I find it difficult to distinguish between common and ruddy even with a natural history museum (FSC) guide. So best guess.
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Ruddy Darters have fully black legs, Common Darters have a yellow stripe running along the length of each leg.
...thank you - is it just legs that differentiates?
There are a number of other features which are useful, but unfortunately most can be somewhat subjective or difficult to see. The main ones are given below:
-The abdomen (when viewed from above) has an hourglass type of shape in male Ruddy Darters, becoming noticeably wider towards the tip.
-Male Ruddy Darters are a much deeper red colour when fully mature and lack yellow on the sides of the thorax (but beware of maturing male Ruddys, which do have yellow on the thorax, and old male Common Darters which can lack the typical two yellow stripes!).
-Ruddy Darters have black extending down the sides of the face, just in front of the eyes (difficult to see unless viewed from more or less in front).
-Also be aware that the legs on old Common Darters can darken making the yellow stripe difficult to see. If you can clearly see a yellow stripe you don't have a Ruddy Darter, if there doesn't seem to be a yellow stripe look closely at this and other features to be sure.
PS. The one in your photos is an old female (older females of both species can start to turn red).
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