Once the trap fully closes, the leaves form an airtight seal so that:
- digestive fluids and insect parts are kept inside the trap
- bacteria and molds can't get in
To make sure that the insects are contained within the trap, the edges of the leaves have finger-like cilia that lace together when the leaves press shut. These long, hair-like projections make the plant look like it has spiny teeth; but the cilia are really only used to latch the trap shut.
There is an upper limit to the size of insect that the trap can accommodate. At most, traps are about 1 inch long, and an insect ideally should be about one-third of this size. If an insect is too large, the trap will not be able to form a seal tight enough to keep bacteria and molds out. Once bacteria and mold have a way in, they can proliferate as they feast on the decomposing insect, and the leaves of the trap will succumb to the assault as well. The trap will turn black as the leaves rot, and the whole thing will eventually just drop off the plant.
Venus Flytraps can tolerate losing a trap here and there, because the plant can eventually sprout new ones. Like a household product manufacturer, nature has engineered the traps with planned obsolescence. After about 10 to 12 closures (partial or complete), the traps lose the ability to capture anything. The leaves remain spread wide open, and instead of going through the ritual of attracting insects and eating them, the former trap devotes its energy to the process of photosynthesis for the remainder of its life span, usually around 2 to 3 months. This way, if a trap is repeatedly stimulated by non-edible objects, the plant can recoup some of the energy and ATP lost to opening and closing the trap during this time spent focusing solely on photosynthesis.
But back to dinner -- once the insect is firmly ensconced in the trap, the process of digestion can begin. The trap now serves as a miniature stomach. Just like our stomachs, the trap secretes acidic digestive juices that:
- Dissolve the soft tissues and cell membranes of the food
- Serve as an antiseptic to kill small amounts of bacteria inadvertently eaten or sealed in with the food
- Enzymatically digest DNA, amino acids and other cellular molecules into small pieces that can be taken up by the plant
These digestive juices are secreted from glands on the inside surface of the trap, right onto the trapped prey. The insect is bathed in these juices over a period of five to 12 days, during which the insect is digested and nutrients are extracted. The time it takes depends on:
- Size of the insect - The larger it is, the longer it takes to break down.
- Age of the trap - The digestive fluid is recycled after each digestion, and an older trap may secrete a somewhat weaker mix of acid and enzymes.
- Temperature - The ambient temperature can affect the rate of decomposition, and up to a point, increasing temperature makes enzymatic processes much snappier.
The process continues until all that's left of the insect is its hard exoskeleton. (Unlike humans and other vertebrates, who have an internal rigid skeleton made out of calcified bone tissue, insects and arachnids use a more flexible, external exoskeleton to both protect and form the framework for their bodies.) Once the nutrients are depleted from the acidic bath, the plant reabsorbs the digestive fluid. This serves as a signal to reopen the trap, and the remains of the insect are usually either washed away in the rain or blown away by the wind.