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The term "ichneumon wasp" is used rather loosely to refer to parasitic wasps in the family Ichneumonidae. There is a genus called "Ichneumon" but it's not easy to distinguish from related genera, so usually if someone says "ichneumon wasp" they are talking about the family as a whole.
Identification of ichneumons is a real challenge. As far as I'm aware there are no introductory publications for ichneumon identification. The family is enormous (c. 3,000 species in the UK) and often requires microscopic examination to make much progress with identifications.
Some of the insect field guides such as the Collins ones are good for getting you to somewhere near the correct species, but inevitably they can only illustrate a small proportion of the total number of UK species, and with ichneumons this approach rarely works. The large brightly coloured ones always look as if they should be identifiable, and maybe some of them are if you know enough about them, but there is precious little published on them and it's hard to know what the range of variation in appearance is for any one species.
You can download Gavin Broad's checklist of the British species of Ichneumonidae from the Biological Records Centre, but it runs to 162 pages (just for the list of names, no pictures!). He also provides a (fairly complicated) key to the subfamilies. Both these can be downloaded from:
(scroll to the bottom of the page under the heading "Taxonomy, phylogenetics and phylogeography")
There's also some useful information on the Yorkshire Naturalists' Union website:
A few ichneumons can be recognised from photos if you know exactly what to look for (which I don't!), but for the majority of these often attractive insects all we're likely to be able to say from a photo is that it is 'some sort of ichneumon wasp'.
I asked Dr Gavin Broad at the Natural History Museum (author of the checklist and key mentioned above) for his comments on ichneumon identification, and here is his reply:
"There are some ichneumons that can be readily identified from photos, such as Amblyteles armatorius, or Ichneumon suspiciosus. Ophion obscuratus is a wasp often photographed and easily recognised (its main confusion species is very rare and can almost always be readily discounted). Some of the larger Tryphoninae can be fairly easily identified and many of the larger Ichneumoninae can be recognised on colour pattern, if one is familiar with the species [that's quite a big 'if'!].
"It is also worth noting that Chinery's photographic guide has a page of ichneumonids entirely incorrectly named.
"I find that usually I can only give the subfamily with any confidence, although fairly frequently I can give a good guess as to genus. Families should always be possible. The main problem is that photos of live ichneumonids very rarely show the features I would like them to show and can be positively misleading if reflectance looks like a white patch.
"Colour patterns vary to an extent, in that larger individuals tend to have more extensive patches of colour (although in some smaller ichneumonids, smaller individuals tend to be more extensively reddish). However, distinctive colour patterns are usually reliable and there will be no need to check morphology in such cases as Ophion suspiciosus, Opheltes glaucopterus etc. An awful lot of ichneumonids in completely different subfamilies have basically black bodies and medially red abdomens. Also, there are a fair few 'Ophion-like' genera which superficially look similar. Almost all photos of 'Ophion luteus' and 'Netelia testacea' on the web are completely wrong.
"I don't know of any good web resources for photos of parasitoids. I am working on an idea at the moment for photographing British ichneumonids but I must stress that in the vast majority of cases it will be impossible to identify the species of ichneumonid from a photograph of it alive. A web resource would, I think, be most useful for getting an approximate identification of a specimen. There are some keys to North American genera of some small ichneumonid subfamilies and lots of useful information about ichneumonids at http://www.amentinst.org/GIN/, but the keys will be of limited use in the UK. I have checklists on my webpage (http://www.nhm.ac.uk/research-curation/staff-directory/entomology/g-broa...), which at least point to the literature.
"The Royal Entomological Society keys (http://www.royensoc.co.uk/publications/index.htm) are definitely worth using; the keys to species of Ichneumoninae, Pimplinae and Anomaloninae are fairly good (if sometimes difficult to use), but allowance needs to be made for some additions and the inevitable name changes."