Moved from Hexagenia bilineata.

Hi David,

Although the males of this species can appear to mimic some traits associated with Hexagenia bilineata, this is not that species but Hexagenia rigida instead. The proof is to be found by looking at the tip of the abdomen in the full resolution image. The slender dark appendage between the clasper and the base of the tails is the unmistakable male member of H. rigida.

This male imago has the typical crown-shaped tergal markings of rigida, but they are infuscated with chocolate brown. The males are almost always darker than the females, but the most common coloration in most rigida males (or females) is a pale to bright yellow with black (usually) or dark brown (often) markings. Another unusual aspect of this specimen is that it doesn't have a dark margin on the hindwing or the few dark spots in the hindwing disc that are typical of most rigida males. I have seen that before in this species, but it is not common.

The combination of easily recognizable genitalia and some unusual traits make this specimen a valuable addition to the Guide; it shows some of the range of variation that is possible in this species. That is particularly useful because this species doesn't vary much - especially when compared to the highly variable Hexagenia limbata.


Best,
Lloyd

Moved from Mayflies.

Hi David-

I believe this male imago to be Hexagenia bilineata.

Moved from ID Request.