Halteres

 

A crane fly we caught in a light trap at work; the halteres are the bulbous, drumstick-looking structures on the back, just behind the wings.

Have you ever wondered what makes a fly a fly? 

The complicated answer is a set of similar physiological characteristics, specific evolutionary relationships, and a dash of arbitrary line drawing by human taxonomists. 

The simple answer is halteres! 

Halteres are a pair of small, club-like structures located just behind the forewings in members of the order Diptera, also known as the "true flies." They function like gyroscopes, providing the fly with information about its position in space. As the fly beats its wings, its halteres beat in time with them, moving along a single, vertical axis. If the fly's body starts to tilt, the halteres maintain their inertia and continue to beat in that original plane. This puts strain on the halteres, which is sensed by receptors at their base and translated by the nervous system into information on the pitch, roll, and yaw of the fly's body. With this information, the fly is able to quickly and constantly makes adjustments to its position and stabilize itself in flight. 

Some flies move their halteres not only while flying, but also while walking on the ground. If you remove the halteres of a fly belonging to a species that does this, it will have trouble climbing up a vertical surface and holding on when the surface is vibrated. This is not the case in flies that do not move their halteres while walking, indicating that halteres probably also play a role in regulating balance in the former group, though we have yet to figure out how (Hall et al. 2015). 

Dipteran halteres almost certainly evolved from what in most other orders of flying insects are a pair of smaller hindwings. In addition to the conspicuous absence of hindwings in all members of the order, this theory is also supported by the presence of similar musculature and a suite of sensory organs at the base of the halteres which are also present at the base of the hindwings in other insects (Yarger and Fox 2016). In an interesting case of convergent evolution, the members of another order of insects - the Strepsiptera or stylops - have also developed halteres. Theirs, however, seem to have evolved from what were once forewings. 

A full body image of the crane fly pictured above; a lot of people mistake these for "the largest mosquito I have ever seen." But while both are true flies, crane flies don't bite - in fact, in many species, adults don't have functioning mouth parts and do not feed. Others drink nectar from flowers.

So, if you're like me and keep finding yourself involved in projects where you have to sort large numbers of tiny, dead insects to order, just remember: if your flying insect has a pair of halteres behind its one pair of wings, it's a true fly; if it has a pair of halteres in front of its one pair of wings, then it's a weird endoparasite that I had honestly never heard of before today, but am now kind of obsessed with. 

And if you're not spending your days looking at dead insects under a microscope...well, I hope you still enjoyed learning about halteres! :)


Sources:

Hall, J. M., McLoughlin, D. P., Kathman, N. D., Yarger, A. M., Mureli, S., & Fox, J. L. (2015). Kinematic diversity suggests expanded roles for fly halteres. Biology letters, 11(11), 20150845.

Hu, D. L. (2018). How to walk on water and climb up walls: animal movement and the robots of the future. Princeton University Press.

Oldroyd, H. (N.A.). Dipteran. Encyclopedia Brittanica.  https://www.britannica.com/animal/dipteran/Form-and-function  

Yarger, A. M., & Fox, J. L. (2016). Dipteran halteres: perspectives on function and integration for a unique sensory organ. Integrative and Comparative Biology, 56(5), 865-876.

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