Carnivory and symbiosis are two topics within the field of botany that are endlessly fascinating. Because they are static entities, plant evolution has gone through some very interesting pathways for survival. Recently, a group of plants found only on the southern tip of Africa have shone a light on yet another interesting plant/insect relationship that is unlike any other yet known to science.
The genus Roridula contains two species that, for all intents and purposes, look like carnivorous plants. They closely resemble sundews in having leaves packed full of sticky hairs that ensnare hapless insects. However, they are neither closely related to sundews nor do they have any sort of digestive enzyme for breaking down their insect victims. Why then would these plants go through the trouble of producing glandular traps? There must be some adaptive benefit to make up for the cost of production. A closer look at these plants revealed that indeed there is.
Living on Roridula plants are tiny capsid bugs that are covered in a special waxy substance that keeps them from getting stuck in the sticky traps. The bugs move about the sticky leaves, looking for trapped insects. When the insects are found, the capsid bugs impale them with their proboscis and suck them dry. As the capsid bugs feed, their droppings end up littering the Roridula leaves. This is how Roridula gets the added nutrients it needs to survive. Although the plants are not capable of actively digesting the full insects, they are capable of absorbing the components of the capsid bug feces. They are literally getting a little bit of fertilizer every time a capsid bug goes to the bathroom. By offering the capsid bugs a place to live and plenty of free, immobilized prey, the plant is able to get nitrogen-rich meals in return!