To me, spring is one of the most exciting times of year in nature. It seems that just about every day there’s a new flower blooming, a new tree leafing out, a new bird arriving back (or passing through) from its wintering area, or a newly emerged insect. With respect to insects, over the last couple weeks I had been noticing two fairly common and unique looking flies around my yard. Both had jet black bodies with noticeable red legs segments, but they looked so different that I assumed they were different species. It turns out I was wrong; they were just different sexes of the same species of March fly, Bibio femoratus (a little anachronistically named since I’m noticing them in May, not March, but I guess the name “Mayfly” was already taken). B. femoratus is just one of approximately 90 different March fly species. 

Sexual dimorphism in this species accounts for the different appearances I observed. Sexual dimorphism is the term for when two sexes of the same species exhibit different physical characteristics. You may be more familiar with this condition in birds: the male mallard duck and northern cardinal are much more colorful than the females, and male peacocks have an impressive set of tail feathers that the females lack. But examples of sexual dimorphism exist in mammals, fish and reptiles, as well as other groups of organisms, including insects. 

I probably never would’ve put all of this together if I hadn’t stumbled across a pair of Bibio femoratus mating – a pretty sure sign that they’re from the same species despite their different appearances. This mating behavior is so common in the Bibio genus that some species of March fly are also known as ‘love bugs’, as males and females remain in tandem, connected at the tips of their abdomens, for prolonged periods of time. And in fact, this is how I found this little odd couple. They remained connected, crawling in tandem through the grass, seemingly oblivious to the giant human looming over them. Although both male and female Bibio femoratus share their main body coloring, they differ dramatically in their wing size and color and their eye size. Females have dark, almost black wings, while the males’ wings are clear. In terms of their eyes, males have large bulbous eyes, while the females’ eyes are much smaller.