Carnivorous Plants Website
Carnivorous Plants in the Wilderness
by Makoto Honda

Carnivorous Plants
III. Snap-Trap
Updated 2018-6-10


There are two snap-trap carnivores today: the Venus flytrap (Dionaea muscipula) and the waterwheel plant ( Aldrovanda vesiculosa). Overwhelming evidence from molecular phylogenetic reconstructions using different regions of DNA sequence indicates that Dionaea and Aldrovanda are sister to each other, strongly suggesting that these extant snap-traps evolved from a common ancestor.

The molecular evidence further suggests that Aldrovanda and Dionaea form a clade that is sister to Drosera (sundews). This implies that the common ancestor of Aldrovanda and Dionaea came from an ancient sundew-like plant (Subgerera Regiae + Arcturia)..

Snap-Trap Evolved Only Once
This most likely means that the snap-trap mechanism evolved only once in the common ancestor of Aldrovanda and Dionaea. Therefore, these snap-traps are very similar --- the basic mechanism behind their snap-trap movement must have its origin in their common ancestor.

That is not to say that these two snap-trap mechanisms we see today in the Venus flytrap and the waterwheel plants are identical. Of course, there are some differences. After all, both lineages had 30-40 million years of completely independent evolutionary paths. Not to forget also that one is terrestrial, the other is aquatic, both traps well fine-tuned in their respective, entirely different environments. We can surely find differences between these snap-traps.

We do well seeking similarities --- rather than dissimilarities --- when we strive to unlock the mystery and wonder of these "most wonderful plants in the world."


An intriguing question remains as to which lineage evolved first --- Aldrovanda or Dionaea ---  from an ancient sundew-like plant? To put it another way, was the common snap-trap ancestor an Aldrovanda-like aquatic plant or Dionaea-like terrestrial plant? 

My take --- Aldrovanda...

When we talk about which came first, we are not suggesting that today's Venus flytrap gave birth to today's waterwheel plant, or vice versa. We are discussing the possible ways in which the ancestor of these extant plants diverged tens of millions of years ago...  

THE THEORY OF RECAPITULATION - "Ontogeny Recapitulates Phylogeny"

1)  Lloyd (p.182) - In Venus flytrap, the trap tilts to the right - mainly in younger plants, just like the clear bend seen in the mature Aldrovanda traps (see Lloyd/Darwin) --- Regarding this trap posture, Lloyd commented on the distinct advantage for Aldrovanda but no benefit for Dionaea. However, Lloyd did not extend this observation he made to an evolutionary context. This tilting trait was acquired in the ancestor of Aldrovanda and was carried over to the Venus flytrap, even if it is no use for the Venus flytrap. It is often manifested in the younger Venus flytraps --- "Recapitulation."

2)  Lloyd (p.181) comments on a young Venus flytrap (2 mm trap) ----> "The number of parenchyma cells ranges between two to four courses with large interspaces, in this feature again resembling the mature leaves of Aldrovanda much more than do the thicker mature leaves of Dionaea."
The structure of the mesophyll of the Venus flytrap leaves is more like that of aquatic plants....

3) Venus flytrap is very tolerant of flooding condition - possibly implying an Aldrovanda-like, aquatic ancestor. It is known that the Venus flytraps grow underwater for a few months without any harm. The Venus flytraps catch prey in the water, no sweat. Hurricanes and violent storms go through North Carolina every year, flooding the region where Dionaea grows. I assume it's been like this for the past 50 million years.

4) Aldrovanda seems to share more common reproductive features with Drosera than does Dionaea.

Comparison of flower parts:  

Flower / sepals / petals
Dionaea                pentamerous (=5), actinomorphic.  flowers protandric (open for 3 days)
Aldrovanda          pentamerous (=5), sometimes 4, actinomorphic obligately autogamous or self-fertilising. (open for 2-3 hrs)
Drosera                pentamerous (=5), actinomorphic (generall)
                               tetramerous (=4)  (Sect Lasiocephalla/1 species + Sect Bryastrum/1 species)
                               8-12-merous (Sect. Ergaleium/1 species) 

Stamens - anthers / filament

Dionaea                10-15 stamens
Aldrovanda          5 stamens (yellow anthers on white filaments)
Drosera                 5 stamens with bithecate anthers ( only a few exceptions)

Pistil - Style / Stigma
Dionaea                single central pistil made up of 5 fused carpels  - fused styles         -  separate/undivided styles  united style
Aldrovanda          pistil made up of 5 fused carpels. 5 separate/undivided styles each terminates in a white stigma
Drosera                 pistil made up of 3-5 (more often 3) fused carpels - separate/undivided styles
                                (In Drosera, styles are major key character)

Dionaea                 superior, unilocular, spherical ovary. basal placentation      Superior ovary - single chamber
Aldrovanda           single spherical ovary, superior ovary / has 5 parietal placentas each bearing 8-13 ovules                                
Drosera                  parietal placentation    The ovary has five parietal placentas ...                          


Dionaea               pollen tetrads are tetrahedral, pollen grains are operculate               
Aldrovanda         pollen tetrads are tetrahedral, 3 pores with an operculum united in tetrads with 4 grains                           
Drosera                multiple operculate pores  (early-branching subgenera Regiae + Arcturia)
                               inoperculate pollen (more derived subgenera Ergaleium + Drosera)                         

Dionaea               8-20 seeds per flower. (1 mm, black, pear-shaped)
Aldrovanda         5 x (8-13) seeds per flower.
Drosera               numerous seeds per flower.

Dionaea               stalked and sessile glands                           
Aldrovanda         stalked and sessile glands                           
Drosera                only stalked glands  (early-branching subgenera Regiae + Arcturia)
                               stalked glands and sessile glands (more derived subgenera Ergaleium + Drosera)                         

Bisexual Flowers:  A flower that has both stamens (male) and pistils (female) in the same flower.
Unisexual Flowers:  A flower that has only stamens (male) or pistils (female) but not both.

Monoecious Plants:  Both the male and female organs exist on the same plant.
Dioecious Plants:  A male plant produces only male flowers and a female plant produces only female flowers..

flower is said to be unisexual if it possesses either stamens (male) or carpels (female) but not both. A plant is said to be dioecious (unisexual) if it possesses only male flowers or female flowers (not both). Or a plant may be monoecious with both male and female reproductive organs on the same plant, either borne in the same flower (bisexual flower) or in different unisexual flowers.


There are two types of self-pollination: (not to be confused with self-compatibility in plants (the ability to form seed using self-pollen).

in autogamy, pollen is transferred to the stigma of the same flower.
in geitonogamy, pollen is transferred from the anther of one flower to the stigma of another flower on the same flowering plant. The pollen transfer uses a pollen vector (pollinator or wind). Wind pollination is a quite common source of self-fertilized seeds in self-compatible species.

Some plants have mechanisms to ensure autogamy:

such as flowers that do not open (cleistogamy),
or stamens that move to come into contact with the stigma.

If a plant is self-incompatible, geitogamy can reduce seed production.

Although geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant.

Monoecious plants like maize show geitonogamy. Geitonogamy is not possible for strictly dioecious plants.


Basal placentation: placenta is at the base (bottom) of the ovary. Simple or compound carpel.
Apical placentation: placenta is at the apex (top) of the ovary. Simple or compound carpel.
Parietal placentation: placentas are in the ovary wall within a non-sectioned ovary. Compound carpel.
Axile placentation: ovary is sectioned by radial spokes with placentas in separate 
locules. Compound carpel.
Free or central placentation: placentas are in a central column within a non-sectioned ovary. Compound carpel.
Marginal placentation: only one elongated placenta on one side of the ovary, as ovules are attached at the fusion line of the carpel's margins . This is conspicuous in legumes. Simple carpel.


Molecular evidence points to a common origin of the two snap-traps (Aldrovanda & Dionaea), suggesting that the snap-trap mechanism evolved only once -- therefore, the basic mechanism for these snap-traps must be similar... actually identical...

It is widely accepted that, in the case of Dionaea, the snapping of a trap leaf involves buckling. This is due to the initial convex curvature (doubly-curved) of the open trap lobes of a mature specimen. This "snap-through" buckling (or "flipping") does increase the speed of trap closure, a little. However, it has to be understood that the buckling, if it does happen, is not the main, driving force of the swift leaf closure, but rather, a result of it. The main cause of leaf closure is the pressure differential created between the opposite sides (upper & lower) of the trap lobes..... The same mechanism is responsible for the swift snap-trap of Aldrovanda (no buckling here, though).

Molecular evidence further indicates that the common ancestor of these snap-traps diverged from an ancient sundew-like plant --- perhaps the ancestor of the basal taxa, such as Drosera regia

This implies the basic mechanism responsible for the snap-trap operation has its root in the tentacle movement (and leaf folding) seen in the majority of extant sundew species (and in D. regia).

The basic mechanism for leaf motion common throughout Drosera-Aldrovanda-Dionaea evolution is most likely to be a sudden (or relatively quick) drop of turgor pressure on one side of the structure in question, creating an imbalance of pressure on the structure to cause it to bend.... In this process, the other side (epidermis) might be forced to stretch a little .... The recovery of the bending (or snapping for that matter) is achieved as a result of the side (epidermis) that lost turgor pressure restoring its lost pressure and then some to counter the stretch of the other side. This is accomplished by slow, normal, actual growth.


Physical motions in plants are caused by different pressures between the opposite sides of the structure in question: one side expands, the other side shrinks, or both --- due to either turgor change or cell growth.

1)  In Drosera tentacle bending, the pressure differential between the opposite sides of the tentacle (stalk) occurs near the base first, and then gradually moves upward in the direction toward the tentacle tip.

2)  In Aldrovanda snapping also, the sudden pressure differential between the inner and outer epidermis in the motor region occurs near the midrib of the trap, and then gradually moves upward (within the motor region) during the tightening phase of the trap closure...

3)  In Dionaea snapping, the sudden pressure differential on the upper and lower epidermis of the lobes seems to occur in the upper half of the lobes --- that effectively causes the snap-through buckling of the trap lobes --- and then the pressure slowly moves downward toward the midrib during the narrowing phase of the closure...


1.  Drosera (sundews) ---- Illustrations

 Trap type: Adhesive - Stalked gland secretes mucilage, exhibits nastic/tropistic movement to secure prey.
 Sensory cells: epidermal cells at the tip of the tentacle (covered by two layers of glandular cells)
 Perception of stimuli: 2 action potentials within 1 minute to cause a tentacle bending...
 Mechanism: Sudden drop of turgor pressure on one side of the tentacle...
                       Tentacle bending (nastic / tropistic) - leaf folding


2.  Aldrovanda (waterwheel plant) ---- Illustrations

 Trap type: Snap-trap (aquatic).
 Sensory cells: trigger hair...
 Perception of stimuli: 1 action potential to snap...
 Mechanism: Sudden drop of turgor pressure on the inner epidermal cells in the motor region...
                       motor region  - narrowing (free-side lobe / bristle-side lobe)


In the illustration below, I chose to show the frontal views of the trap (all views except one lateral view) as if the leaf blade is extending toward you (not from the tip of the trap, as in the video). You can see that by noting the bristles shown on the left of the trap.

                      Aldrovanda vesiculosa  snap-trap closure                         

Well, based on my repeated trials of measuring the screen image of this video, I concluded that, indeed, the A-B distance got shorter (in the left view of the video) after trap snapping, and therefore, the Aldrovanda closure is driven by warping of the motor zone of the lobes. This is in concert with the traditionally-held view by many past investigators, including  ...

3.  Dionaea (Venus flytrap) ---- Illustrations

 Trap type: Snap-trap.
 Sensory cells: embedded in the indentation at the base of the trigger hair
 Perception of stimuli: 2 action potentials within 20 seconds to snap...
 Mechanism: Sudden expansion of mesophyll and loosening of the outer (abaxial) epidermal cell walls...




Copyright (c) 2017 Makoto Honda. All Rights Reserved.