What are halteres?

Halteres are small club-shaped structures behind the forewing of all Diptera. They are vestigial structures that were once wings and have now reduced and changed form and function. Below are some diagrams and some real examples to take a look at. You can see how they differ within species, but all contain a stem and club-shaped end, with muscle attachments at the hinge point that contain sensory organs transmitting information on aerial forces to the fly nervous system. Find out more about how they move and collect sensory data below...

Blow fly Calliphora vicina, Image credit:(Hengstenberg, 1998)

Diagram showing the haltere sensory fields (campaniform sensilla) Image credit:Yarger, Alexandra & Fox, Jessica. (2016).

(dorsal Hick's papillae (dF1), basal plate (dF2), dorsal scapal plate (dF3), unbestimmte Papille (dSi), ventral Hick's papillae (vF1), and ventral scapal plate (dF2) (Gnatzy et al. 1987; Chan and Dickinson 1996). 

Cranefly (Brachypremna dispellens Spp.), Image credit; https://matthewwills.com/2023/04/24/revisiting-a-crane-fly/

left marginated calligrapher fly (Toxomerus marginatus) right soldierfly (Stratiomyidae Spp). halteres labelled, Image credit: https://www.knowyourinsects.org/glossary.html ,

How do they move?

The muscles at the base of halteres stimulate constant oscillation in an off-set direction to the remaining two wings. This is done at a wingbeat frequency of more than 100hz (Diora et al. 2021). Although not in time, crucially the frequency of the wingbeat and haltere is the same (if it wasn’t, the torque effect would be too great disabling flight - Fry et al., 2003).

Sensory information is collected from receptors called campaniform sensilla and sent via neurons to the brain that in turn sends motor neurons to the remaining wings adjusting flight. (Nalbach, 1994; Nalbach, 1993; Nalbach and Hengstenberg, 1994; Pringle, 1997). The diagram below shows a model of a fly and how the halteres interact with the wing-pair and indirect flight muscles. These indirect flight muscles deform the thorax and help generate regular wingbeat motion.

Image credit: (Deora et al., 2015)

Watch this amazing video showing slow motion diptera action - look for the halteres...

Why are these structures present on Diptera?

Here are four of the key reasons why the development of halteres was beneficial….

1.                   There is a significant aerodynamic and visual advantage – Halteres enable extreme agility and control, allowing evasion from predators or webs and have led to the success of dipterans as an order of insects (Simmons, 2024). Being able to drive visual feedback by effecting a change in gaze is another advantage – quickly seeing a predator mean it can be avoided.  (Chan and Dickinson, 1996; Dickerson et al., 2019).

2.                   They are able to respond more rapidly – It is not their overall speed that is improved (birds like swallows and dragonflies are likely much faster) but the speed and acceleration (and deceleration) with which they make fast motor adjustments. This speed of movement allows both evasion of threats, advantages against less speedy competitors. Wardill et al. (2017) investigated the incredible speedy and controlled flight trajectory of the robber fly when catching prey (video link here).

3.                   They have improved stability and accuracy in flight - Studies show that the aerodynamic forces halteres experience are small, but the inertial forces (including Coriolis forces connected with body rotation) are significantly higher. (Yarger & Fox, 2016). This gives them clear and instant information. This stability gives advantages of predator evasion and control in windy weather. Stability can also give pollination advantages, think of the bee-fly hovering expertly to reach nectar from a spring primrose. (Shown in the photo below).

Bee-fly (Bombylius Spp.) Image credit: D Browning 2024

4.                   They opened up ecological niches - Increased flight control and manoeuvrability allowed diversification and specialisation of diptera, ranging from pollinators to blood suckers, detritivores to carnivores. The sheer diversity of Diptera is incredible, allowing them to evolve and change to fill specific niches. This diversity is described by Erica McAlister of the National History museum. She describes brilliantly how they diversified to co-evolve with some plants species such as the cocoa tree that depend on them for pollination - see the video below;