Published by Patrick Mutisya · 14 days ago
Describe the rapid response of the \cdot enus fly‑trap (*Dionaea muscipula*) to stimulation of the trigger hairs on the lobes of its modified leaves and explain how the closure of the trap is achieved.
| Step | Physiological Process | Time Scale |
|---|---|---|
| 1 | Mechanical deflection of a trigger hair | ≈ 0.1 s |
| 2 | Generation of an action potential (AP) in the epidermal cells | ≈ 0.2 s |
| 3 | Propagation of the AP across the lobe (via plasmodesmata) | ≈ 0.3 s |
| 4 | Opening of voltage‑gated Ca²⁺ channels → rapid influx of Ca²⁺ | ≈ 0.4 s |
| 5 | Activation of mechanosensitive ion channels → efflux of K⁺ and Cl⁻, water follows osmotically | ≈ 0.5 s |
| 6 | Loss of turgor in the outer epidermal cells of the lobe margin | ≈ 0.6 s |
| 7 | Snap‑buckling of the midrib (elastic instability) – the two lobes snap together | ≈ 0.7 s |
| 8 | Sealing of the trap by interlocking marginal cilia (if prey is present) | ≈ 1 s |
Stimulus detection – When an insect touches a trigger hair, the hair bends, stretching the plasma membrane of the sensory cell. This mechanical stress opens mechanosensitive ion channels, causing an influx of Na⁺/Ca²⁺ and depolarising the membrane.
Action potential generation – If the depolarisation reaches the threshold (≈ −30 mV), an action potential is fired. A single hair touch produces a sub‑threshold AP; a second touch within \overline{20} s produces a second AP, and the combined signal reaches the threshold needed for closure.
Electrical signal propagation – The AP travels through the lobe via plasmodesmata, reaching the midrib and the opposite lobe. The rapid spread ensures a coordinated response.
Calcium signalling – The AP opens voltage‑gated Ca²⁺ channels in the motor cells of the lobe margin. Cytosolic Ca²⁺ rises sharply, acting as a second messenger that activates downstream ion channels.
Osmotic changes and turgor loss – Elevated Ca²⁺ triggers K⁺ and Cl⁻ efflux from the motor cells. Water follows osmotically out of these cells, causing a rapid loss of turgor pressure in the outer epidermis while the inner epidermis remains relatively turgid.
Snap‑buckling – The differential turgor creates a curvature stress across the midrib. The stored elastic energy is released, and the lobes snap together in a bistable “snap‑buckling” transition. This mechanical instability allows movement faster than could be achieved by gradual cell expansion.
Sealing and digestion – If prey is trapped, the marginal cilia interlock, forming a watertight seal. The trap then initiates enzymatic digestion (not covered here).
The \cdot enus fly‑trap avoids wasting energy on false alarms (e.g., wind‑blown debris). Two separate stimulations of the same or different hairs within a short time window (≈ 20 s) are needed to generate sufficient cumulative depolarisation to trigger the full cascade described above. This “count‑and‑wait” strategy is a simple form of signal integration.