These are general tips on how to approach the keys and good practice methodology and preparation for keying a tachinid:
- Using a graticule (an addition to the eye-piece that superimposes a grid on the image) will help you measure relative distances. This is important when judging such things as the relative height of the eye or the width of the vertex.
- Go through the keys and underline important words – it is very easy to read a couplet, understand the choices but then pick the wrong answer. I underline words like “has” or “has not” and “with” or “without”. It is easy to do and can save errors.
- Try to set male specimens with their genital capsule well extended – this can help later if genitalia are important to the identification. If your specimen has not been set well enough it can always be relaxed at a later date and reset – or the genital capsule can be removed, boiled to soften it and preserved in glycerol.
- If possible ensure that the head and front legs don’t obscure to area on the underside of the thorax called theprosternum. This isn’t used in Belshaw (thankfully!) but is used in most other keys – and is very important! The prosternum is a small disc (or shield) of chitin just in front of the fore coxae – under the fly’s ’chin’! On some species this is setose or slightly ’hairy’, while on others it is bare.
- I pin specimens laying on their sides – head facing left and wings usually held dorsally. I use a fine 15mm micro-pin and stage the specimens using a short plastazote or nu-poly strip. This isn’t a ’pretty’ way of doing things but it does enable you to see most identification features easily, it takes up minimal space and protects the specimen from damage. If you set flies in the classic way (’dorsal uppermost, wings at 90° to the side’) you take up more space and risk obscuring some of the important lateral (side) and underside features.
- Rubbed or missing bristles can cause problems but remember that you should still be able to see the socket where the bristle joined the exoskeleton – hairs do not have an obvious socket.
- Species with protruding mouths are usually very obvious – there are only a few borderline cases. Try viewing the head from below – it makes seeing the edge of the mouth and vibrissae much easier. Also, it helps to have a specimen of Eriothrix rufomaculata (a very common mid-summer species) to hand so that you can remind yourself what the feature looks like. I have also written a mini-article on how to judge the mouth edge - here.
- Scutellar bristles figure heavily in most keys and sometimes it can be difficult to work out whether they are parallel or crossed. My rule of thumb is that ’crossed’ bristles also means those that are obviously curved towards each other – parallel or diverging bristles are either straight or curved outwards. If it doubt I tru crossed first – most specimens you will come across have crossed bristles.
- When discussing hairs on the parafacial area Belshaw’s keys are refering to the area of face above the vibrissae but below the lowest parafrontal bristle. This definition is tucked away in the figures and is easily overlooked.
- When checking bristles (especially dorsocentrals) always make sure you check both sides – you occasionally get aberrants with asymmetrical arrangements of bristles.
- When checking for the presence of hairs on the eyes or aristae it is often best to view against a dark background. Also, when checking for eye hairs try looking just in from the edges of the eye and the area of eye above the gena (jowls) – this is where hairs often persist longest on old or tatty specimens. See also my mini-article on this subject - here.