Dysderidae: Woodlouse hunters

My second spider family, Dysderidae, is a much smaller family in the UK, with only four species of wandering, nocturnal hunters. They are unusual in having just six, not eight eyes, arranged close together. They don't build a hunting web, but use their silk to spin cells under stones or logs where they hide during the day. Females wrap their eggs in silk and guard the egg sac. The most widespread is Dysdera crocata, a species that is actually cosmopolitan as a result introductions in North America and elsewhere, and that is often associated to human habitations.

Fangs!
It is a brightly colourful spider, with a flattened, elongated appearance. However, their most striking feature are their massive chelicerae projecting forward. These, together with their long fangs enables them to be able to hold the woodlice like pincers, and inject venom in the softer underside of the animal. Woodlice likely comprise the bulk of their diet.

I disturbed this D. crocata as it was feeding on a woodlouse under a pot.

 A mature female roaming in the open at night

A male and a female individuals in threatening position: fangs wide open, bodies and front legs lifted and keeping their distance from one another.

Araneidae: typical orb weavers

It's #arachtober, a whole month dedicated to the celebration of spider awesomeness and diversity! For this reason, I will be writing a daily post on the natural history of a British spider family.

For the first post, I have chosen Araneidae, the typical orb weavers. This is one of the largest spider families and include many familiar species, such as the garden spider (Araneus diadematus), the window frame spider, Zygiella X-notata, the wasp spider Argiope bruennichi, and the small cucumber spider Araniella sp. There are 33 species in the UK.  The drawing on the left shows the eye arrangement in the family (based on a photograph by Kiron Basu)

In foggy weather, orb webs become very visible, heavy with dew. The spider is sitting in the central hub.

A. diadematus spinning its web.

The spider's web
The most characteristic feature of araneids is that they spin flat, rounded webs with radial threads connected by a spiral net-like pattern of threads, which they attach to vegetation or other structures  The threads are sticky, and able to trap flying insects. The spider either sits in the middle of the web (often at night) or hides in a retreat on a corner of the web, with one of their legs touching a signal thread connected to the hub of the web, which allows them to detect the vibrations produced by any struggling insects. Orb spiders often eat their web at the end of the day, and build a new one in the morning. As they adopt a sit and wait strategy to catch prey it is easy to find particular individuals in the same spot every day.

Dangerously close. The male (on the left) was dispatched before he had the chance to mate, after a lengthy and careful approach.

Mating and cannibalism
Adult females are readily distinguished by their larger size and swollen abdomen. Females often cannibalise males (often right during mating), and males might die after mating a couple of times so at the end of the season, only mature females might remain.

Female Araneus diadematus with its fresh egg sac.  Once their egg sac is finished, the female sits on it and will die in a few days.

Egg sacs and spiderlings
Females lay their eggs, often over a thousand of them, and spin an egg sac around them. Females might die shortly after egg laying. The spiderlings hatch, moult inside the egg sac and then they emerge. In some species, like the garden spider, they spin a communal web and cluster together in it until they are ready to disperse.

A spiderling ball, newly born A. diadematus

At the end of the summer, full size Araneus diadematus can subdue quite large insects such as droneflies, butterflies and wasps.

A. diadematus are very variable in background colour with a range from sandy to chocolate brown, although the white markings on the abdomen tend to be more uniform.

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.

Stretch spider identity revealed

There is a population of Stretch spiders Tetragnatha sp., on the wildlife garden. On Saturday I popped in with my youngest daughter and there was an adult female by the pond, sitting in the centre of her web, which was almost horizontal, about 20 cm from the surface of the water. She noticed the disturbance and retreated to one end of her web. Fortunately still exposing her underside so I could photograph the areas that make it possible to identify the species: the sternum with a paler centre and the epigyne or genital opening. Matt Prince and Dr Richard Pearce on Twitter confirmed her ID as Tetragnatha extensa, a species likely to be associated to water.