Speckled Wood feeding on ripe plum

When I took this photo I didn't realise that this Speckled Wood had landed by one of the few over ripe plums left on the tree. This butterfly is rarely seen nectaring on flowers, they feed instead on honeydew, but they appear to be partial to the sweet juice of plums.

Here be dragons!

This year I'm taking a special interest on dragonflies. They are a relatively small group, species are easy to identify in the field and they offer a range behaviours relatively easy to watch. In July I atended an FSC course on identification of aquatic macroinvertebrates and I immediately wanted to learn to identify dragonfly and damselfly larvae. A few days after the course I visited my local wildlife garden and went to the pond to check invertebrates. Something on the leaves of an aquatic plant called my attention. It was a dragonfly exuvia, the moulted skin of a nymph as the adult dragonfly emerges! 

My first dragonfly exuviae.

Dragonflies have incomplete metamorphosis, which means they lack a pupal stage. The dragonfly eggs hatch into nymphs which grow though several stages as aquatic predators. Nymphs lack functional wings or reproductive organs. The final nymphal stage will creep out of water and moult into an adult. There wasn't just a single exuvia though, there appeared to have been a mass emergence of Common Darter dragonflies and I counted over 50 of them. 

Several visible exuviae on iris leaves (13/07/17).

These are all the the exuviae I could reach from one end of the pond.

A close up of one of the exuvia looking quite intact. The adult emerges though the thorax.

The following week I had the chance to pop in the wildlife garden one morning. I checked the leaves for more exuviae and I was surprised to find two emerging dragonflies!

A Common Darter just emerging, head down, showing how the old nymph skin holds it to the leaf. On the top shot, one already drying its wings, folded back and still lacking pigmentation (both photos on 21/07/17).

This fresh adult male Common Darter, perhaps from the first emerging batch, was sunbathing by the pond.

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.

Red Admiral's admirable camouflage

A Red Admiral flew into the cherry tree as I was in the garden. I had seen where it had landed and how it positioned itself head down and as I looked for it I realised how wonderfully camouflaged, matching the colours and patterning of a branch. Can you spot it? If you can't I have cropped a photo at as slightly different angle, showing the antennae. I then made a sudden movement and the butterfly slightly opened its wings, exposing the red marks on its forewings (bottom photo). It had started to rain, and it was much cooler so the butterfly will probably spend the night roosting like that in the tree, well concealed from visual predators.

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.

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.

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.