Dorsocentral Area – the major setae, dc & acr
The dorsocentral area of the scutum can be defined by a forward bounding from the scutellum base (Fig. 1b, blue area) — the major dorsocentral area setae are taxonomically quite important in the Acalyptratae and Calyptratae. As a generalisation, the dorsocentral setae pairs (dorsocentrals, dc) are positioned just inside. The acrostichal setae pairs (acrostichals, acr) are formed down the center (Fig. 1).
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The numbers of both dc and acr are often presented as a formula of presutural pairs plus postsutural pairs (Fig. 1c) — note the transverse suture is usually medially complete in Calyptratae (e.g. Fig. 3) but typically not elsewhere in the flies but can be imagined from the lateral sutures (Figs 1b, 2a). A simple difference in the formula can often separate similar looking genera (Fig. 2) or species (Fig. 3).
Notes on dc line shape….
The dorsocentral setae line can run entirely straight, or the last pair (prescutellar pair) can be offset outside the line set by the anterior pairs, which can be important taxonomically. Extreme cases of offset prescutellar dc occur where they fall outside the dorsocentral region as defined by the blue patterned area in Figure 1b (e.g. Notiphila, Ephydridae) — historically such examples have resulted in different postsutural numbers offered in the literature where early workers did not include the last grossly offset pair. It should also be noted that for some flies a dc or acr pair can sit on the transverse suture, in which case they are considered ‘sutural’ (e.g. Antissa, Ephydridae).
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Form of dc !
A difference in the form and/or relative placement of the bristle pairs can also separate similar looking flies. Figure 4 shows two similar looking Sapromyza species (Lauxaniidae), both with dc 0 + 3, however the form and placement of the two anterior postsutural dorsocentral pairs differs greatly between the species.
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Finally, figure 5 demonstrates a peculiarity in Rutilini flies (Tachinidae) where many groups within show diminished and variable form of dc and acr — in Rutilini, the numbers of these pairs are not considered important taxonomically, but instead whether they are stable, equally spaced and strong (as in Fig. 5a) or show the unstable form (Fig. 5b) — a further extreme to the unstable form is where the prescutellar acr are indistinguishable among a transverse line of supernumery setae (e.g. Rutilia simplex and some Rutilia (Donovanius) spp.).
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…and acrostichals !
Acrostichals (acr) are similarly important taxonomically as the dorsocentrals (dc) shown above for Acalyptratae and Calyptratae. Thus acr number, relative placement and form should also be noted when comparing species in these groups (e.g. Fig. 6).
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