Pirate wolf spiders in retreat

This afternoon I walked to local pond in search of dragonflies. Only Azure damselflies were active, half a dozen males bobbing about searching for females on the pond edge, a pair mating. As I looked for exuviae on the plant leaves, I noticed two female Pirate wolf spiders, possibly Pirata piraticus, in silky retreats at the base of branched burr-reed leaves, both carrying their bright white egg sacs. Apparently, unlike other wolf spiders, they build silky tubes, sometimes partially submerged, from where they hunt passing insects.

The Migrant Hawker unusual life history

Migrant Hawkers have been around since the beginning of August, and they should be around until October or early November. They are often seen flying away from water. They hawk at about 3 m high, patrolling an area repeatedly, occasionally rising to catch an insect, their abdomen held at an upward angle, with a slight droop at the end. 

Migrant Hawker in flight.

Their late summer appearance and social, non-aggressive behaviour are unlike other hawker dragonflies in the UK. When they hunt, it is not unusual to see several individuals together, either ignoring each other or not overtly aggressive. Occasionally they form large hunting aggregations in favoured habitats. They also often rest near each other in pairs or small groups in sheltered glades or hedgerows after hunting or during overcast conditions, hanging from branches orienting to the sun (top shot, a female in the foreground, with a male on the background). 

The reasons behind their distinctive life history lie in their ecology. Migrant Hawkers have a widespread distribution, from the Mediterranean to Asia to Japan. In the south of their range they often develop in temporary ponds, which recurrently dry in the summer. For this reason they have a very fast life cycle: after laying in the autumn, eggs undergo a period of winter rest (diapause). Larvae hatch in late winter and develop through spring, before metamorphosing and emerging as adults in late summer. The whole cycle lasts a single year, unlike other dragonflies that take at least two years to develop. When the adults emerge, the ponds where they developed quickly dry, so the adults do not mature immediately. They will hunt away from water for a while - up to 4 months in the south of their range in Algeria - and only return to ponds and mature when autumn rains may have fill the ponds, therefore they have one of the longest adult lifespans in dragonflies. During their immature stage they are great wanderers and migrate in both latitude and altitude to new areas where new ponds can be found or weather conditions more benign. In the summer, when they are away from water, immature migrant hawkers have no reason to be aggressive to others, there is no territorial behaviour or competition for receptive females, hence the congregations in feeding swarms on the plentiful insects. 

Male Migrant Hawker.

 Migrant Hawkers used to be rare in the UK before the 1940s, but they are now widespread in England and Wales, and also found in Scotland. In the early 2000 they colonised Ireland together with two other dragonfly species. It is likely that increasing temperatures are allowing this species to complete its cycle in the UK, where previously it was just a migrant species and their range has spread north as most other British dragonflies.

More information
Hickling, Rachael, et al. A northward shift of range margins in British Odonata. Global Change Biology 11.3 (2005): 502-506.

Samraoui, B., Bouzid, S., Boulahbal, R., & Corbet, P. S. (1998). Postponed reproductive maturation in upland refuges maintains life-cycle continuity during the hot, dry season in Algerian dragonflies (Anisoptera). International Journal of Odonatology, 1(2), 119-135.

A Stretch spider by the pond

I spent some time in the wildlife garden this morning looking and photographing pirate wolf spiders (post to follow) and watching pond skaters in the pond. I had noticed a pair of legs coming from under a yellow flag leaf, hugging it and I decided to inspect. As I turned the leaf I spied a large, wonderful female stretch spider of the genus Tetragnatha. The spider had its front and rear legs stretched to the front and rear, and held onto the leaf with her middle legs. She walked up and down the leaf, rearranging her silk threads, which I had possibly dislodged. She was quite a large sized and very colourful spider, with yellow and red stripes on her elongated abdomen, it looked so exotic and tropical!
  Out of the six British species of Tetragnatha, two are larger and more common and one of them associates with water, T. extensa, although detailed genital examination is needed for a secure identification.
 Tetragnatha have very large jaws, and the males using their modified jaws to lock the female's in place - and keep out of danger - when mating.
 You can see the folded fangs in this shot

And this is a scanned figure from Bristowe's The World of Spiders (1958), drawn by Arthur Smith, demonstrating the locking mechanism (male at the bottom)

I took this shot to give you an idea of the size of the spider

For a wonderful account of the mating behaviour and natural history of these spiders, go to the Spiderbytes blog.

Pirate spider

I spotted this Pirate Wolf spider yesterday, which goes with wonderful latin name of Pirata piraticus on the pond of my local wildlife garden. There are several British species, most of them found in marshy habitats. These spiders are not only able to move easily on the wet vegetation, but they can run, walk and stand on water, as this one shows. They can even submerge, surrounded by a bubble of air, in case of danger.

Two freshwater flatworms

The garden barrel is one place where some invertebrate activity is guaranteed any time of the year. Snails graze algae in the winter and Water Slaters, Asellus aquaticus, munch the decayed leaves. There are predators too: flatworms. These innocent looking, slug-like animals, covered in sticky mucus slither on the surfaces of leaves looking for prey. Today I found two species, a small, dark one, probably Dugesia lugubris (top) and a large white one, Dendrocoelum lacteum (bottom). Both species shown here have a pair of simple eye spots, other species have more eyespots.

The LED lights of the hand held microscope reflect on the water surface and bring out the colour of the branching gastrovascular system of Dendrocoelum lacteum. The white band in the middle of the body is the evertible pharynx, on one end of which is the mouth. Small organisms, including Water Slaters, are often prey of flatworms, and are subdued with the mucus and wrappen on the flexible bodies of the flatworm.

More information

Richard Fitter & Richard Manuel 1986 Collins field guide to Freshwater life. 

Helen Mellanby 1975 Animal life in freshwater life. Chapman & Hall.

Marsupial water slaters

 Water slaters or water-lice, Asellus aquaticus, scurry about the leaf litter of my indoor pond. They are fascinating pond inhabitants, always active, sometimes three individuals walking in a line across the tank walls. Most of the water slaters had broken antennae when I collected them, but they have now grown back, the new segments were still thin and transparent a few weeks ago (above), but have now fully developed. Unlike insects, which have very limited regeneration abilities, crustaceans can regenerate lost or broken appendages - as even when adults they carry on molting, and in each molt, they regenerate a bit more of the missing leg or antenna.

Water slaters tolerate a wide range of ecological conditions and are distributed across much of temperate Europe, Russia and North America. They are often abundant in polluted water and live in lakes, rivers, springs and even caves - where albino subspecies with smaller eyes have evolved. Being scavengers, they just need some detritus and fallen leaves and organic material to subsist.

In the last few days, a female has laid eggs into her brood pouch or marsupium. While she climbed over the leaves beside the tank wall, I took a photo where her yellow eggs are visible through the transparent brood pouch.

The marsupium is made of overlapping flat blades coming out of the female's anterior four pair of legs, and allows the mother to hold the brood under her body. This structure is a common feature of all Isopods - which include also terrestrial woodlice. The female not only carries the eggs but also the developing larvae. The larvae will emerge as miniature versions of the adults (see the figure below).

Fig. I. Marsupial stages of Asellus aquaticus. Top plate: A, stage A showing two membranes (arrows). B, stage B showing one membrane (arrow) and cleft yolk mass. C, stage B shedding membrane to reveal lateral outgrowth (arrow). D. stage C with its comma shape surrounded by a membrane (arrow). Bottom plate: A, stage C shedding membrane (arrow) to reveal stage D with free appendages. B, stage D with lateral outgrowth (arrow) and appendages covered i n a closely-fitting membrane. C, stage E showing setose body and appendages (arrows). It is at this stage that the juvenile leaves the marsupium to become free-living (from Holdich & Tolba 1981)

Occasionally females expel some larvae from the pouch - possibly due to physical limitations of how many she can carry as they grow - and larvae can also develop normally outside the marsupium from developmental stage B. Probably they need the ventilation and extra oxygen produced by the mother in the initial stage. Carrying the eggs and larvae is also bound to offer them some protection from predators, so it probably evolved as a form of maternal care.

Although water slaters normally stop reproducing during the coldest months of the year, my indoor pond is warmer than outside, and this might have triggered the start of reproduction; a situation that also occurs in wild populations downstream thermal power stations where water is warm all year round. 

More information

Vitagliano, G., Fano, E., Marchetti, E., Colangelo, M., & Vitagliano, E. (1991). Importance of longevity, growth, and diapause in the evolution of Asellus aquaticus Bolletino di zoologia, 58 (2), 113-117 DOI: 10.1080/11250009109355740

Holdich, D., & Tolba, M. (1981). The effect of temperature and water quality on the in vitro development and survival of Asellus aquaticus (Crustacea: Isopoda) eggs Hydrobiologia, 78 (3), 227-236 DOI: 10.1007/BF00008519

A little red underwater planet

 I have brought a few animals from my half barrel (=mini pond) to see close up to a small tank. Most of them are detritus feeders or vegetarians, and regularly I top up the tank with fallen leaves. Today's water was stained red and the reflection of the light in the water and the leaves made the photos take a surreal tint. A Great Pond Snail (Lymnaea palustris) fed on the algae growing on the tank walls (above).

Another snail, Planorbis planorbis

A group of ostracods, small bivalved crustaceans, and a copepod towards the left of the image.

There are also a few water slaters and planarians, but they didn't show as well today.

UPDATE: Thank you to Richard Comont for the pond snail ID, now corrected.

More information
Online aquatic snail identification guide from the Conchological Society. here.

Freshwater Flatworms

 If you kneel down on the edge of a clear water pond and look towards the bottom, even at this time of year, you might be able to see some flattened elongated creatures slithering over the sediments, like they are levitating (above). They are planarians or flatworms. On closer inspection they have a quizzical look due to their simple eyespots or ocelli, which appear to be both looking inwards. They are relatively simple organisms, which move over a carpet of cilia and have simple guts, with a single opening, the mouth, which is sometimes placed in the middle of the body. I am trying to get some shots of water slaters and I have set up a little tank at home. Most of the time when I get some decaying leaves from my half barrel (=minipond) a few flatworms are accidentally transferred with them, and then I can take some photos of them while they climb on the walls. There appear to be several species, but yet have to find out some resources to tell them apart.

One of the most striking features of flatworms is their capacity for regeneration. If you cut them in half, each half grows the missing parts. Even a tiny part of a planarian can regenerate a complete individual.

Pond Skater

One of my earliest bug memories is watching pond skaters moving effortlessly and magically over the water. Pond Skaters, Gerris sp., also known as Water Striders, are true bugs: they piercing mouthparts are used to suck the juices of the insects they predate. Their legs are clothed with water-repellent hairs and they propel themselves with their middle legs in a flicking glide. Their front legs are small and kept tucked ahead of them, sensing the vibrations in the water from other ponds skaters or potential prey that have fallen in the water, and also grasping and holding their prey. They are seven similar species of Pond Skaters in the U.K., and they are sometimes found in the same pond. Pond skaters are active in these mild days of autumn and will overwinter as adults, moving away from the water with the first frosts. Many pond skaters species are polymorphic and have long-winged and short-winged morphs. In species with two generations per year, the long-winged form tends to be the overwintering generation - which is also the dispersive one - while the summer generation tends to be short winged. When the winter comes, short-winged individuals, unable to fly, will crawl out of the water and hide in the bank, while long-winged ones can fly away from water, to sites that are free from risks of flooding or freezing. In the spring, emerging pond skaters will fly back to ponds and lakes, using the reflections of the water - or other shiny surfaces - as guide.
Pond skaters, as other true bugs, go through five instars before becoming adults. The individual of the top photo is a nymph (24 Jun 2011), the one at the bottom, an adult (2 Jul 2011). Although the photos are from earlier in the year, smaller nymphs were still in a local pond last monday.

The Great Ramshorn Snail

There is still a lot of activity in ponds. We haven't had a frost yet and Pond Snails, water-lice and ponds skaters go about their lives as usual. This is a Great Ramshorn Snail, Planorbarius corneus, inhabitant of still or slow moving waters with a high calcium content and plenty of vegetation. Unlike land snails, Ramshorn Snail's eyes are at the base of their long tentacles. They graze algae and plants or feed on dead leaves. They are hermaphrodite, although they prefer to cross-fertilise one another. They will lay their gelatinous egg batches on the underside of leaves. I photographed it on the white bowl with some rain water.

Bees in ponds

When you think about a pond, bees do not normally spring to mind. Bees, however, need water. In sunny days honeybees travel in numbers to water sources to take water to cool down their hive, and they also use it to dilute honey to feed their larvae. These three were drinking by the edge of the pond in my local wildlife garden, near their hive.

On the other side of the pond edge there is a muddy site. Several female Red Mason Bees (above) were collecting mud to line and cap their cells. A female would dig up the mud and shape it with the  help of her jaws and two little facial horns into a little ball, and then fly to the nest holding the ball in her jaws. It took me a few attempts to get a half decent shot.

Osmia rufa female with her ball of mud

Water slater embrace

Our pond does not even qualify as a pond. It is half a wooden barrel, almost full of dead leaves and overgrown irises and marsh marigolds. Still, it's got some water snails and frogs have bred the last couple of years. A few days ago, I was watching the very active ramshorn snails when I noticed this paired water slaters, Asellus aquaticus. Water slaters are isopods, like woodlice, but unlike their terrestrial relatives they display a behaviour which is common in crustaceans, mate guarding, by which males spend some time holding onto the female before the actual copulation occurs. This mate guarding, or pre-copula, is thought to have evolved in response to a very short fertile time window in females. As in many crustaceans, females can only mate right after moulting, so this guarding behaviour provides males with the reward of a fertile female at the end, and is more effective than searching randomly for receptive females.
 Unlike most sexually dimorphic crustaceans, male water slaters are larger than females. This is thought to have evolved by sexual selection. Larger males are more efficient dislodging other males already guarding females, or avoiding being taken over by other males.  Females seem to offer little resistance to being guarded, so sexual selection by females can be discounted. However larger males also pair faster, suggesting they might be better at finding females, and experimental manipulation of antenna length has shown that it is their longer antennae which increases the rate of encounters of larger males with females.
 Mate guarding has also been suggested to have an antipredatory function in water slaters. Water slaters are often predated by newts, and research by Paul Verrell indicated that paired males had a lower predation risk than both unpaired males and females in laboratory trials. Female predation rates are unaffected by being paired (see Table 1 below). According to Verrell two factors explained these results, first, males are much more active when unpaired, as they move about searching for females, so they might be much more exposed to predation. Second, when they are paired, after an attack males have a quick escape response from the female which might reduce their predation risk.

(from Verrell, 1985)


References
Bertin, A. & Cezilly, F. (2003). Sexual selection, antennae length and the mating advantage of large males in Asellus aquaticus Journal of Evolutionary Biology, 16, 698-707 DOI: 10.1046/j.1420-9101.2003.00565.x
Verrell, P. (1985). Predation and the Evolution of Precopula in the Isopod Asellus aquaticus Behaviour, 95, 198-202 DOI: 10.1163/156853985X00127

The footballer hoverfly

With its yellow and black stripy thorax and brightly marked abdomen, Helophilus pendulus is a handsome hoverfly. It is also easy to photograph, as it likes to rest on leaves or the edge of pools and occasionally it can be seen feeding on flowers. This is the most common of a group of closely related species, some of them migrants and some found as north as the Arctic. Their larva are of the long-tailed type and are aquatic, found in small bodies of water with abundant organic material, where they feed. They have also been recorded in cow dung. I have found the adults from April to October. Today, an adult patrolled around a puddle on the pavement on a sunny spell after the last rainy days.