Now for something really tricky...

Over recent months I've been posting occasional forays into the world of taxonomic morphology i.e. how an understanding of structure (mainly in invertebrates) allows us to identify them. So far all my postings have covered species I'm pretty confident about - however I thought this was the time and place to have a go, sort of publicly - at something where I'm definitely not so sure - the Ichneumonidae.

As a group, the ichneumons are fairly familar - elongate parasitic wasp-like insects, often black with a bit of red, orange or yellow, plus the long spike at the back of females' abdomens - the ovipositor for laying eggs which is long in those species that lay eggs in host larvae hidden deep inside wood or similar. However, their broad familiarity masks the difficulty in identifying them - in Britain alone there are around 3,200 species and few are readily identifiable by single characteristics, even at subfamily level, let alone genus or species (Broad 2011). Instead, varying combinations of features are often needed, plus there is a lack of readily available literature and some of what has been published can be difficult to use (e.g. Perkins 1959; Townes 1969), though some more recent works are a great improvement if not perfect (e.g. Wahl 1993). Here I will be relying on Broad (2011). This doesn't give an overview of the terminology and anatomy used with ichneumons, but useful information can be found in Fitton et al. (1988) which covers a single subfamily, the Pimplinae but gives the names of wing veins and other body parts.Anyhow, enough background, so what have I found in my garden this time?

A female ichneumon, length 7mm (not including antennae and ovipositor)

So, I have a female ichneumon - black with orange legs and a fairly long ovipositor. So, time to consult Broad (2011) to see if I can at least determine the subfamily. The starting point is easy enough - the wings are not reduced, and vein 2m-cu is present.

Forewing with vein 2m-cu indicated.

The top of the thorax is shiny and unwrinkled and the spiracle (air-hole) of the 1st metasomal tergite (the 1st abdominal segment) is found about halfway along it, with the hardened part of this segment extending more than halfway along it. Some of this can be tricky to see, so experiment with lighting and when you get it right, the spiracle shows up as a tiny but sharp-edged round hole...

1st metasomal tergite with spiracle marked. The hardening is not that clear in side view, but is present along the top (there is a tiny ridge just visible here which is the edge of this feature). The underside of this and the next segment is membranous - a feature of the Ichneumonidae.

This specimen is female but does not have eggs conspicuously hanging from the ovipositor, and the antennae have numerous (more than 16) segments. The next bit is trickier however - the hind and mid tibiae of the legs have spurs and these need to be counted.

Mid and hind tibiae with spurs indicated.

At first glance this looks straightforward - there's one spur on each tibia. However, following this route through the key soon arrived at couplets that simply didn't work - I'd clearly gone the wrong way. So, I went back to this point and looked again. When I did so, by rotating the specimen and looking more carefully, I saw that there were in fact two spurs on each tibia - in the photo the first hides the second in each case because of the angle of view when the specimen is on its back and the legs are tucked in (which is often the case). I should know better, but this is a good example of a feature that really needs care...

Moving on, I looked at the mesopleuron (the side of the large thoracic segment) and could see no sternaulus (a groove running across it on the lower half). From here, the large ovipositor precluded the possibility of this being in the genus Euceros (subfamiliy Eucerotinae) as this has a tiny ovipositor, while the areolet (the tiny cell in the wing, near the top of vein 2m-cu) is triangular.Here another tricky point is reached. Are the ovipositor sheaths stiff or 'flexible-looking'?

The bristly ovipositor sheaths.

Side (and upside-down!) view of the sharply pointed ovipositor plus sheaths.

To me they look stiff and this would indicate the genus Cidaphus - however, Cidaphus does not look like this, being orange with a shorter ovipositor as far as I am aware. So, moving on again, the next couplet requires a close look at the face.

The ichneumon's face. Please ignore the bits of household fluff/dust...

The key here asks whether the clypeus is separated from the rest of the face by a groove. Personally I find this hard to see, even trying different lighting under the microscope, but having learned from the tibial spurs, I did try again...

Another view of the face, this time with different lighting and removal of sone of the fluff...

There is a groove here (running across a little above the lower edges of the eyes), and although the face itself is somewhat convex, it is not smoothly bulging as suggested by the key; rather it is slightly angular. Taking this route, there is no triangular pattern of grooves on top of the abdominal segments, while the hardened part of the 1st metasomal tergite does not extend far past the spiracle (at least not past about three-quarters back); also this segment broadens towards the rear.

An abdominal segment - rather nice puncturation, but no triangular groove pattern.

From here, the hindwing venation and the presence of an 'epicnemial carina' (a narrow ridge along the edge of the mesopleuron) return us to the ovipositor which is clearly not strongly down-curved. The mandibles are not twisted as far as I can tell and the ovipositor has small teeth on the underside at the tip. Also, vein 2m-cu has two bullae (tiny faint patches) although these are very faint in the photo above. And so, if all the above is correct, this is a member of the subfamily Pimplinae.

This is handy as it falls within the remit of Fitton et al. (1988). I won't go through the same process here, but the key takes me towards the genus Scambus (my initial thought was Pimpla but the ovipositor is too long) via various features such as an ovipositor 3-4 times the length of the hind tibia (this ration is important in pimplines). Continuing to the species level, features such as colour become important (such the lack of yellow markings on the pronotum, plus orange-red coxae) and the specimen readily keys out as S. buolianae. This is reasonably common in England and, unusually for a pimpline, has a wide range of hosts, including some present in or near our garden.

So, to conclude, a tricky identification, but one that is possible with care (and a willingness to back-track occasionally) and the right literature - and the result is a record for the local biological records centre covering a group that is likely to be under-recorded.


References

Broad, G. (2011). Identification Key to the Subfamilies of Ichneumonidae (Hymenoptera). Test key available for download here. [accessed 05/08/2011]

Fitton, M.G., Shaw, M.R. & Gauld, I.D. (1988). Pimpline Ichneumon-flies. Hymenoptera, Ichneumonidae (Pimplinae). Handbooks for the Identification of British Insects 7(1): 1-110.

Perkins, J.F. (1959). Hymenoptera. Ichneumonoidea. Ichneumonidae, Key to Subfamilies and Ichneumoninae - 1. Handbooks for the Identification of British Insects 7(2ai): 1-116.

Townes, H.K. (1969). The genera of Ichneumonidae, Part 1. Memoirs of the American Entomological Institute 11: 1-300.

Wahl, D. B. 1993. Key to subfamilies of Holarctic and Neotropical Ichneumonidae. Pp. 396-509 in: Goulet, H. and J. T. Huber, eds. Hymenoptera of the world: An identification guide to families. Agriculture Canada, Ottawa.