by Matthew Cobb
Oh hear and attend, my Best Beloved, and let me tell you the story of how the fly lost a pair of wings. Or rather, what it did with the wings that it lost (here endeth the attempt to channel Kipling again).
Flies are technically grouped in an Order known as Diptera – ‘two wings’. This makes them very different from other insects like bees and dragonflies and butterflies, which have two pairs of wings. The ancestral state in insects is to have two pairs of wings, so insects with one pair of wings, like flies and beetles, have apparently ‘lost’ one pair over the course of evolution.
However, nothing actually disappears in evolution – there is always some vestigial trace, be it genetic or morphological, of a past adaptation that has since been ‘lost’ in a particular lineage. So for example, in beetles, the front pair of wings has turned into the ‘shell’ of the beetle, which protects the flying wings during ground locomotion but which flip up during flying, as shown in this great photo by Steve Weeks of a male stag beetle about to take wing (photo taken from here):
In flies, it is the front wings that do the flying, while the rear wings have changed their form and function – they have become compressed into slender dumbell-shaped structures known as halteres, as you can see on this photo of a Tipula cranefly in Georgia, by Wayne and taken from here.
What do the halteres do? Well one (rather cruel) way of finding out is to snip them off – relatively easy in a crane fly. The result is that the fly now bumbles about in a completely undirected way. Flies use the haleteres for balance, and to help direct their flight. There is also a suggestion that they provide information to motor neurons in the fly’s neck that is correlated with visual input, so the fly knows how to respond to visual stimuli appropriately. (This raises the not entirely facetious possibility that haltere-less flies might also suffer from a kind of motion sickness.)
This fantastic hi-def video of a Syrphid (hoverfly) taking off was doing the rounds on Twitter a month or so ago as part of a ‘high-speed arthropod week’ by Sean McCann on ibycter.com.
You can see that the halteres beat in counterpoint to the front wings. Sean notes: ‘the plane in which the halteres move differs by an angle of up to 30 degrees from that of the wings’. It appears that this involves Coriolis force, as suggested in this diagram from this paper, which you may or may not find informative:
Reader Dom has noticed another behavioural use of halteres, although it’s not clear exactly what’s going on. In these two videos, flies are wiggling their halteres in a coordinated manner while they are sitting on the ground. This may be a consequence of some internal movement that leads to changes in pressure and therefore their halteres moving, or it may be a more immediate use of the halteres – but for what?
This one shows what appears to be a Sepsidae fly looking pretty chuffed about having found some lovely dung (look at it rubbing its little hands in glee!) while brief flashes from behind its wings show the halteres wobbling about:
This crane fly is sitting on the side of a tent (?) and is wagging its halteres too and fro:
So – what do we make of this? What are they doing with their halteres?
Oh, and the answer to ‘what did insect wings evolve from’ is the usual answer to the best questions – we don’t know. The best bet seems to be from gills (don’t forget all insects came out of the sea). Patterns of gene expression of pdm/nubbin in insect wings are also seen in book gills in other arthropods, but this isn’t proof. Interestingly, the same evidence suggest that those gills turned into spinnerets in another arthropod – the spider.
h/t to Dom for prompting all this, and taking and sending the videos!
Mmm… brainfood.
And sub.
Cool videos. Sepsids routinely lek at dung falls, so I’d guess the one in the video is displaying for a mate in some way (or preparing to do so). The brightly coloured haltere knobs would make sense if this is the case.
As for the crane fly, I have no idea. Maybe just random movement, maybe related to respiration (helping pump air in and out of thoracic spiracles, one of which is located adjacent to the base of the halteres), maybe some as yet unknown purpose. Interesting nonetheless!
There was a mating couple there as well. They are very small & I did not spot them until I looked at the photo later.
While I was lying on the ground a few Homo sapiens walked past – I felt obliged to explain why I was dipping my nose towards a cowpat!
Ha Ha Ha! I’m sure your explanation made perfect sense to them as well.
Such and interesting subject! I’d never thought much about halters–presumed they were some sort of stabilizer or something. Had no idea they even moved!
Great vids, Dom!
Some sort of sexual display was going to be my guess, but I’m really just guessing.
I don’t see the dynamics of the halteres in the later videos. So I’m going to assume they are pretty much the same as in the first.
Then my hypothesis (which is mine) is that it could be an adaptive trait for the fly to have its “gyros” (and perhaps visual system) prepped for flight while vulnerable.
It would take some energy perhaps but the waste heat could maintain work temperature of the close by wing muscles. (That is way out there, I dunno about the biology.)
But foremost it would save reaction time to be immediately in flight dynamics when the fly jumps from the surface (as I hear some do) and starts its wings.
I don’t quite buy this. The wings are far more massive than the halteres; if the wings can be instantly ready for flight from a standing start, why can’t the halteres be ready even quicker?
What I might buy is that the halteres function as sensory organs to detect ambient air motion even while the fly is at rest. That would be something worth knowing about before takeoff. Perhaps that’s what you meant?
I’m with you Torbjörn.
It could be a part of the response system towards predators.
It keeps its engine running in neutral, so to speak.
Just like humans benefit from warming up before engaging in a demanding physical feat.
But humans don’t warm up for a footrace by wiggling their fingers. They warm up the parts that are going to be doing the heavy work.
And I’m not sure the concept of warming up applies at this tiny scale. Unlike humans, these flies are all surface; their muscle temperature is essentially at equilibrium with ambient air temperature at all times.
Good points.
Isn’t it fun to be completely obsessed with flies.
sub
“The best bet seems to be from gills (don’t forget all insects came out of the sea). Patterns of gene expression of pdm/nubbin in insect wings are also seen in book gills in other arthropods, but this isn’t proof.”
— Just a note that, IF insect wings were derived from gills like those in the aquatic immature stages of mayflies, etc., this is not an “out of the sea” story. It’s pretty clear now that insects are most closely related to higher crustaceans, which do not have book gills.
The first insects [here I mean Hexapoda, including things like springtails, as well as flying insects and their nearer relatives] were fully terrestrial with tracheal respiratory systems. The most primitive living and fossil winged insects have/had immature stages living in fresh water, and many of these groups have articulated, winglike abdominal gills. Those could well be serially homologous with wings, but note that these are always tracheal gills — again emphasizing that this is a return to water, and not something reaching back to marine arthropod ancestors.
Reblogged this on Mark Solock Blog.