Atlant’s Column: The Ruminant Fly

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Welcome to the section on which explores, analyzes and explains fascinating microscopical structures.

You are looking at a fly’s tongue. The wave-like structures in the upper half are part of a pad that resembles the floor brush of a vacuum cleaner. The lower part of the image displays the beginning of the ‘hose’. So we take the perspective of a food particle that is about to be sucked in.

Text by science writer Atlant Bieri
Photography by Martin Oeggerli

Scientists call the pad labellum. The fly dips it into a wet surface and takes a sip. That’s all a fly can do. Chewing is impossible. If the food source is dry, such as crystalline sugar, the fly regurgitates saliva and wets it until the surface starts to dissolve. Then it sucks up the nutrient laden liquid which travels along the half pipes displayed in orange and yellow colour. They are called pseudotracheae (meaning ‘false windpipe’, because they are not for sucking up air but liquid matter). Their diameter is half a micrometre which means flies are unable to feed on any particle bigger than this.


Having a Taste for Faecal Bacs

Pollen, yeasts, or fungal spores make no meal for the fly since they are way too large to pass through the pseudotracheae. Instead, the flies main dish consists of faecal bacteria which in small amounts are found pretty much anywhere – a door handle, the kitchen table, or on the floor. And of course flies gorge themselves on any sugary or salty solutions such as syrup, sweat or spilled ketchup. How do they know what to eat? The labellum is peppered with tiny rods (here displayed yellow) which are nothing else than taste buds. They send a firework of electrical pulses to the tiny fly’s brain as soon as they come into contact with a favourable food source.

The pseudotracheae all end up in the mouth of the fly a millimetre or so up from the pad. But before the fly transfers the food to the gut for digestion, it stores it in the crop, a pouch somewhere inside its abdomen. After feeding for a while, it takes a break sitting somewhere. During this time it regurgitates the crop content and puts a droplet down in front of it. Water evaporates from the drop concentrating whatever nutrients are in there. The fly ingests the broth once more and only then does it start to digest it.

Another way to concentrate liquids is to bring up a droplet and hold it with the labellum. The scientists call this bubbling. The fly waits for a while until a sufficient amount of moisture has evaporated before it sucks the droplet back up the pseudotracheae. Thus, flies act in a way like ruminant cows processing their food twice.

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Continue reading Atlant’s Column: David vs. Goliath, September 2011, or go back to the archive