Wings: Fly Halteres:
Banded-legged Gasteruptiid Wasp
Wasp Mimicking Fly (Agapophytus aterrimus)
Retired Professor of Anatomy, Ian Gibbins, has kindly shared his thoughts with us to help people learn:
I don't think the halteres physically help with balance. They beat in anti-phase with the wings (ie when the wings are up, the halteres are down and vice versa). According to the most recent info I've got at hand, they measure torque on the body during flight. At their base, there are hundreds of sensory endings that are activated by rotation of the body during flight. They feedback directly to flight control neurons to control steering.
Birds have a similar direct feedback system from some of their wing feathers: turbulence on the feathers is detected by sensory endings that feed directly to the spinal flight motor neurons to adapt the shape of the wing / spread of the feathers to minimise turbulence to prevent stalling.
It's not clear if the halteres are providing directional stability or flight stability: probably both, given the size of flies.
At the speed the wings are beating, the flying velocity, and the size of the fly, the apparent viscosity of the air is enormous. It would feel like water a motor boat prop... :)
I don't know, but I suspect the relatively larger size of the halteres in smaller flies might have something to do with scaling effects vs apparent air viscosity (Reynolds Number etc) vs sensitivity of the mechanoreceptors themselves.
Another parallel we have is the way we swing our arms when we walk = apparently redundant evolutionary hang-over from quadrupedal gait, but it turns out it significantly helps balance re momentum / torque. So the halteres movement itself may or may not be important, but they probably need to do it in order to preserve wing-beat control, and along the way, give a dynamic reference point for maintaining balance / orientation.
I'd expect there to be some sort of interaction between this system and the visual system in orientation control. The latter has been studies a lot in hoverflies, so I'll have a look there as well.
The halteres are acting more like accelerometers rather than gyroscopes. Their base is flexible and distorts relatively easily. The knobs at the end of them have high mass compared with the stalks. Inertial forces mean that the ends of the halteres will continue to oscillate in their original plane, but that's no longer aligned with the body of the fly => bending at the base of the haltere which activates the mechano-receptors => information to the wing motor neurons and the central nervous system.
It turns out that the info to the head mainly goes to motor neurons controlling neck angle (22 of them in blow flies) and these neurons control head position / gaze direction. They are controlled mainly by inputs from the visual system. However, the input from the halteres becomes important during high speed changes in direction.
It's recently been discovered that in some species (especially Calyptratae), the halteres are active during walking as well as in flying. They seem to be important in keeping balance and orientation when walking on vertical surfaces.
It's also now well established that the anti-phase oscillations of the halteres is not universal in Diptera, with much variation / lack of phase locking in more primitive families. This correlates well with the high level of flying skill and visual systems of more advanced Diptera.
Finally (for now!), re the relationship between relative haltere size and body size, there's been a very comprehensive study of this (and a lot more...). In general, haltere length correlates reasonably well with body size in absolute terms, though this relationship doesn't hold so well for some mosquito groups. Overall, relative stalk length is greater in advanced groups, especially the Calyptrates, again in line with their highly developed flying skills.