Wednesday, 16 August 2017

Pollen gathering spiders

If you go out into the garden and search for orb webs amongst the bushes, you are likely to surprise a garden spider holding onto a fly, an aphid or other small insect tightly wrapped on silk. In the autumn, when they are at their largest, they will be able to subdue large prey including butterflies and droneflies. Indeed, spiders are generally known as predators and the purpose of the almost invisible orb web is to trap flying insects with the sticky droplets that cover the silk threads, right? Orb spiders, including the common garden spiders, Araneus diadematus, have challenged this assumption. When garden spiders hatch in spring, and after spending a few days in a tight ball with their siblings, the tiny, millimetre long spiderlings will disperse and make their first orb web (top shot). Until then, they have been living of the yolk from their egg stage, but now they have to find food. At that time of the year there are not many flying insects, and spiderling mortality due to starvation is high.
Young orb weaver, Zygiella sp, with an aphid prey.

  Now, when you look at a spider web under the microscope you can see not only insects and other small invertebrates trapped on it, but also pollen grains and fungal spores. In spring there is an abundance of pollen of wind pollinated flowers and trees. Given that orb spiders rebuild their web at regular intervals, pulling down and eating the web before spinning a new one, the young spiders could potentially be feeding on the pollen collected, benefiting of this plentiful resource. Risa Smith and Thomas Mommsen, from the University of British Columbia and Dalhousie University carried laboratory experiments to test exactly this. They compared the survival and web building frequency of individually housed spiderling, which were fed exclusivelly on birch pollen or fungal spores with unfed controls and spiderlings fed on aphids. Their results show that spiderlings not only feed on the pollen, but pollen feeding increases their survival compared to starved spiderlings or spiderlings given fungal spores. however, only the spiderlings fed on aphids moulted into the next instar. Smith and Mommsen reckoned that given that pollen from wind pollinated trees is highly deficient in the aminoacid tyrosine, essential for cuticle formation, spiderlings could be unable to thrive only on pollen, but would need to capture the occasional insect to moult into the next stage. The researchers noted that pollen grains are too large to be consumed whole by the spider, therefore, they couldn't have been accidentally swallowed. The spider indeed treated pollen grains like insect prey: grabbing the pollen grain, dissolving a hole in its coating and sucking the contents! More recent experiments estimate, using isotopes, that about 25% of the diet of young orb weaving spiders can be made of pollen. Spiders might be best considered omnivores rather than carnivores!

More information
Smith, R. B. & Mommsen, T. P. Pollen feeding in an orb-weaving spider. Science 226, 1330–1332 (1984).

Eggs, B. & Sanders, D. Herbivory in spiders: the importance of pollen for orb-weavers. PLoS One 8, e82637 (2013).

Thanks to Robert Jaques for sharing the paper that started this blog.

Friday, 30 June 2017

Roosting Wool Carder Bees

After a warm couple of weeks the temperature dropped suddenly on Tuesday and we've had quite a long of rain. From the conservatory window, I noticed a roosting Wool carder bee (Anthidium manicatum) on a Purple Toadflax flower spike, one of their favourite flowers. As I took a photo I saw there were actually three others roosting nearby, all females. Today it was far too cold for them to be active, so they are braving the weather holding onto the flowers with their jaws, so when the sun shines again they should be ready to feed straight away.
 Although we've had a few poor years for wool carder bees, this year they have come back en force and two males have been defending their territory in the garden.
Spot the roosting bees. A carder bee, Bombus pascuorum, feeds on the flowers unmolested. Bumblebees generate their own body heat and their dense hair coat helps them retain it so they can be active at lower temperatures than the wool carder bees.

Wednesday, 14 June 2017

The cost of long-horned moths long antennae?

As I walked past the university woodland area, in the usual clearing, a swarm of 7-8 Yellow-barred Long-horn moths, Nemophora degeerella in their bobbing flight. A few other males sat on the leaves facing the swarm. I searched for females to no avail, but something caught my eye, a male that had been caught in a spider web by his antennae (top shot), still alive, but kicking hopelessly. I have covered these moths in previous posts, the extraordinary antennae of their names only applies to males: the females have a much shorter antennae. This sexual dimorphism suggests that the male's antennae have evolved in response to sexual selection, possibly in relation to pheromone detection by males.
Two males rest on leaves (1/06/2017).

The evolution of exaggerated sexually-selected traits often is accompanied by costs and in the case of these moths, increased predation risk is a likely prize the males are paying for their oversized antennae.
A large swarm of long horned moths on 1/06/17.

Friday, 2 December 2016

Two Woolly Jumpers

There is a thriving population of the small jumping spider Pseudeuophrys lanigera in my office building at University, which, with its species name meaning 'woolly' I think it deserves the common name of woolly jumper. I spotted one on the windowsill as I was going to close the blinds, and then another one descended from a thread of silk and ended up near the first one. Both were males and, after some swaggering and lifting of the front legs, one of them, the newcomer, retreated and followed on his way. It is the females that truly look 'woolly'. Males are very colourful, with dark and large front legs, red markings around the eyes and white moustaches and palps. These spiders are found on and inside buildings. They must consume minute prey, such as bark lice, springtails or small flies, although jumping spiders are able to tackle prey their own size. The adults can be found year round, although their peak season is June for females and September for males.
The smaller spider number 1, who held his ground.
The newcomer.

Friday, 26 August 2016

A Walnut orb weaver

It's not every day I find a new species of spider in the garden, so I was pleased this afternoon, when opening the garden gate I disturbed a young Walnut orb weaver, Nuctenea umbratica (above). It crawled up and hid in a crack between the planks of wood. This is a large and distinctive species, the females can reach 14 mm.
umbratica means 'of the shadows' in latin, which refers to their liking for dark cracks and crevices, where they hide during the day, aided by their wide and flattened abdomen. When discovered, they are often crouching in a characteristic position, which is the only explanation I can see to another common name: 'toad spider'. They can be very dark, almost all black, but this individual shows the wavy leaf-shape pattern on the abdomen, surrounded by a pale rim and pinkish sides with annulated legs. They like dry microhabitats, including wooden structures, like fence posts, dead wood, window frames (e.g. in birdwatching hides), greenhouses, and cliffs. They are mostly nocturnal, they emerge as it gets dark and make their stout, large orb web, of a similar shape to that of its relative Araneus diadematus, although they can also sit on their web during the day. Their tattered webs can be useful to detect the spider when found in suitable habitats during the day.
 Males peak in July and August, while females are found year round. I wonder if being a very large spider, like the garden spider Araneus diadematus, females may take two years to reach maturity.
Although I might have overlooked it due to its retiring habits, I keep finding it more often in recent years.
Adult N. umbratica in a window frame of a hide. (11/7/16, Tophill Low)
One of several N. umbratica adult female inside bird hide (11/10/15) Alkborough Flats.
A fresh egg sac found inside the same hide with several N. umbratica. It looks very similar in colour, shape and size to an Araneus diadematus egg sac.
An adult female N. umbratica sitting on her web during a dark November day inside a greenhouse (Thwaite Botanical Gardens, 3/11/15).