Friday, 29 September 2017
Tackling Diptera families – where to start and how to progress
In Britain, the fly fauna now comprises around 7,100 species. The number has risen dramatically in the last twenty years, by around 350 species. It is currently the largest single component of our fauna but may eventually be surpassed by the Hymenoptera if the taxonomy of the Parasitica is ever resolved!
The sheer numbers give us a lot of headaches. Can one individual really tackle them all? If you cast around it is clear that even the most competent Dipterists tend to specialise.
The big question is how to get started and how to progress from there onwards? The trouble starts with finding a reliable key to families; we don't have a published key that is simple.
Life gets complicated because there are now two systems for naming the parts: for example the Comstock-Needham system of naming the wing veins, which has been around for a century or more, and McAlpine which is more recent. Older Royal Entomological Society keys use the former and later ones tend towards the latter. So, when you start with Diptera there is an added level of confusion with two completely different sets of anatomical names! We do make life difficult for ourselves!
Most Dipterists dislike the AIDGAP key by Unwin, and there is agreement amongst the leading specialists that it does not work in some places. The best readily available key to families is Pjotr Oosterbroek's key to European families of Diptera but it is not for the faint-hearted. There are many technical terms and unless you are familiar with, for example, the nomenclature of wing veins, it is often hard work switching back and forth from key to illustrations.
Stuart Ball and John Ismay have been working on a key that deals with the British fauna. It is now pretty well developed, but cannot be published because it relies very heavily of illustrations cribbed from other publications. Stuart is in the process of photographing all of the relevant features so something may emerge eventually.
Most people tend to start by noticing animals that are obvious. Leaf-baskers and flower visitors. They naturally gravitate towards families such as the Syrphidae for this reason. But it is not just Syrphids that visit flowers: lots of Calyperates do too, especially Tachinidae, Calliphoridae and Muscidae. Unfortunately, user-friendly modern keys are not available for all of these families and modern entomologists are somewhat spoiled by the keys to Syrphids. Stubbs & Falk does an amazing job of turning a family that was once considered too hard for all but the museum specialist into one that can be tackled with relative ease. [I do stress relative ease – parts of this family are far from straightforward!]
The big advantage of Syrphids is that once you have learned what they look like, you have also been introduced to quite a few non-Syrphids and a certain amount of comparative anatomy – not a hoverfly but what is it? If it has bristles then a lot of people are rapidly turned off because the keys rely on the relative positions of the bristles or bristle scars. Nevertheless, once you have mastered hoverflies you may want bigger challenges. Soldierflies and their allies sit comfortably with hoverflies so they too get tackled quite early on. Thereafter, it is a question of whether there are accessible keys but, perhaps more importantly, also whether you can actually use the key and have some confidence that the point you arrive at is reliable.
In my early days running ecological surveys I had some interesting discussions with colleagues who took the view that the end point they had reached was always right – whereas I felt that one should check further and remain cautious. If there is a species description, read it! If the description says that your species is confined to the far north of Scotland and you have recorded it in Dorset you are as likely as not wrong! We see an awful lot of duff data that can be eliminated quite quickly because the geography is wrong.
In my early days as a Dipterist, there was a lot of interest in hoverflies because Stubbs & Falk had just been published and even the seasoned 'experts' were breaking new ground. Over time, those people have moved on from hoverflies and into other families. The challenge is breaking new ground and finding species you've not seen before – it is a sort of 'collector' approach that is utterly understandable: unless you are interested in some form of data interpretation the greatest thrill is something new.
Today, many of those former hoverfly enthusiasts tackle other families and only take a passing interest in hovers. But, they do this as a progression: there is a moderately workable key to Dolichopodids and they are quite attractive flies; Male Empis and Rhamphomyia, with their exhuberant genitalia, are also interesting and Collin's key is very workable (in parts); there is a reliable key to Sciomyzidae and they are eminently doable; the Tephritidae are pretty doable (but I do have trouble in places), as are Conopids, Otitidae and Uliidae.
Thereafter one starts to enter a minefield. There are keys to Phorids, but I would not touch them (very few Dipterists will) – they require slide mounting, as do Psychodidae. Likewise very few people tackle Sciarids or Sphaeroceridae. More importantly, when starting with a new family you really don't have the markers that help you find your way around the key. If you have access to a reliably determined collection then you can get to grips with a new family; if not, how do you know whether you have got the right ID? Your fly is pink with blue spots, but all the key does is to tell you that it has two notopleural bristles and crossed post-ocellars! You are none-the-wiser!
Where the keys are sparsely illustrated and lack species descriptions potentially doable families are as yet under-attempted. The Tachinidae are a case in question. They are often big and seemingly obvious, but try using the key! Unfortunately there are a lot of single species genera that make the key a long procession that becomes very confusing. I've yet to master it but have this on my to-do list for the winter! I've been collecting Tachinids all year and now have three or four hundred to tackle. Hopefully that will get me further!
It is not really viable to tackle a key with just a single specimen or indeed a handful of specimens. Within moderately large families you just don't have the comparative material needed to understand what the key is talking about and even some small families can be a problem without relevant comparative material.
The system a lot of the more adventurous Dipterists use is to collect for a couple of seasons before attempting to get to grips with the key. When I do this, I attempt specimen one and see how I get on? If I hit a block then I put it to one side and try another, and another and another. Each time I familiarise myself with another facet of the key and start to see how it is constructed and how the writer has interpreted the morphology. Over time come a few small successes: those start to be the markers in the key – I know what that is and the next specimen is not it! We try to do the same when running training courses – make sure that critical parts of the key are embedded and the student has some simple markers to work from.
Crucially, this all takes time. Finding your way around the families, recognising features such as costal wing breaks and head chaetotaxy requires infinite patience and access to comparative material. So, the follow-up is a need to maintain a collection. That in turn becomes the limiting factor – store boxes and cabinets are expensive, take up a lot of space and require curation, so in the end you have to specialise!