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The dorsocentral bristles (DCs), the large paired bristles on the central thorax. The Suture is rigid joint between hard elements in this case on the dorsal surface of the thorax. We count how many bristles are forward of the suture (presutural), and how many bristles behind the suture (postsutural) in the case of your fly there is one presutural, and 3 postsuturall. We abbreviate this to 1+3. If it were Suillia this would have been 1+4.

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