One of the most remarkable facts about butterflies is that they can taste with their feet. When a butterfly lands on a surface, sensory organs on its tarsi (the equivalent of feet) immediately detect the chemical composition of whatever it has touched. This ability — known as contact chemoreception — is essential for finding food, selecting egg-laying sites, and avoiding toxins.
The Anatomy of Butterfly Taste
Butterflies possess specialised sensory structures called contact chemoreceptors (or gustatory sensilla) on the tarsi of all six legs. These structures are tiny hair-like projections, each containing several sensory neurones whose dendrites extend to a pore at the tip of the hair. When the hair comes into contact with a dissolved chemical, the molecules stimulate the neurones, which send signals to the butterfly’s brain.
How Tarsal Chemoreceptors Work
- Structure: Each sensillum is a hollow hair (5–30 micrometres long) with a pore at the tip
- Neurones: Typically 4–5 gustatory neurones per sensillum, each tuned to different chemical classes
- Detection: Sugars, salts, amino acids, alkaloids, and specific plant secondary compounds
- Response time: Near-instantaneous — the butterfly can assess a surface within milliseconds of landing
- Sensitivity: Up to 200 times more sensitive to dissolved sugars than the human tongue
Why Do Butterflies Need to Taste with Their Feet?
This adaptation serves two critical functions: food detection and oviposition (egg-laying) site selection.
Food Detection
When a butterfly lands on a flower, fruit, or other potential food source, its tarsal chemoreceptors immediately detect whether sugars are present. If the concentration of sugar exceeds a threshold, the butterfly automatically uncoils its proboscis (the long, straw-like tongue) and begins to feed. If no sugars are detected, the proboscis remains coiled and the butterfly moves on. This is called the proboscis extension reflex (PER) and has been well studied in laboratory experiments.
| Substance | Tarsal Response | Behaviour Triggered |
|---|---|---|
| Sucrose (sugar) | Strong stimulation of sugar-sensitive neurones | Proboscis extension; feeding begins |
| Sodium chloride (salt) | Moderate stimulation at low concentrations | Puddling behaviour (drinking from damp ground) |
| Quinine (bitter alkaloid) | Stimulation of deterrent neurones | Rejection; butterfly moves away |
| Specific plant chemicals | Stimulation of specialist neurones (in females) | Egg-laying on correct host plant |
Oviposition Site Selection
For female butterflies, tarsal tasting is even more critical. A female must lay her eggs on the correct host plant species, because her caterpillars can typically feed on only one or a few plant species. By “drumming” her forelegs on a leaf surface, the female releases plant juices and evaluates their chemical composition through her tarsal chemoreceptors. She can identify specific secondary metabolites — such as the glucosinolates in brassicas (detected by cabbage white butterflies) — that confirm she has found the right plant.
Did you know? Female cabbage white butterflies (Pieris brassicae) can detect sinigrin — a glucosinolate compound found in brassica plants — at concentrations as low as one part per million through their tarsal chemoreceptors. This allows them to identify suitable host plants with extraordinary precision.
Comparison with Human Taste
Human taste buds are located exclusively on the tongue, palate, and throat. We detect five basic tastes: sweet, sour, salty, bitter, and umami. Butterflies, by contrast, combine taste with touch — they literally taste every surface they land on. Their tarsal receptors are estimated to be roughly 200 times more sensitive to sugar than human taste buds, enabling them to detect nectar rewards that we would perceive as plain water.
Other Insects That Taste with Their Feet
Butterflies are not the only insects with tarsal chemoreceptors. Flies (Diptera) also possess well-developed contact chemoreceptors on their tarsi — which is one reason flies are constantly landing on food. Honeybees have tarsal taste receptors that help them evaluate nectar sources. Parasitoid wasps use tarsal receptors to detect chemical cues from host insects hidden within plant tissue.
- Landing: The butterfly alights on a surface, placing its tarsi in contact with the substrate.
- Drumming (females): The forelegs tap or drum the surface, releasing plant chemicals.
- Chemical detection: Gustatory sensilla on the tarsi detect dissolved chemicals through the terminal pore.
- Neural processing: Signals travel to the suboesophageal ganglion (the insect brain’s taste centre) for evaluation.
- Behavioural response: The butterfly either extends its proboscis (food), lays eggs (correct host plant), or departs (unsuitable surface).
Key Takeaway
Butterflies taste through chemoreceptors on their tarsi (feet), detecting sugars, salts, and plant-specific chemicals within milliseconds of landing. This ability drives two vital behaviours: the proboscis extension reflex for feeding, and host plant identification for egg-laying. Their tarsal taste sensitivity far exceeds that of the human tongue.