54. Hybotidae – form & function

Hybos femoratus

At last! A fly of character. Although small, it has plenty of style – the thighs on the hind legs worthy of a sprint cyclist, though unlike those of a serious cyclist, they are unshaven and delicately feathered with bristles. The front legs are more delicate, the thorax nicely humped, the abdomen and wings long, the eyes the full height of the head with polkadot reflections (more bulging in life), bead-like antennae with a long terminal arista, long hairy palps sticking out in front and a sharp proboscis below – a fly with panache. But what is it all for?

On my occasional forays into mending things in the house, I am often rewarded when I take the time to look at the engineering of the thing I am trying to repair – why is the piece I am trying to fit back into place that particular shape with a screw hole just there? There is often a good reason, and so it is with flies. Did you guess that a fly with a needle-like proboscis and large eyes might be a predator? Those long palps are in the perfect place to sense when the prey is in the right place for piercing, and the long wings and humped back suggest a strong pursuit flier, capable of outgunning smaller fry. And the monstrous hind legs – could they be used for catching or crushing the prey?

They could, but unfortunately for the neatness of this story, there are plenty of other flies in the family that are also aerial predators but have normal-sized hind femurs. The images I can find of females in the genus suggests that their legs may be more elegant than the puffed up ones of the male. So perhaps hind femur girth is a character that is sexually selected, and the panache of this fly is that of Henry IV of France who wore a white plume in his helmet, the famously extravagant gestures of Cyrano de Bergerac, and the peacock’s tail. Form and function have a more complicated relationship in a fly than in an electric kettle.

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