Insects in flight

Today it was a summery, hot day, with a light breeze, ideal for insect watching. Somehow, I managed a few decent in flight shots, which usually evade me as I am not patient enough. The first one happened when I spotted two hovering Volucella pellucens. They are large hoverflies, which live near trees and are identifiable even at several meters of distance due to their translucent abdominal belt. Males hover incessantly on the same spot, and inspect or chase off other hoverflies and even speckled woods invading their territory, so maybe this way they spot female hoverflies to mate. Hoverflies also hover during courtship. I tried to encourage them to sit on my finger by slowly raising it towards the hoverfly. This technique often works with hovering males, like this Eristalis intricarius from yesterday shows:

but the Volucella refused several times...

So I tried to get some shots while it hovered and got the shot at the top of the post, and I was pretty pleased with that!
 A bit later, then, in my street, a couple of migrant hawkers were hunting over the verges. Migrant hawkers often hunt together with other individuals, and they may settle to bask near each other. They tend to hunt from 4-5 m above ground. I managed some records shots, this one the best.

I popped in the wildlife garden later to release a grasshopper (a matter for a different post) and watched a large Ectemnius digger wasp (probably the common E. cavifrons) inspecting a rotten log. She had to fend off a Tegenaria spider. Ectemnius are hoverfly hunters and are skilful hovers. Females dig their nests in soft wood, and look by suitable nest sites, often hovering in front of the wood. This allowed me an opportunity for the third flight shot for the day.

Drama under the barbeque

This morning, while watching the hoverflies feeding on the fennel I realised I wasn't the only one interested in them. A female digger wasp Ectemnius, was poised on a cherry leaf, attentively following the action, and at one point the spotted a suitable hoverfly victim and attempted a capture.
 In the afternoon I came across Ectemnius again. Another female inspecting potential nest holes in a bee hotel, going in and out of them.

 I decided to sit and watch the action, faintly hoping the wasp would start digging a nest. I noticed the characteristic carded silk threads of Amaurobius spiders (top shot), and thought that spiders and digger wasps must come across each other with some frequency as female wasps inspect nesting sites. Then the wasp moved to the log pile under the BBQ and walked in and out amongst the logs. She appeared to have walked onto a silk thread and a false widow spider, Steatoda bipunctata, promptly came out of her refuge. A fight ensued, the wasp trying to bite or sting the spider, the spider spreading silk on the wasp and retreating, and repeating the procedure as the wasp became more and more entangled and buzzed intermitently in her futile attempts to get herself free.

 
The commotion got an Amaurobius out of her retreat, and at some point both spiders were attacking the wasp, although the Steatoda managed to secure it and, when the wasp stopped fighting, the spider cut free the threads that attached the prey to the log, and pull her to the safety of the retreat.

Early stages of the fight

The Amaurobius' legs are visible behind the abdomen of the wasp in the shot above, while the Steatoda secures the jaws of the wasp with more threads of silk

The wasp front end is now tightly wrapped on silk, now the spider focuses on the rear end

The spider drags the wasp to her retreat

 I watched all this with amazement and struggled to get decent photos as the action happened almost behind the log. The whole sequence of events happened within 15 minutes!

Chewing me timbers!

From June to October, I find Ectemnius digger wasps in the garden. They are formidable hoverfly hunters and, close up, they look so fierce: all enormous, forward facing eyes and long, sharp jaws. Females bore nests in soft, dead wood using their jaws: a central burrow with lateral cells which they stock with flies. They will lay eggs on the flies and leave their nests for their larvae to feed on the flies when they hatch. They use dead branches on trees, laying wood and even visit bee hotels. Males are often found inspecting holes in dead wood, in search of emerging females.

Year after year, these wasps come back to my log pile under the barbecue. A few days back, I watched a female searching for suitable nest sites amongst the logs. She settled at the end of a lime log and started chewing the wood, producing abundant sawdust in the process (top photo). She carried on for a while, although at the end I think she settled for a different log, as today a large pile of sawdust had accumulated underneath (below).

For more posts on Ectemnius digger wasps click here.

The hoverfly nightmare

Yesterday morning I found this Ectemnius wasp on a plant pot. Usually, they are very active and are flighty, but this one was still cold and let me take some close ups. Ectemnius are digger wasps that hunt flies - including hoverflies - for their larvae to feed. They dig a tunnel in wood and stock cells with flies, laying their eggs in them. They are several similar species, that can only be distinguished under the microscope. The microscope, however, is not needed to appreciate two of the weapons that these wasps use for hunting: their amazing, large, envolving eyes - note the antennae nicely folded between them, not occluding their field of vision, and their fearsome jaws, folded under their head.

Face off

Ectemnius wasps are skilled fly hunters. Not only they hunt flies, but some species specialise in hoverflies, those masters of controlled flight. Like their prey, Ectemnius wasps are able to hover. As their large eyes suggest, they hunt visually and inspect flower patches that hoverflies frequent, hovering a bit, changing body direction, inspecting their terrain thoroughly. Once they detect a hoverfly, they try to approach from behind, and when at about 10 cm they aim and attack. Ectemnius provisions its nest - dug in dead wood - with hoverflies, which will serve as food for its larvae. The adults themselves feed on nectar. I was trying to photograph this digger wasp stalking hover flies, when it came across a male Syritta pipiens. The tiny hoverfly male, which often confronts other hoverfly males on this patch of wild rocket, instead of fleeing, it confronted the wasp, mirroring the predator movements and keeping his distance, both insects hovering perfectly still in front of each other for a few moments. Surprisingly, the wasp did not attack the fly and just moved on. Some insects have been shown to display a stealth strategy called "motion camouflage" by which an individual (the shadower) can conceal its movements to another individual (the shadowee) by maintaning its position in the retina of the shadowee. The shadower then appears as an stationary object from the point of vision of the shadowee. Some male hoverflies (and Syritta pipiens in particular) use this flying strategy to track females undetected and dragonflies use it to avoid being detected when intruding in other territories and it is also suspected to be used by visual insects when approaching prey. A range of models have been developed to simulate this strategy. Unsurprisingly, engineers are developing motion camouflage strategies to be applied to robots, satellites or with military purposes.
 According to Justh & Krishnaprasad:

Motion camouflage can be used by a predator to stealthily pursue the prey, but a motion-camouflage strategy can also be used by the prey to evade a predator. The only difference between the strategy of the predator and the strategy of the evader is that the predator seeks to approach the prey while maintaining motion camouflage, whereas the evader seeks to move away from the predator while maintaining motion camouflage.

I wonder if this was the case in the above photo. A temporary stale mate, but the prey got away.

References
Justh, E., & Krishnaprasad, P. (2006). Steering laws for motion camouflage Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 462 (2076), 3629-3643 DOI: 10.1098/rspa.2006.1742

Mizutani, A., Chahl, J., & Srinivasan, M. (2003). Insect behaviour: Motion camouflage in dragonflies. Nature, 423 (6940), 604-604 DOI: 10.1038/423604a

The hoverfly hunting machine: watch digger wasps from your garden bench

In 2003, quite a warm summer, an interesting wasp kept watch of the cloud of hoverflies (mainly Episyrphus balteatus) on the nearby flowering Hebe bush from my garden bench. It was there every day and I called it 'Benchy'. A browse in the Chinery guide - the 'Bible' for insects in the UK - showed it was a wasp of the family Sphecidae - a digger wasp. Digger wasps are hunters specialising on various insects, which they paralise and stock on nests to feed their larvae. They excavate their nests, often on the ground, and thats why they are called digger wasps. Wild About Britain provided the ID for the genus, Ectemnius, and even a few guesses for the species ID: E. cavifrons, E. cephalotes and E. sexcinctus, with the first two being the most common. The wasp has returned year after year and I have managed to assemble a collection of photos illustrating a few aspects of the natural history of this fascinating animal.The wasp is yellow and black, with a big head and very large eyes, which make a large area of the head - a very different feel from a common wasp. It is indeed a highly visual animal and it nervously scans around in search for its favourite prey. Oher species of the genus like to hunt other types of flies but Ectemnius cavifrons favours hoverflies.

 When a potential prey is detected, the wasp hovers like its prey, staying stationary at around 10-20 cm from the fly. You need to be quick to manage to focus it, as the wasp is wary of anything large approaching it. The prey is then attacked swiftly, and if the capture is successful, the fly is taken to the nest.

I managed to shoot a hovering Ectemnius wasp, unfortunately a blue plastic car ended up being the background, doh!
For a few years 'the nest' seemed to be located far away from my garden as the wasp disappeared into the distance carrying its booty. But last summer I got lucky and I managed to locate a nest in a dry log in the pile of wood under the BBQ.

Ectemnius wasp about to leave its nest hole.