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.