It doesn't take long to learn to seperate tachinids from other calyptrate flies by the characteristic bulging subscutellum. However, there is a small family of flies called the Rhinophoridae, which also have a subscutellum and these will really confuse you if you can't spot them amongst the tachinids.

Some species of rhinophorid are fairly common and you are bound to find them - even in your garden - where they are also parasitoids of invertebrates (mainly woodlice). Rhinophorids behave very like tachinids - they scurry around on low vegetation - and they are quite bristly, like tachinids.

Although rhinophorids have a subscutellum their subscutellum is usually reduced in size with the upper side partially membranous. However, this isn't easy to see so I have taken some photos down a microscope to illustrate the point - first the 'real thing', 2 tachinid subscutellums:

(above) Exorista rustica (male, subscutellum) looking from behind and slightly above.

(above) Phasia hemiptera (male, subscutellum) looking from behind and slightly above.

Now the tricky ones - the rhinophorids. Note the subscutellum is less evenly curved and often shows a well demarked membranous area on the upper surface:

(above) Two images of Paykullia maculata (male, subscutellum - lateral view) - note the light-brown membrane that joins the darker chintinous part to the underside of the scutellum.

(above) Two images of Rhinophora lepida (male, subscutellum - lateral view). This species is much smaller and consequently it is more difficult to see the light-brown membrane. But you can see that in profile the subscutellum is angled and not curved smoothly.

(right) Lastly, the common rhinophorids have a large petiole on the wing and an angled vein-M - as on this picture of Paykullia maculata. This species also has very distinctive shading along the wing veins, which makes it quite easy to identify.

If the cell is petiolate but vein-M is curved then consider that you might have a Phasia sp.