Halteres

Crane fly, with a pair of halteres visible below the wings as appendages, about as long as the animal's antennae, with knobs on the end

Halteres (pronounced /hælˈtɪəriːz/; singular halter or haltere) are small knobbed structures found as a pair in some two-winged insects. They are flapped rapidly and function as accelerometers to help the insect maintain stability in flight.

The halteres evolved from wings. The ancestral insect species had two pairs of wings (like dragonflies and most other flying insect species have). In the Strepsiptera the forewings evolved into halteres, while in the Diptera (flies, mosquitoes and gnats) the hindwings changed into halteres.

In Diptera, the formation of the haltere during metamorphosis is dependent on the homeotic gene Ultrabithorax (Ubx). If this gene is experimentally deactivated, the haltere will develop into a fully developed wing^[1]. This is an excellent illustration of an important mechanism of evolutionary development; a simple homeotic gene change can result in a radically different phenotype.

Halteres operate as vibrating structure gyroscopes. They flap up and down as the wings do and tend to maintain their plane of vibration. If the body of the insect changes direction in flight or rotates about its axis, a Coriolis force develops on the vibrating haltere, deflecting it from its stroke plane. The animal detects this deflection with sensory organs known as campaniform sensilla located at the base of the halteres. The planes of vibration of the two halteres are orthogonal, each forming an angle of about 45 degrees with the axis of the insect.

Halteres thus act as a balancing and guidance system, helping these insects to perform their fast aerobatics. In addition to providing rapid feedback to the muscles steering the wings, they also play an important role in stabilizing the head during flight.

References

External links

• A magnificent flying machine, The Telegraph, 23 April 1998
• Photo of a haltere of a mosquito (Nematocera)