standard deviation. (figure modified from Zagrobelny et al. 2007).

The researchers then compared the cyanogenic compounds of virgin males and females, as well as mated males and females they paired up in the laboratory (see figure above). Virgin males and females had roughly similar levels of cyanogenic compounds. In contrast, after mating, females had larger levels, whereas males had lower levels. This indicates that during mating, males transfer some of these chemicals to the female, likely with the sperm. The levels of cyanogenic compounds in rejected males (males in an experimental pair that the females refused to mate with) were lower than average levels in virgin males, which suggests that females will mate preferentially with those males better loaded with chemical weapons. Why would the female benefit from acquiring more cyanide compounds. Possibly because the more she puts into eggs, the better defended they will be from predators, so this nuptial gift might be seen as a form of paternal behaviour. Alternatively, the female might gain through using this nuptial gift to produce more pheromone, attract further males and increase the vigour of her offspring.

A Six-Spot Burnet on Bird's Foot Trefoil

References
Zagrobelny M, Bak S, Olsen CE, & Møller BL (2007). Intimate roles for cyanogenic glucosides in the life cycle of Zygaena filipendulae (Lepidoptera, Zygaenidae). Insect biochemistry and molecular biology, 37 (11), 1189-97 PMID: 17916505

Snake millipede

I was out and about this weekend taking advantage of our "mini-spring". When repotting some self-seeded daisies I came across a few millipedes in the soil. I seem to always encounter the same species (maybe the same genus I should say, as there are several similar looking species) of millipede: a long, thin type of various sizes, which, upon encounter rolls itself into a spiral. This is the aptly named millipede Cylindroiulus. The commonest UK species is C. punctatus and it is likely this is my garden's species. Millipedes (Class Diplopoda) like insects and spiders are arthropods, and they are thought been amongst the first animals colonising land, some time in the Silurian Period, over 400 million years ago. There are around 10,000 described species, although there are probably several times that number of species to be described. Just 52 of them are known from the U.K. Millipedes are mainly detritus-feeders, and are often found in the soil, leaf litter, or on dead wood, where they contribute to the recycling of vegetal matter.
 Millipedes means "A thousand legs" but millipedes, although having more legs than centipedes, don't have a thousand legs, the record holder is, apparently, 750! Each of the millipedes body section has two pair of legs, except the few rings behind the head. Their antenna are elbowed and clubbed, and they constantly tap the ground while they move. Cilindroiulus has eyes, but many millipedes, especially those permanently living in the soil or in caves, lack eyes. A curious feature of millipedes eyes is that they add ocelli as they moult, so that adults have much large eyes, and likely much better eyesight, than juveniles. The ocelli are added so precisely to each moult than the growth stage can be determined in some species by counting the rows of ocelli.

 Millipedes can deter predators, physically, curling up into a spiral which shelters the head and ventral area (above), exposing only the chitinous, hard dorsal rings, in a similar way to an armadillo or pill woodlouse. At the same time they curl up, they display their chemical defence: they secrete cocktails of repellent and/or toxic chemicals through glands present in each body ring when attacked. In large centipedes the smell may be quite obvious, in small ones, you have to sniff them to detect the smell. Some small predators might be killed by these chemicals if confined with the millipede in a small space, or deterred by the smell. Despite this defence, millipedes fall prey to birds who like to explore the leaf litter, such as blackbirds.
 During winter, Cylindroiulus moves down into the soil, and sometimes several individuals roll up together during winter or hide under bark. That is what the usually wood dwelling millipede was doing under the pot soil.

More information

Stephen P. Hopkin, Helen J. Read. 1992. The biology of millipedes. Oxford University Press, 1992 233 pp.

Gordon Blower. 1985. Millipedes: keys and notes for the identification of the species. Brill Archive. 242 pages.

Why do ladybirds overwinter in groups?

You have probably come across ladybirds clustered under leaves or bark during winter. To spend the winter, seven spot ladybirds - otherwise solitary creatures -   they seem to actively seek each other. I took the photo above a few minutes ago in my garden. I counted 16 ladybirds - most were 7-spots, with two Harlequins - on the shady side of an agave killed by this winters' harsh frosts. Before I go on to explain this communal overwintering behaviour I have to explain why ladybirds are so colourful. Ladybirds are aposematic, a term describing an antipredator adaptation by which organisms have evolved bright, contrasting colours (think on the yellow and black stripes in cinnabar moth caterpillar or wasps) to warn predators of dangerous behaviour (stinging) or distastefulness. Ladybirds belong to this later group. Their beautiful glossy red and yellow elithra with black spots is the first line in a complex defence system, a warning signal to predators, probably birds, of their foul taste. Their bodies contain a bitter tasting alkaloid. If the predator ignores the warning signal and attacks the ladybird - or if you handle them a bit roughly - they release a yellow liquid from their leg joints, with high concentrations of the alkaloid, coccinelline, in what is called "reflex bleeding".
  Given that ladybirds can secrete up to almost a quarter of their body weight during the reflex bleeding, it is an energetically demanding defence mechanism. During the winter, when ladybirds do not feed and need to save precious resources, they do not reflex bleed, although they still taste bitter. It is because of this that they cluster together: as other distasteful prey that lack a mechanism to let know of their foul taste to a potential predator, clustering allows individuals with warning coloration in a group - even if they are unrelated - to benefit from just one of them being injured or killed by a predator, as the predator is unlikely to attack further ones in the cluster.

Reference
Holloway, G., Jong, P., Brakefield, P. & Vos, H. (1991). Chemical defence in ladybird beetles (Coccinellidae). I. Distribution of coccinelline and individual variation in defence in 7-spot ladybirds (Coccinella septempunctata) Chemoecology, 2 (1), 7-14 DOI: 10.1007/BF01240660