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.

Late bumblebee collecting pollen

It is not unusual to see bumblebees in Autumn. Queens are still about, storing up some reserves before hibernation. What I found unusual is a worker of a Bombus pascuorum collecting pollen in my patch of Lamium maculatum today. The bumblebee visited each flower inserted its head deep into it and as exiting, wiped its furry back with its rear legs collecting the pollen on its corbicula or pollen basket. I understand that bumblebees collect pollen for their larvae, so there must be still be B. pascuorum active nests this late in the year.

Tree bees. 1. Holly.

These days I am looking up trying to find bees in trees. Insect pollinated trees can attract large numbers of bees, as they offer a highly concentrated resource: hundreds or thousands of flowers are present in the same spot. The drawback for the bee watcher is that, unless you carry binoculars, identification is not easy. There are two holly trees in my garden and I had noticed patrolling Red mining bees and feeding Bombus hypnorum in them before, but I could not take good shots. Today I found a holly hedge in bloom and I could get close to the bees that pollinate this tree. Holly flowers are small and inconspicuous and I have rarely noticed bees visiting them, but this little bush had many bees in it. Hollies are dioecious, which means that there are male and female trees, self-pollination is not possible and fertilisation requires insects visiting first a male tree and then a female tree. I am not sure if male holly flowers produce nectar as well as pollen. On the photo above a male Red Mining bee sits on the flowers of a male holly, its antennae covered on pollen.
In their Holly monograph, G. F. Peterken and P. S. Lloyd stated:

Entomophilous. Apis mellifera L. is the commonest insect visitor, but the following bees have been observed in southern England (O. W. Richards): Andrena wilkella Kby. (Andrenidae), Osmia rufa L. (Megachilidae) and Bombus lucorum L. (Apidae). B. lucorum and syrphid flies have been seen at the flowers in northern England. Nectar is secreted from tissue at the base of the ovary.

A honeybee visiting the flowers.

An Red Mining bee at the left of the flowers with scopa full of yellow holly pollen

Reference:

Peterken, G., & Lloyd, P. (1967). Ilex Aquifolium L. The Journal of Ecology, 55 (3) DOI: 10.2307/2258429

Pollen gathering face brushes

Most non-parasitic female bees collect pollen as well as nectar to provision their brood cells. Pollen is brushed from the plant anthers using their front legs and stored on specialised structures on the bee body or legs. Some bees, however, specialize on collecting pollen from flowers with raised anthers, which touch over the bee's head or thorax when bees land on them. These are called nototribic flowers and include species from the Lamiaceae (the mint family) and Scrophulariaceae (the figwort family). Although bee-pollinated plants benefit from bees taking nectar - exchanging nectar for inadvertent pollination - they do not benefit when potential pollinators efficiently gather the pollen for their offspring consumptions instead. Nototribic plants in response to specialised pollen gathering by bees, have flowers that make pollen hard to collect, even when they have plentiful nectar. Only bees equipped with either specialised behaviour or morphological modifications, or both can effectively make use of their pollen.

 Mueller found that only 13 species of Central European bees (including Anthophora furcata, Anthidium manicatum and Osmia caerulescens) possess specialised structures to brush and collect pollen from nototribic flowers. In several of these species the specialisation is on the face of the bee and consists on a think covering of peculiarly shaped hairs, of wide bases and sharply bent ends. These specialised hairs seem to have evolved independently several times as the bees belong to different genera also containing species lacking such modified structures.

A photo of Anthophora furcata clypeus (top lip of the bee) and on the right a portion of it showing the specialised hairs (from Mueller 1996).

One of such bees, Anthophora furcata, the Fork-Tailed Flower Bee is a summer bee, active in June and July in the British Isles. These are long-tongued, relatively specialised species, which collects pollen mostly of Lamiaceae (and to a minor extent of Scrophulariaceae) flowers. They are often present in gardens where they feeds on Lavender, Tree Germander and Lamb's ears (Stachys byzantina) as well as foxgloves. A couple of males I spotted yesterday, the first of the year, were feeding on a patch of Stachys sylvatica, and the photo of one of them on top of the post shows the position of the top lip of the flower - which hides the stamens and pistil - touching the bee's head. 

Mueller also investigated the brood cell contents and pollen load of bees to ascertain the degree of specialisation of the bees and also looked into the actual bee's behaviours to gather the pollen to account for the position of the facial hairs. In the case of A. furcata:

[it] presses its head against the anthers and, by vibrating its indirect flight muscles, buzzes the pollen amongst the transformed hairs. No further head movements can be observed.

The 'buzzing' releases the pollen from the anthers, and the specialised hairs help retain the pollen grains on the face. The bee then removes the pollen grains adhering to the specialized pilosity by rapid stroking movements of the forelegs in flight after leaving the flower.

Female of A. furcata feeding on Lambs ears.

Although the bee behaviour could be thought as a form of 'pollen robbing', pollination can still take place while nectar sucking, as the pistil of the flower still touches the bee's thorax, making no contact with the specialised brush on the bee's face and the flower constancy of the bees makes it more likely that she will be carrying some of the same species pollen on her back.

More information
Muller, A. (1996). Convergent evolution of morphological specializations in Central European bee and honey wasp species as an adaptation to the uptake of pollen from nototribic flowers (Hymenoptera, Apoidea and Masaridae) Biological Journal of the Linnean Society, 57 (3), 235-252 DOI: 10.1111/j.1095-8312.1996.tb00311.x