Insectivory “Insectivory” translates as the eating of insects. While this is not technically accurate - plants do not actually “eat” insects - it is true that certain species of plants assimilate nutrients directly from decomposed insects. The most important element afforded to insectivorous plants is nitrogen in a form that is useful to plants. Furthermore, it is also true that these insectivorous plants have an active role in 1) trapping insects (and presumably other small organisms) and 2) digesting their soft tissues, often using digestive enzymes produced by the plants themselves. Insectivory has apparently evolved several times among flowering plants. The evidence for this is that there are insectivorous plant species in several distinct/unrelated lineages within the angiosperms. Under what kinds of conditions might insectivory have evolved and persisted? In other words, what does insectivory provide that is not provided by normal metabolic functions of plants? Well, the answer is really “nothing” - most plants can get nitrogen without digesting insects. However, there are certain environmental conditions where nitrogen availability is diminished. Most plants receive nitrogen from the soil in which organic nitrogen-rich molecules are broken down by soil bacteria. What would happen if the soil bacteria were not able to break down the organic molecules into useable (to the plants) nitrogen? In fact, insectivorous plants generally grow in soils that are less conducive to growth of nitrogen-converting bacteria (for example, very low-pH soils). The ability for the insectivorous plants to augment their nitrogen supply, via small insects that are rich in nitrogenous organic molecules, allows these plants to circumvent the dangers involved in low-nitrogen soils. Let’s leave the physiological assimilation of nitrogen to another course, and concentrate on the physical modifications of plants to trap insects for part of their nitrogen supply. The mechanisms for trapping can be sorted into three basic patterns:

• Pitfall Traps. Here, leaves are modified to pitcher-like shapes into which unwary insects fall and out of which they cannot escape. The epidermis of the inner part of the pitcher may be very waxy (i.e., slippery), and/or it may have down-pointing trichomes that allow the insect to crawl down (with the trichomes) but not back up (against the trichomes). Once the insect has fallen into the bottom of the pitcher (which generally has fluid in it), it’s soft tissues are digested, either by bacteria that live in the leaf, or by digestive enzymes produced by the plant itself.

• Glandular Traps. In this case the plant produces sticky substances in glands, commonly at the tips of trichomes. The glands also produce digestive enzymes. When a small enough insect comes into contact with the glands it is stuck in a glue-like substance, and the more it struggles, the more it becomes entrapped. The common sundew plants represent this syndrome.

• Thigmotropic Traps. “Thigma” refers to touch, and when a thigmotropic plant part is touched it moves (you’ve all probably played with the sensitive plant, whose leaves collapse when touched). Thigmotropic traps are alarmingly similar to human-made animal traps. They snare the insects when the insect inadvertently touches a responsive structure. The response of the trap must be fast enough to snare the insect before it flees. The commonest example to most of us is the Venus’ fly-trap. Sadly, while this species is commonly available in stores and is grown by many people, it is extremely rare in the wild, known only from coastal swamps in North Carolina and adjacent Virginia.