1. How Do Flies Dissolve Dry Food with Liquid Before Sucking It Up?
Sometimes a fly lands on a candy crumb and stays still for a while. During this time, the fly is thought to be doing something completely different from how humans eat. Adult houseflies cannot gnaw on solid food. Instead, they spit liquid onto the food to dissolve it, then suck up the dissolved part with their sponge-like mouthparts. A 2025 paper states that even dry food can be eaten if it is first melted by this wet spit [2].
Humans eat in a "chew and swallow" order, but flies use a "dissolve and suck" order. The big difference in a fly's meal is that the food itself becomes liquid first. So, what is the tip of a fly’s mouth like?
2. Why Is a Fly's Mouth More Like a Sponge Than a Straw?
At the tip of a fly's mouth, there is a fleshy, swollen part called the labellum. Its surface has fine grooves lined up, which are called pseudotrachea. It is said that these pseudotrachea help suck up liquid through capillary action [1].
Capillary action is when liquid naturally moves upward through narrow gaps. It is similar to how water slowly soaks up into the end of a wet cloth dipped in water. It is easy to imagine if you think of the fly pressing its sponge-like labellum against the food, letting the liquid soak into the grooves as it takes it in. However, the detailed mechanism is described with the phrase "it is said that."
The first half of the system for dissolving hard food and sucking it up is handled by the spit. What exactly is this liquid?
3. What Digestive Fluids Does a Fly Spit Out to Dissolve Food?
The liquid spat out is said to contain digestive enzymes and partially digested food mixed together. Regarding where the liquid comes from, general explanations only mention that a sac called the crop, which stores liquid, is involved; details are not fully understood [3].
This means flies begin digestion on top of the food, not inside their stomach or intestines. Since they cannot chew, they do the "preparation for digestion" on the food first. The explanation of the liquid's contents is based on general commentary, so it remains as "it is said." On the other hand, the point that dry food can be eaten if dissolved by spit is also stated in the paper [2]. So, what kind of food do flies choose to turn into liquid and suck?
5. Why Do Fly Larvae (Maggots) Grow in Decaying Organic Matter?
Larvae (maggots) grow inside decaying organic matter, such as livestock feces or garbage piles [6]. Documents from Nagoya City also state that many types of flies have larvae that grow in decaying matter like kitchen waste, barns, and compost, and adults gather around these places too [5].
While adults only consume liquids through their mouths, larvae grow inside decaying matter. Even for the same insect, what they eat and where they live change greatly depending on the growth stage. The size of a mature larva is about 10 mm (about 0.4 inches) [6].
Larvae avoid light and prefer warm, moist, dark places. For example, in pig feces, there is a report that larvae hardly grew in dried feces where the moisture was less than 30% after maturation [1]. This is one example and not a number that applies to all environments.
The number of days from egg to adult varies greatly with conditions like temperature. Since descriptions of the larval period differ even within the same sources, no specific day count is shown here. The lifespan of an adult is usually 2 to 4 weeks [1].
By the way, you often see flies rubbing their feet together. What are they doing?
6. How Do Flies Taste with Their Feet, and What Do We Know from Other Species?
Flies have chemoreceptors on their tarsi (foot segments) that sense taste, and it is said they can identify sugary foods just by walking [1]. The rubbing of feet is also explained as being for cleaning.
However, the evidence for this "tasting with feet" comes from studies on blowflies and fruit flies, not specifically on houseflies themselves. For example, in fruit flies, taste sensory hairs on the mouthparts, wing edges, and legs are reported to be used for "tasting" before eating food [7].
Whether houseflies have the same mechanism cannot be stated definitively here. It is best to accept it as something that may be common among flies.
7. How Can You Check Fly Mouthparts at Home with Books and a Magnifier?
Here are two ways to check things at home. First, imagine what the fly does to food by copying it with sugar. Place a grain of sugar on a dry plate, drop one drop of water on it, and watch how it dissolves into liquid. You can see how sugar dissolves in water. However, this is just sugar dissolving in water; it does not reproduce the action of fly digestive fluid.
Second, look at the shape of a fly's mouthparts using encyclopedias, museum specimens, or school microscope photos. Look for the sponge-like labellum and the grooves on its surface. You do not need to catch and touch a live fly.
In this article, we saw that adults and larvae eat different foods, and that the evidence for foot taste comes from studies on other flies. If you are curious whether there is research directly testing housefly foot taste, you might find clues by looking at insect books in libraries or academic paper search sites.