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Benoit

Presutural ac is me being lazy. The expression in full is pre-sutural acrostichal setae. Setae are bristles, and acrostichal setae are those set in a double row longitudinally down the dorsal centre of the thorax (not to be confused with the dorso-central bristles, which sit to the left and right of the acrostichals). Calyptrate flies have a 'suture' or indentation which runs across the dorsal thorax from side to side - clearly visible in your photo. Pre sutural bristles sit in front of this suture ie between the suture and the head, and post sutural bristles sit behind the suture ie between the suture and the scutellum. Sometimes you will see the bristles expressed as eg 2+3 ac. This means there are 2 pre and 3 post sutural pairs of acrostichal bristles. In the case of your fly it would be 0+1 ac.

If you look at your fly and Jan's first fly in his thread, they do not appear to have any pre sutural ac, but if you go to the gallery and look at Nikita's pics of D albofasciata and E cyanicolor you will see that the D albofasciata has (looks like) 2+1 ac and E cyanicolor has 0+1 ac.