(1) After removal of the halteres equilibrium during flight is upset, and as a consequence of this the fly cannot keep in the air.
(2) The flies are perfectly able to take to flight and to fly for long periods when suspended in mid-air.
(3) Removal of one haltere alone has either no effect at all on flight or only a very slight one.
(4) Immobilization of the halteres by sticking them fast with a small drop of glue has the same effect as complete removal. This has been previously reported by Weinland (1891), and confirmed by other authors.
(5) Cutting off the end knob alone has the same effect as removal of the entire haltere including the base.
(6) The halteres are vibrating rapidly during flight in a plane which is fixed relative to the body of the fly.
These facts argued against the balancer theory. If a fly with only one haltere can fly relatively normally, this discounts the idea of halteres acting as balancers. Earlier critics had noted that the weight of the halteres is not significantly large to act as a balancer.
Frankel proposed that the halteres must work as a sense organ to stabilize flight. They vibrate up and down with the fly wingbeat in a single plane. Changes in attidude of the fly during flight would apply force to the halteres which would send feedback to the fly. Our modern understanding of haltere function is built on this simple idea.
*Fraenkel, G. (1939). The function of the haltere of flies (Diptera). Proc. Zool. Soc. Lond. A109, 69-78.
So amazing that even in such a huge group of animals in this world, insects, we still know so little! Thanks for the interesting look at halteres.