The thing with sharing one’s enthusiasm about the natural world, is that those profound “Wow” moments are often all-too brief and lose something of their impact through interpretation. The mental picture and the associated story may well be vivid and compelling, but as we have all become so accustomed to the privileged insights of programmes such as BBC1’s Life, we need a bit more to really whet our appetite.
The great thing about digital photography is that is makes capturing those precious moments more convenient. Not easier, just a bit more expedient. I continue to receive images from local photographers and they stitch a tapestry of incredible natural wonder in my email inbox.
A recent stunner once again came from the lens of Peter Vernon, who captured this image of a helpless small white butterfly pupae being attacked by an ichneumon wasp. A moment such as this reveals a grizzly and utterly compelling aspect of natural history and such a vivid image helps bring it into sharp resolution.
Ichneumon wasps (pronounced ick-new-mun) are members of the largest insect group the Hymenoptera. Meaning “membrane-wing” the Hymenoptera include bees, wasps and ants and is a group I am very fond of. Ichneumon wasps are typified as parasites, and often possess a gruesome looking appendage which is often misconstrued as a “stinger”. The long, thin needle-like structure is in fact an ovipositor, which is how the female wasp injects her eggs into a host. The term host conjures up images of conviviality and hospitality, in this instance a parasite’s host is a lot less reciprocal in nature.
Parasitic wasps with the longest ovipositors use them for injecting an egg into a grub inside a rotting log and therefore have to be very long and very mobile to negotiate their way towards the host. How she knows there is a little white larvae within the rotting log in the first place, I’ll leave to another article!
Back to our photo. The shot shows the female injecting a pupa, what looks like a seventh leg there is actually the ovipositor being manoeuvred directly down, positing the ova, or indeed ovum. As it is very difficult to tell categorically which wasp this is caught in the act, it is similarly tricky to say with any degree of accuracy exactly what ecology is being enacted, is one big egg being laid, or a collection of smaller ones? Suffice to say that what happens next is anything but pleasant.
The egg or eggs will sit in the soupy gunk inside the pupa as it overwinters. This ingenious use of a host to get your eggs through the worst the British winter possibly hints towards the evolutionary origins of the behaviour? The caterpillar continues to metamorphose and when spring comes and it’s time to emerge, the wasp’s grub leaps into action. Firstly it consumes its fill of semi-butterfly, metamorphoses within the pupa and emerges by eating its way out.
If the butterfly was free of the parasite and continued with its development unchecked, the pupa would split along a line of weakness at the front of the case when the butterfly emerged; a parasitised pupa will look discoloured and have a more clumsy escape route; anything from a small hole in the shell, to the entire head end being chewed off!
So next spring, keep an eye out for empty or emptying butterfly pupae and perhaps you will be lucky enough to see the results of this fascinating natural history.