Planet of the Bugs: Evolution and the Rise of Insects

by Scott Richard Shaw

  Over the past 120 million years, insects have coevolved and explosively diversified in tandem with the angiosperms—the dominant forms of plant diversity in modern ecosystems. They are essential as pollinators and seed-dispersers for most of the flowering plants, whose communities would be vastly diminished if all plant-associated insects were eliminated. We often tend to think of plant-feeding insects in general as pests, but I like to point out that only a miniscule small fraction (less than 1 percent) of the total number of insect species are actually significant pests. In fact, most of the plant-feeding insects should be considered beneficial for two reasons. First, they reduce the reproductive output of particular plants by putting stress on them. That sounds bad if the plant is an agricultural crop, but in a natural setting, such as a tropical forest or a mountain meadow, that plant feeding has a very desirable outcome. It prevents particular plant species from becoming superabundant and weedy, allowing vastly more species to coexist in much smaller spaces. Plant-feeding insects are a driving force in the evolution of plant community species richness, and so the extraordinary plant diversity of tropical habitats is largely due to insect diversity, not despite it. Second, but of no less importance, the majority of plant-feeding insects are themselves edible to other kinds of wildlife. Many insects are a fundamental and nutritious food source for most kinds of vertebrate species, including fish, amphibians, reptiles, birds, and most mammals, including primates and even humans. Not many organisms totally depend on humans for their continued existence, but a large part of living plants and terrestrial animals depend partly or entirely on insects for their survival.

  Whether or not they rule the planet, insects certainly have largely overrun it. They can be found in abundance in virtually every kind of terrestrial habitat, from tropical rain forests to deserts, in meadows and prairies, from sea shorelines to alpine tundra and Andean páramo. Aquatic insects not only inhabit mountain streams, rivers, waterfalls, seepages, lakes, ponds, swamps, and salt marshes, but they even occupy mud puddles, sewage ponds, craters in rocks, tree holes, pitcher plant leaves, and bromeliad leaf bases more than a hundred feet above the forest floor. Semiaquatic insects exploit the force of surface tension to skate across still ponds and lakes, while the ocean water strider, genus Halobates, has been seen walking on the ocean surface hundreds of miles at sea. Clouds of millions of African migratory locusts have flown across the entire Atlantic Ocean to land in the Caribbean Islands. The insect macro-societies, ants and termites, are essential soil movers in the Amazon basin, where their biomass outweighs the biomass of vertebrates. But sheer insect abundance is not strictly a tropical phenomenon. Even near the Arctic Circle, the combined weight of biting flies and midges outweighs that of the mammals.

  Insects and their relatives have evolved and adapted to some of the most extreme conditions on the planet. Stoneflies have been recorded at an elevation of 5,600 meters in the Himalayas, while subterranean species of beetles, crickets, and cockroaches have adapted to life in caves deep underground. Some aquatic stream beetles breathe across the surface of an air bubble and can stay underwater indefinitely. Brine flies, shore flies, seaweed flies, and deer flies have developed extreme tolerance for high levels of salt and live in salt marshes and salt flats and along ocean shorelines. Springtails have evolved antifreeze compounds in their blood, and some are among the most abundant organisms on sub-Antarctic islands. At high elevations worldwide, species of icebugs, springtails, snow scorpionflies, and some flightlesstipulid flies are active on the frozen surfaces of snow fields and glacial ice. Living chironomid midge larvae have been dredged up from the depths of Lake Baikal in Russia, where they have adapted to a low-oxygen environment by evolving hemoglobin-like blood pigments. The adaptability of water boatmen bugs is remarkable: some inhabit salty water below sea level in Death Valley, California, while others live high in the Himalayan Mountains. Some swim in frigid water under ice, while others thrive in hot springs at temperatures up to 35°C. The Yellowstone hot springs alkali fly develops in the edges of scalding hot water pools with temperatures up to 50°C. Other fly larvae living in arctic ponds are known to survive winter cold temperatures as low as −30°C. One of the most impressive organisms is the South African chironomid midge fly, Polypedilum vanderplanki, which has adapted to extreme drought conditions by evolving cryptobiosis—a suspended-animation condition where larvae become dehydrated and tolerant to the most extreme conditions. It has been reported that these dehydrated fly larvae can tolerate immersion in boiling water as well as being dipped into liquid helium.

  FIGURE 1.1. Common denizens of the leaf litter, springtails (order Collembola) tolerate many environmental extremes. (Photo by Kenji Nishida.)

  Most insect species are not nearly so tolerant of a wide range of extremes, and indeed, many fresh water stream insects have such a narrow range of acceptable conditions of water temperature and oxygen levels that they are very valuable to us as bioindicators of good water quality. On the other hand, hundreds of thousands of tropical plant-feeding insects have evolved physiologies that allow them to feed on and metabolize plants that are highly toxic to mammals and most other animals. Many tropical caterpillars are able to feed on toxic plants containing hundreds of chemical compounds that would kill a human. Other insects are remarkably tolerant of exposure to heavy metals, and even to poisonous chemicals specifically engineered to try to kill them. Hundreds of insect species have been reported to have evolved resistance to insecticides, and despite our best attempts to eradicate certain pest species over the past century, we have not exterminated a single one to extinction. Ironically, we can’t seem to eliminate any of the ones we would really like to be rid of, like the malaria mosquito, the human body louse, the rat flea, or the house fly, while at the same time probably millions of nontarget tropical insect species may be immediately threatened with extinction by our unfortunate habit of sheer habitat destruction.

  Perhaps it is easy to sound impressive by saying that there are more than one million insects, or anything else for that matter. Most of us don’t own a million of anything, so in practice we don’t count that high very often. But what really makes insect species diversity remarkable is not just the astronomically large number but the fact that we are talking about unique and different entities. To really grasp how extraordinary that is, one needs to begin with a clear understanding of what it means to be a species.

  “And Whatever the Man called Every Living Creature—That Was Its Name”

  In biology, the species is the most fundamental category for defining the kinds of living things. Since there are millions of different kinds of living organisms, you might not be surprised to learn that even biologists have a hard time coming up with a single definition for species. What works well for defining species of butterflies and beetles might not work as well for defining species of flowers, fungi, protozoa, and bacteria. Among the more popular ideas for defining species are the biological species concept, the evolutionary species concept, the ecological species concept, and the morphological species concept.

  The biological species concept defines species as populations of individuals that are able to interbreed and produce viable offspring and are reproductively isolated from other such populations. In other words, biological species consist of groups of individuals that will mate with one another but will not normally interbreed with other species. This concept works very nicely for most sexually reproducing insect populations, such as butterflies and bees. To cite a familiar example, the monarch butterfly (Danaus plexippus) is a very well-known and widely recognized insect species. The viceroy butterfly (Limenitis archippus), the well-known mimic of the monarch, looks superficially similar in color patterns but is a distinct and separate species. If you are patient and an observant naturalist, you will see male monarch butterflies courting and mating with female monarch butterflies, and you can observe male viceroys courting and mating with female viceroys. However, you won’t find monarchs and viceroys interbreeding with each other or
with any other species, for that matter. The biological species concept attempts to recognize and name the fundamental groups into which organisms naturally segregate themselves. In that regard, the species category is interesting, because it attempts to recognize groups that are not arbitrarily defined but have an underlying reality in nature.

  The main problem with the biological species concept is that it does not apply well to species that reproduce asexually, such as many plants, fungi, bacteria, protozoa, and even some kinds of insects. Many aphid species, for example, reproduce rapidly by having several generations of females that asexually produce more females without mating. Among the parasitic wasps there are many known species where females simply produce more females by asexual reproduction and males are totally unknown. The evolutionary species concept attempts to solve this issue by defining species as separate biological lineages that share a unique evolutionary history and are genetically distinct. As a theoretical concept this definition is more broadly applicable to all groups of organisms, but in practice it can be difficult to apply. If we see male and female monarch butterflies mating, that provides compelling evidence that we are observing two individuals of the same biological species. Getting DNA samples from those same two butterflies and assessing that they belong to the same evolutionary species is still an expensive and challenging technological task. While our technology may be moving in this direction, the fact is that we only have assessed DNA “fingerprints” for a small fraction of insect species.

  The ecological species concept defines species based on their ecological niches, that is, the unique combination of their habitat, feeding, environmental quality, and behavioral requirements. While the monarch and viceroy butterflies might at times occupy the same habitats in Canada, monarch caterpillars will feed only on milkweeds, while viceroy caterpillars will feed on willows, something a monarch would never do. The two species differ in their degree of cold tolerance and solve the problem in different ways, monarchs by migrating southward to Mexico, and viceroys overwintering as cold-tolerant, partly grown caterpillars. So the two species occupy different habitats at different times, and they utilize different resources for their development. A key part of the ecological species concept is the idea that no two species can occupy exactly the same ecological niche. Because they compete for living space and resources, species tend to diverge so that they adapt to use the world in slightly different ways. While this seems to provide a satisfying definition of how monarchs differ from viceroys, even the ecological species concept has a fundamental practical flaw: we don’t know the ecological niches of many of the species that have been discovered.

  Most named insect species were proposed based on the morphological appearance of collected specimens, size, color patterns, body form, and other distinctive anatomical characteristics. This brings us to the oldest and perhaps most fundamental definition: the morphological species concept, which characterizes morphospecies based on their anatomical appearance. This may seem old-fashioned or somehow less satisfying than the other species concepts, but in most cases it is extremely practical. I don’t need to observe mating behavior, gather DNA evidence, or observe the larval food plants to tell the difference between a monarch and a viceroy butterfly. Just put a specimen in front of me, or even a photograph, and I’ll tell you correctly which species it is, based only on its morphological appearance. Those two species each have unique and distinctive wing patterns, and people have been successfully recognizing monarchs and viceroys for more than two hundred years. Admittedly, there are some issues with the morphological species concept. Ranges of variation need to be assessed and understood, such as differences between sexes and variations between immature and adult stages. Also, we understand that in some cases there are such things as cryptic species that appear morphologically identical but can be differentiated by behavioral or genetic evidence. But the vast majority of living species can be defined based on their morphological appearance, and, as a practical matter, the species of most fossilized organisms can be defined based only on their morphology. This operational definition once prompted the paleontologist David Raup to remark, a bit cynically, that “a species is a species if a competent taxonomist says it is.”3

  While it is important to conceptualize what a biological species is in theory, it is also valuable for you to realize what a species is, in practice. For the past 250 years or so, biologists have been naming new species, and since 1961 this has been done according to various rules set forth in the International Code of Zoological Nomenclature. To describe and name a new insect species, the rules do not require you to have DNA samples, know the ecological niche or the evolutionary history, or even to observe mating biology. The code does require that you have a specimen, or part of a specimen, that can be observed and described and archived for reference in a museum collection. The actual process of naming a new insect species involves describing the morphological characteristics of the proposed new species, giving it a name, and publishing this information in a scientific journal; the date of publication is what makes the name official. Our system of naming species always uses binomial nomenclature requiring two words to state the full scientific name of a species: the first word is the genus name and second is the species name or epithet. Those two words form a unique combination, so that the species name for every living species is unique and distinctive. The species name is always Latinized but need not be complicated or difficult to learn (you probably already know your own species name, Homo sapiens). The specimen is kept in a museum collection for future reference, but for the most part, species become known by what is published about them in the scientific literature. So under the taxonomic species concept that is universally used for naming and discussing insects, a species is first defined as a set of organisms with a certain stated series of shared characteristics.

  I prefer to think of naming new species as making a species hypothesis. When we define a species based on morphology, we are essentially hypothesizing that the same biological, evolutionary, and ecological species exists with that form. The species hypothesis is tested with each addition of new information. We hope and expect that in the future we will learn the biology, evolutionary history, and ecological niche of every named species, thereby corroborating the morphological species that have been proposed. With the discovery of new specimens, we gain new information about variations, and the taxonomic concept of an organism may be modified and expanded to include this new information. If the discovery of new information suggests that a named animal or plant is merely a population of some other species, then the older name is preserved and the more recent name is “sunk” and becomes a junior synonym of the older valid name. Or, if a population is discovered to consist of multiple cryptic species, then new names can be added to recognize the newly discovered biological species. Naming a new creature is just the first step in a long scientific process of developing a fuller understanding of the organism, but that process necessarily starts with simply providing a morphological description of the appearance of the beast. Having mentioned that scientists have been attempting to do this work for 250 years maybe makes it sound as if we might be nearly done with discovering and describing species. But keep in mind that given the current rates of new discoveries, description, and publication, it could take an estimated 500 more years just to provide names and morphological descriptions for the remaining insect species.

  However you define them, there are at least several million insect species on this planet, and they all have one fundamental common feature: they are unique populations. It’s not like having millions of the same thing. In some cases, sheer abundance of one insect species is amazing by itself. It’s impressive to know that one ant super-colony might have millions of nearly identical workers. But it’s mind-boggling to understand that this planet has millions of distinct insect species populations, each with its own unique reproductive biology, ecological niche requirements, genetic and evolutionary history, biochemistry, anatomy, and beh
avior. It’s hard to conceptualize even the one million or so named insects. To visualize how many that really is we might look at it this way: since each species name consists of two words, it would require printing two million words just to state the names of the known insect species; I’d need twenty books about this size just to list all the insect names in print, in sequence. That’s just the living modern insect species, not the extinct ones known from fossils or the millions of other unnamed ones living high up in tropical forest canopies. I can’t hope to tell you something interesting and unusual about every single insect species, although every one certainly has a fascinating tale to tell. So, we need a system of grouping species together into meaningful categories for discussion.