Flies definitively taste the world through their feet, possessing a sophisticated sense of taste on their legs that differs from the human experience. This ability to taste by simple contact is a form of chemical sensing, serving as a rapid, on-the-spot assessment of the environment. For an insect whose survival hinges on quickly identifying nourishment and avoiding poisons, this distributed sense of taste is a highly effective biological strategy.
Sensory Organs on the Feet
The fly’s “foot,” known anatomically as the tarsus, is the primary location for its external taste sensors. These last segments of the leg are densely covered with specialized, hair-like structures called chemosensory bristles, or sensilla. Each of these tiny bristles functions as a self-contained taste bud, capable of instantly sampling the chemical makeup of any surface the fly lands on.
When a fly steps onto a liquid or dissolved solid, the chemicals enter a pore at the tip of the bristle. Inside this structure are several sensory neurons, each one typically tuned to detect a specific taste quality. These neurons react to different dissolved compounds, primarily sugars (sweet), salts, and toxic substances (bitter).
The process is called contact chemoreception, which is the direct sensing of non-volatile compounds. The sensilla also contain a mechanosensory neuron that detects pressure, allowing the fly to differentiate between simple touch and a chemical signal. Information from these receptors is swiftly relayed through the nervous system to the subesophageal ganglion in the brain.
Research has shown that the sweet-sensing neurons in the legs are divided into two functional groups. One set of neurons connects to the ventral nerve cord and serves to immediately stop the fly’s movement upon stepping onto a food source. The second group projects directly to the fly’s brain, initiating the feeding process. This specialized wiring ensures that the fly both anchors itself to the food and begins preparation for ingestion almost simultaneously.
Other Fly Tasting Locations
The ability to taste is not limited to the tarsi; the fly utilizes a widely distributed sensory system across its body. The most concentrated and primary gustatory organ is the proboscis, the fleshy mouthpart that extends to ingest food. The tip of the proboscis, called the labellum, is covered in taste sensilla that function in much the same way as those on the feet.
This multi-location system provides a gradient of sensory assessment, from an initial landing to the final decision to swallow. Taste receptors are also situated along the anterior margin of the wings, which allows the fly to sample the air or surfaces during flight or while resting. Furthermore, female flies possess taste sensilla on the ovipositor.
While taste receptors detect non-airborne chemicals through contact, the fly’s antennae and maxillary palps are its main olfactory organs, detecting volatile, airborne chemicals (smell). These two senses work together to guide the fly to a potential food source, with smell providing long-range guidance and taste confirming edibility upon contact.
Taste and Fly Survival Behavior
The extraordinary sensitivity of the fly’s taste receptors translates directly into survival behaviors. The most immediate and observable response to a positive taste signal on the feet is the proboscis extension reflex. When the leg receptors detect an attractive compound, such as sugar, this reflex causes the fly’s proboscis to automatically extend.
This reflex allows the fly to confirm the food source by touching it with its mouthparts and begin the process of liquid ingestion without delay. Conversely, detecting bitter compounds with the feet triggers an avoidance response, allowing the fly to immediately lift off and search for a safer source of nutrition. This preemptive tasting prevents the ingestion of toxic or distasteful substances.
For female flies, the taste sensilla on the ovipositor are used to evaluate potential sites for depositing eggs. The female samples the substrate to ensure it contains the necessary nutrients for the larvae to develop after hatching. By tasting with its body, the fly makes rapid decisions about feeding, poison avoidance, and reproductive success before committing to a surface.