What do you think of when you think of flies? Annoyance at picnics or in the house? Maybe you think of the itching bites that some species inflict? In reality, flies are an extremely diverse group of insects that lead complex and often dangerous lives. Flies can be predators, pollinators, pest-controllers, blood-suckers, herbivores, and even caring parents. But in the bogs of the southeastern United States, there is a group of flies that spend their entire lives in and around an organism that has spent the past 3 million years perfecting a way to eat them. And they’re not alone. Why put themselves in such mortal danger? To find out, we’ll first need to explore the fly-eating creature itself: the carnivorous Pitcher Plant.
Pitcher plants are native to North Carolina and various species occur throughout the southern and eastern coastal plains of North America, from Texas in the west all the way to Canada in the north. Despite this wide range, pitcher plants are relatively “young” in evolutionary time scale. The 3 million years or so that they’ve spent on Earth are nothing compared to most other plant groups. For example, oak trees first appeared around 30 million years ago! Despite only just showing up to the party, pitcher plants have undergone remarkable adaptation to the nutrient-poor environments in which they live. All plants get energy from the sun and turn it into sugars via photosynthesis, but they also need nutrients from the soil in order to grow and stay healthy, just like we need vitamins from the food that we eat. The soils found in bogs have very little nutrients, and so pitcher plants have had to turn to insects to get the materials that they need to survive. Several plants have evolved to eat insects, like the famous Venus Fytrap (also a North Carolina native) and they each have their own way of trapping prey. Pitcher plants use modified, tubular leaves lined with slippery wax and downward facing hairs; a one-way ticket down for any insect unfortunate enough to fall in. Down at the bottom of the pitcher, bacteria and enzymes break down prey into a nutritious soup that the plants absorb. This pitcher leaf, evolved for the sole purpose of trapping and digesting small animals, seems like the last place any insect would willingly venture, but there are actually several insects and arachnids that use this trap as a way to catch food, a place to live, and even as a nursery for their young! The pitcher leaves may seem dangerous, and indeed they are, but the space that they create and the prey that they contain are ecological resources that are worth risking life and limb to exploit. There are spiders that use the luring nectar of the pitcher to attract prey, moths that spend their entire lives clinging to the interior wall of the pitcher’s trap and eat it from the inside, and of course, the Pitcher Plant Fly.
Pitcher plant flies are flesh flies, the sort of fly you’d expect to find on roadkill or buzzing around a poorly-cleaned dog park. As such, they have a particular fondness for dead and decaying things, and are important decomposers. But these flies see rotting flesh less as food for themselves and more as a nutritious meal for their developing young. Their larvae live in and feed on decaying material until they are ready to pupate, and ultimately emerge as adults who go off and find rotting food for their own young. As rotting flesh tends to last for only a short time, they don’t want to waste a minute of potential feeding time, including time spent hatching from an egg. To solve this problem, flesh flies give live birth, and even have internal “milk” glands to nourish their unborn offspring, who can begin feeding as soon as they are born. Unlike their fetid cousins, pitcher plant flies have no interest in flesh or feces. Instead, they deposit their larvae directly down the maw of the pitcher plant. Infanticide? Quite the opposite, in fact. Once down in the pitcher plant’s trap, the larva feeds on the prey that the plant has so patiently captured, safe from other animals that would eat or parasitize the defenseless maggot. Though it may seem that the larva is stealing the plant’s catch, in fact it is likely that the larva is doing the plant a service. As the larva eats, it also excretes, and that excretion is much easier for the plant to digest than an intact insect. (After all, that’s the active ingredient in many mulch mixes.) But why isn’t the larva digested by the plant like the other insects that fall inside? Though the answer isn’t known, scientists have a few hypotheses. As mentioned before, other flesh flies live in harsh environments like rotting meat where bacteria and other microbes secrete strong flesh-melting compounds to break down tissue. To defend against these chemicals, flesh fly larvae have a very thick skin, called a cuticle, that protects them. This skin may also have enzymes that neutralize the compounds given off by the microbes. It may be that this same defense protects the larvae of the pitcher plant fly from harm down in the digestive pits of the pitcher plant. Whatever the reason truly is, the larvae face no great threat from their host. Once the larva has fed and grown, it’s time to pupate and become an adult. The larva chews a small hole in the side of the pitcher, crawls through, and completes its pupal stage on the moist soil of the bog. As a fully-fledged adult, the fly never truly leaves the pitcher plants. Though they no longer venture down into the trap, adult flies frequently rest on the top or sides of the pitcher plants and drink the sweet nectar that exudes from the pitcher’s lip. This extraordinary relationship means that these flies are only found where pitcher plants thrive, and this habitat is becoming increasingly scarce and fragmented due to human activity.
From our southern Appalachian Bogs in the mountains to the southeast coastal plain, East Carolina University is busy studying the flies that live in the pitchers, like the pitcher plant fly genus Fletcherimyia. This genus has eight species, each of which only deposits its larvae into a few species of pitcher plant. A few fly species only live in a single pitcher plant species! But how did this happen? Why so many different species with such particular behaviors? This series of relationships may have been a result of cospeciation, a process that occurs over evolutionary timescales. In this scenario, the first pitcher plant fly may have developed a relationship with the first pitcher plant, likely behaving in a similar way to the flies that exist today. Over time, geological forces and shifts in climate may have isolated groups of pitcher plants from one another, and therefore the flies would have been split with them. In their new separate environments, the plants may have faced different pressures and therefore developed different adaptations to face them, and the flies changed to adapt to the plants. The result is two new species of pitcher plant, each with its own species of fly. If this process happened multiple times over millions of years, it may have resulted in the eight fly species and ~10 pitcher plant species that live in North America today. It isn’t known exactly how this relationship developed, but researchers from ECU are keen to turn back the evolutionary clock to begin to find out. North Carolina is a hotspot of diversity for the pitcher plants and the flies that inhabit them, and The Nature Conservancy maintains a wide variety of habitats that allow them to continue their death-defying relationship in the face of human activities such as habitat destruction, pollution, and poaching.