I was wondering what the little orbs were on this big, beautiful cranefly I was observing on the outside of a window one sunny morning….
A deep dive into Wikipedia revealed that they are halteres. They oscillate while the creature flies, acting like little gyroscopes. They’re attached to specialized sensory organs that detect minute changes in the position of the halteres and thus allow the fly to detect its position and orientation while flying. Have you ever held a gyroscope and turned it in your hand, and noticed the feeling in your hand kinda resisting the turn? I imagine the sensory stimuli would be something like that, except much more intense.
Apparently most flies have halteres, including houseflies. The halteres evolved from the rear wings of their ancient ancestors.
I wonder how this feature evolved. I Imagine how flies with mutated smaller rear wings might have been better at sensing their position and thus better able to evade predators, increasing their survival rate. Maybe over millennia as subtle gene transcription errors resulted in slight variations of these lil’ rear wings—maybe the wing muscles became more specialized for oscillating instead of beating those wings, and maybe a concurrent gradual series of mutations made the wings more bulbous and another set of mutations made them shorter and more rigid… All the while, other mutations modifying other systems to establish the neuronal mechanisms for these sensations to be integrated with other senses.
What a wondrous thing a fly is. I imagine integrating those gyroscopic signals with thousands of images from compound vision, and with the proprioceptive feedback from hundreds of wingbeats per second, and with whatever incredible chemical sensory receptors flies have on their antennae and feet (a subject for another post)…
Evolution is incredible. I’m still gonna swat houseflies though.
And, just so this post both scratches both the entomology itch and satisfies the etymology urge: “Halteres” comes from a Greek word for metal weights that athletes would hold in their hands to add inertia to aid in leaping (or maybe something else).