The amazing flies of the genus Richardia: sexual selection taken to extremes

I have a decent backlog of readers’ wildlife photos, but not enough to make me comfortable, so be sure to keep sending in your good pictures.

Today we’re taking a hiatus and featuring the amazing photographs of photographer and entomologist Gil Wizen, taken from his eponymous website (with permission; note that he also has a Twitter page and a Facebook page). I especially like these photos because they show the effect sexual selection can have on flies: in this case flies in the genus Richardia (the post from which I took these photos is “Photographing Richardia: a long way to victory“).

Gil’s photos were taken in Ecuador, and feature some really cool flies. (Note that these are copyrighted, and you must ask permission for both commerical or noncommerical reproduction.) Here, for instance, is an “antlered fly”, with Gil’s description (indented):

Males have antler-like projections from their eyes, which are used for pushing an opponent during a combat over territory or a mate. The female Richardia lacks those projections, but is characterized by a telescopic ovipositor at the tip of her abdomen, used for injecting eggs into fruits and other plant tissue.

The site also has an awesome close-up of the antlered head itself, so go over and see that. Here are the males: dorsal and frontal views. Note that the “antlers” are projections of the head itself, and are not antennae or aristae, which stick out straight in front in the first photo:

Richardia also includes ‘hammerheaded’ flies, in which the males (but not females) have their heads elongated laterally, resembling (but not related to) the “stalk-eyed flies” (diopsids).  The fact that only males have wide heads is a clue that sexual selection is going on, and indeed it is: in the form of male-male competition. As Gil notes:

The hammerhead Richardia can sometimes be seen on the underside of broad leaves such as those of banana and heliconia plants. Males engage in head-pushing tournaments while a single female usually stands by watching and waiting for the winner to approach. He will then display a short dance, running in circles and waving his decorated wings, before mating with her.

Clearly males with bigger heads have an advantage here; that’s what’s driven both the elongated heads and the sexual dimorphism. Here’s a male:

And a female of the same species, having a “normal” head:

Just to show the lengths to which sexual selection can go, below is a male from a different fly genus. Yes, those are the eyes on the tip of its head, and surely this design is not only maladaptive for fly vision, but also for flight. (If it were visually and aerodynamically good, the females would have it too.) Gil’s caption:

Male hammerhead fly (Plagiocephalus latifrons), dorsal view. One of the most amazing fly species out there in my opinion!

The behavior of this fly isn’t described, but I would bet $100 that the males engage in head-butting contests or “my head is bigger than yours” comparisons, with bigger-headed males generally winning. Of course, selection will only proceed to the point where the sexual advantage of having an even longer head is counterbalanced by natural selection against that lengthening, probably based on metabolic, visual, or aerodynamic constraints.

Look at that head!!:

Gil wondered, as did I, how these huge heads could possibly fit into a pupal case. I guessed, based on the fact that flies also expand their wings after they hatch, that these males can also expand their heads after “eclosion” (hatching from the pupal case). That in fact is what happens. In the amazing BBC video below, also posted by Gil (narrator sounds like David Attenborough), you see a stalk-eyed fly right after hatching. It gulps air bubbles and forces them into its head to expand the eyestalks!

Thanks to Gil for permission to use the photos and Matthew Cobb for calling my attention to Gil’s post.

Snarky aside: as one reader below noted, some misguided souls might suggest that this sexual dimorphism isn’t the result of evolution, but is simply a social construct: males are raised to have long eyes! Well, we know that can’t be true (how do we know that?). At any rate, male-male competition is also a likely a behavior that, imposing sexual selection, led to sexual dimorphism in body size in our own species, with males being larger and having more muscle mass than females.  Imagine what human males would look like if they had to head-butt to win a mate!