Monday, 22 July 2013


It has been suggested (and rightly so) that I don't say very much about the techniques that I employ in order to get some of my shots. Yesterday I managed to get this rare shot of an Ichneumonid ovipositing into fly larvae hidden inside the Knapweed flowerhead:

Ichneumonid ovipositing

Ichneumonid close-up
These shots are very elusive, so it occurred to me that it might be useful to explain what went on before, during, and after the photography session.

Firstly, background information is required. Ichneumonids are parasitic wasps that lay their eggs into the bodies of the larvae of other insects: the most commonly targeted species are flies, moths, butterflies or sawflies. Timing is crucial: if the target larvae are too small, then the egg-laying process will be too difficult, but if the larvae develop too much they might already have pupated, or become too mobile, or even be able to defend themselves.  The timing is also often dictated by the foodplant of the target larva: if the plant isn't in the correct state then the female of the target species won't be able to lay her eggs. So we clearly have (at least) two timing dependencies. This means that the opportunity to take these photographs is severely limited - to perhaps a few days in each year. So in order to plan for this kind of photograph, we have to be aware of the potential timing of these events, and to monitor relevant food-plants at the appropriate time. I noticed that some females were investigating the Knapweed on Saturday, so I planned the photography session for the next day.

The female operates by flying from flower-head to flower-head, quickly testing each one with her antennae. If nothing is detected, she will quickly fly to a nearby flower-head and start to test again. The ovipositor is stored in a sheath that is held behind her, and if she detects a target, she will quickly unsheath the ovipositor and position herself so that she will insert it in the correct direction. This unsheathing and positioning takes less than a second. She then follows the unsheathing with a series of thrusts, each a little deeper, as she reaches the target larva. This thrusting might take two or three seconds: this is the opportunity for the shot. So we have to follow the female from flower to flower and take great care to observe when she appears to be taking a little longer than usual to inspect the flowerhead: this is a sign that she might be about to deploy. That's the clue to get close and be ready to focus. A fast-focussing lens is essential at this point, and I have my focussing system set to a single point. If you are focussing on a scatter system, the lens will have difficulty focussing on the insect and will get distracted by background leaves, stems or flowers. I use a Canon ultrasonic macro lens, which can focus in less than a second: essential for this kind of shot. A quick press of the shutter release when near the subject will prime the lens to be suitably-focussed: it is already nearly in the right position. As soon as she starts the injection, I am already in place with the camera primed and the lens in approximate focus. Then I take repeated shots, perhaps two shots in three seconds, so that I might get five or six opportunities from each injection sequence. Just occasionally, she will make more than one attempt in the same flowerhead - presumably to target further larvae: those are the best opportunities for a shot like this.

Most of the shots are bad: either out of focus or blurred due to hand movement, or obscured by stray, windblown leaves, or the female might have suddenly turned to get a better direction of injection. This photograph took 20 minutes to get, with probably 50 shots that went straight to the recycle folder when I was sorting the photographs later. Fortunately digital shots are cheap, so we can take as many as we like. Another reason for the long time is that the female might well decide that the local area has nothing left for her, and vanish over a hedge. This means that we have to wait for another female to come along. In addition, because we don't want to lose sight of the female, we can't pay too much attention to where our feet are going, so tripping is a frequent hazard, especially with Bramble runners nearby. Finally, when she's finished laying, she will quickly seek another flowerhead. Since we are down and close to the previous flower, we need to quickly refocus our attention to follow her movement. These can all add up to missed opportunities for the shot.

I think I might have achieved perhaps half a dozen good opportunities for a decent shot in those 20 minutes.

One final tip enables us to get that elusive shot: I call it plant-twisting. When the injection is taking place, the female is so focussed on the process that you can hold the plant stem and rotate it for the best angle for the shot without alarming her and scaring her off. (She actually couldn't fly away if she wanted to: she is firmly attached to the plant). That gives us the opportunity to rotate the plant, not only to achieve the best angle, but also to select the best background for the shot. It is no coincidence that the yellow flower at the rear of the shot highlights the ovipositor: I selected that background as I held and rotated the stem.

So all-in-all a very tricky process to get that elusive shot.

It's worth noting here that there are many hundreds - probably thousands - of species of Ichneumonid in the country, each with a unique or severely restricted range of target species. Many of the target species live in places that simply can't be photographed, but there are a few species that use prominent and available plants: these are the ones that present us with photographic opportunities.

To complete the story, the wasp's egg will hatch out, and the wasp larva will eat the host larva. The wasp larva will then pupate and hatch at the appropriate time next year (more synchronisation!). The requirement to fool the host's rejection mechanism ( the egg has to resist rejection when inside the target's body) explains why most parasitic wasps have such a small range of target species: it would be too difficult to trick all the rejection mechanisms. This is analogous to the restricted range of host plants for leaf-miners.


Gerry Snape said...

brilliant info...I tried to catch an emergent dragonfly today...large wing span and bright burnt orange in colour......failure ...too fast for me and my panasonic DMC-FZ45 sadly.

Gill said...

Fantastic page - thanks Stuart (and a wonderful shot of the wasp of course).