Flying animals have had a major impact on nonflying organisms. Briefly consider the ecological and evolutionary interrelationships between pollinators and flowers, or between mosquitoes, the parasites they transmit and humans. Even a cursory glance at the manifold relationships flying insects have with all other forms of terrestrial life evaporates any doubt whether the world would be a very different place if they had never evolved.
The evolution of flight in insects, as well as the other three groups that independently evolved flying (bats, birds, and pterosaurs), was somewhat of a conundrum for biologists around the time On the Origin of Species was published (1859) and for Darwin himself. Criticism focussed on the question of how complex structures can arise when a simple version of that structure cannot do the job of the complex one; this argument boils down to What good does half a wing or half an eye do? No good at all! Therefore, how do these structures arise? Predictably, the explanation came from Darwin and is still generally accepted to this day: that structures may have different functions and advantages in response to different selective pressures at various times in their evolution (Gould, 1990). Essentially, the structures that developed into wings at a later evolutionary time might have had a function other than producing lift and thrust. Evolutionary scientists use the catchy term “functional shift with structural continuity” to describe this principle (Kingsolver and Koehl, 1994).
There are several hypotheses that attempt to explain the process by which wings were evolved in insects. Each hypothesis suggests wings developed from outward growing body parts that had a previous function and later became larger and more specialised toward flight, only differing in which body parts wings originated from. The idea with the most convincing evidence is the “paranota with leg gene recruitment hypothesis” (Staniczek et al., 2011). It predicts wings first originated as lateral outgrowths of tergal plates (strong dorsal plates on insects) called paranota, which then only later became mobile and articulated because of a recruitment of leg genes. However, there is still considerable speculation and disagreement in the area because of the very poor fossil record during the period between 400-340 million years ago. Unfortunately, the fossil record of insects before they evolved flight is nearly completely absent; only a few specimens – from 400 million years ago – have ever been found, all of which are wingless (Engel and Grimaldi, 2004)[*]. The few early fossils indicating flight are ironically of isolated, detached wings, and seem fully functional (around 340 million years old) (Alexander, 2002). Thus, most of the very earliest winged insects we know of were already proficient in the air. Palaeobiologists can therefore conclude that the evolution of flight in insects and all of the related important milestones took place during a 60 million year period between 400-340 million years ago in the Carboniferous.
Ergo, insects have been flitting around for at least 340 million years. Interestingly, it may have actually been a self-contained evolutionary arms race that stimulated a range of insects to develop advanced proficiency in flight. In a three-dimensional environment, such as a dense collection of fern-like plants in the mid-Devonian (e.g. Wattieza spp.), which could grow to about 8m tall, early insects likely went from plant to plant and fed on spores (Stein et al., 2007). Other plants quickly developed woody buttresses, and by the end of the late Devonian several other clades grew to heights of up to 30m, such as Archaeopteris, a group of trees with fern-like leaves (Retallack, 1985). This structural variety may have posed problems for wingless insects, as moving from one plant to another would either involve hurling themselves off a 30m plant with reckless abandon, or climbing down the trunk of one plant and up the trunk of the other. It may be that insects saved time by jumping off and, as we can see in many modern species of vertebrates (e.g. gliding lizards – Agamidae: Draco; gliding snakes – Colubridae: Chrysopelea; flying squirrels – Sciuridae: Pteromyini; sugar gliders – Petauridae: Petaurus; colugos – Cynocephalidae: Cynocephalus; flying frogs – Hylidae: Ecnomiohyla), structures already present that happened to aid in horizontal movement whilst in the air may have gradually been under different selection pressures: to increase the horizontal distance these falling insects can travel. Many of these ancient insects had fairly unspecialised dicondylic (two-jointed) chewing mandibles (Blanke et al., 2015) and some became predatory, feeding on their winged comrades whilst they tried to consume spores. Predators would need better flight capabilities than their prey, and this is supported in the later Carboniferous fossil record by insects like Meganeura, the large predatory relative of dragonflies. The advent of complex flight behaviour can then be explained by the well-established ‘arms race’ between predators and prey (Dawkins and Krebs, 1979).
Despite this early level of self-contained co-evolution, winged insects have influenced almost every aspect of terrestrial life and changed the course of global evolution since their proliferation. The most obvious example of this is the complex relationship between angiosperms (flowering plants) and insects. Angiosperms have existed for 160 million years, overtaking gymnosperms as the dominant plant group at least 66 million years ago (APG, 2016), and are, by the majority of measures, the most common and successful plants today, comprising roughly 300,000 described species (Christenhusz and Byng, 2016). By the beginning of the Eocene (60 million years ago), angiosperms were already extremely diverse, showing clear characteristics they were pollinated by specific insects (Crepet, 1983). The anatomy of flowers similar to snapdragons and larkspurs, in part pollinated by bumble-bees, and tubular flowers like morning glories, pollinated by butterflies and moths, are common in the fossil record from the early Eocene. Interestingly, the rapid radiation of angiosperms coincides with the presence and radiation of the Hymenoptera (ants, bees, wasps, and sawflies) (Alexander, 2002). Bees in particular are specially adapted for consuming pollen and nectar – the latter adapted as a “reward” for insect visitors in flowering plants. The benefits visiting insects afford the flowers by pollination must outweigh the cost of producing nectar. Not just that, but pollination biologists suspect the fitness advantage angiosperms receive from pollinators was very likely present before the production of nectar and the proliferation of flowers similar to those found in the modern day (Crepet, 1983). Like gymnosperms, the ancestors of flowering plants were wind-pollinated. However, pollination was certain to occur frequently if insects began routinely visiting flowers of the same species to feed on the protein-rich pollen around the anthers (pollen-producing organs). The reproductive organs of plants can increase this beneficial effect by becoming more attractive to those insects eating the pollen (e.g. presenting bright colours and producing nectar, like flowers found today). Enticing insects to aid in pollination gave early forms of flowering plants a huge competitive advantage; by delivering expensive, protein-rich pollen directly from the anthers of one flower to the stigma (pollen-germinating organs) of another, insects increase the chances that a flower becomes pollinated, which in turn means less pollen needs to be produced than the vast quantity required for wind-pollinated plants. Some modern-day angiosperms have secondarily evolved wind dispersed pollen, but the vast majority are pollinated by insects. In temperate regions, 78% of angiosperm species are animal pollinated, but this rises to 94% in tropical regions (Ollerton et al., 2011)[†]. Seeing as almost all species of pollinator are winged insects, the success of angiosperms – by far the most dominant plants in temperate and tropical regions – can be in part attributed to their associated insects. It’s not an overstatement to claim that where angiosperms dominate, whole ecosystems would collapse should they be removed. A large majority of these in turn depend on their insect pollinators. Is it therefore a hyperbolic claim to say that temperate and tropical ecosystems are so structurally diverse and species rich because of the co-evolution of insects and angiosperms, giving rise to the current plethora of species we all require to not just to enrich our lives, but to exist? Do we owe our survival, nay, our very existence to insects!? Whilst our current-day survival is without a doubt dependent on insects, nobody can confidently predict what the course of evolution would have led to if the insect-angiosperm relationship never took off. I suppose we can only speculate…[‡]
Although angiosperms and their associated insects have affected the course of evolution on a global scale, winged insects profoundly influence the current state of the world, ecologically and anthropocentrically. Here, I shall concentrate on the anthropocentric effects winged insects have. Even a brief overview of the ecological effects insects have on the environments in which they are found are far too numerous to tackle in a blog post; an MSc thesis, PhD, text-book or life-long career would be far more appropriate formats to explore that subject.
Though the three most important crop species – wheat, corn, and rice – are wind-pollinated, the majority of crops require or benefit from pollination by insects. Citrus fruits, blackcurrants, pumpkins and squash, cranberries, blueberries, strawberries, apples, pears, passionfruit, cherries, peaches, plums, alfalfa and clover are all primarily pollinated by bees, though many other insect groups also pollinate these crops (Free, 1993). Dipterans (true flies) are the primary pollinators of cacao and cashews (Free, 1993). Many of these crops are also damaged because of herbivorous insects, including species in the Orders Hemiptera (aphids, leaf- and planthoppers), the larvae and slightly less commonly the adults of Lepidoptera (butterflies and moths), Coleoptera (beetles), and Diptera, thrips (Thysanoptera), Orthoptera (locusts and crickets), as well as the non-flying ant (Hymenoptera) and termite (Blattodea: Isoptera) workers. These in turn are controlled by insect predators and parasitoids such as ladybirds, lacewings, and various species of parasitoid wasps that can occur naturally or be purchased by farmers to disseminate at precise intervals throughout the growing season. This is an effective treatment in the era of pesticide resistant crop pest species and large scale environmental damage caused by overzealous spraying of insecticide. In addition to the detrimental damage of crops by insects, they may be vectors of diseases that can devastate crop plants. Aphids transmit numerous viral diseases, including squash mosaic disease, bean mosaic disease, and potato leafroll; thrips may transmit tomato spotted wilt virus and at least 19 other viral diseases to crop species (Nault, 1997; Morse and Hoddle, 2005).
Of course, disease transmission is not limited to herbivorous crop pests. People are significantly affected by diseases spread by haematophagous insects, most of which are winged. Few animals are as globally hated as the mosquito. Some of the most important vectors and the diseases they transmit are listed below (all data below sourced from WHO, 2017):
- Anopheles spp.
- Plasmodium, the apicomplexan parasite responsible for malaria in humans and other vertebrate species. In 2015, an estimated 214 million people were infected with malaria, of which 500,000 died.
- Filariasis, infestations by filarial worms, transmitted by various mosquito species (not just Anopheles). This can come in many forms due to the variety of parasite species and immune responses (Farrar et al., 2013). The commonest form of the disease, lymphatic filariasis, infects an estimated 40 million people, with 13 million incapacitated or disfigured by the disease.
- Aedes aegypti
- Yellow fever, caused by a species of RNA virus in the Flavivirus genus. 200,000 cases year, causing 30,000 deaths.
- Dengue fever, again caused by a species of RNA virus in the Flavivirus genus. A recent estimate suggests 390 million people become infected by the virus per year, of which 96 million manifest clinically (i.e. dengue fever), of which 20,000 die.
- Zika fever, caused by another species in the RNA virus genus Flavivirus. No known deaths caused by Zika virus; similar, but much weaker, symptoms than dengue fever.
- Chikungunya, caused by the alpha virus in the family Togaviridae. Whilst decreasing rapidly in frequency, just 3 years ago, 600,000 people were infected, killing roughly 600 people. Only 31,000 cases were reported for 2016.
- Culex spp.
- Various viral diseases in the Flavivirus genus (e.g. West Nile Virus and Japanese encephalitis).
- Various filarial worms and parasites, often quite important diseases in wild and domestic animals.
- Anopheles spp.
- Tsetse flies (Glossina)
- Vectors of Trypanosoma brucei, the cause of African trypanosomiasis or sleeping sickness. As of 2015, only 2804 cases were recorded, down from the 9,000 recorded deaths just five years before.
- Sand flies (Lutzomyia in the New World and Phlebotomus in the Old World)
- Vectors of Leishmania protozoan parasites, which cause leishmaniasis. As of 2016, 900,000-1.3 million new cases per year, resulting in 20,000-30,000 deaths.
- Black flies (Simulium yahense)
- Vectors of the parasitic nematode Onchocerca volvulus, causative agent of “river blindness”. 17-25 million people infected, 800,000 of which have impaired or lost vision.
- Kissing bugs (Hemiptera: Reduviidae: Triatominae)
- Vectors of the protozoan parasite Trypanosoma cruzi, which cause American trypanosomiasis or Chagas disease. 6-7 million people infected, 12,500 deaths per year.
Indeed, such a list shows the staggering effect flying insects can have on humans when they transmit diseases.
We must remember that the insects are being somewhat hijacked by the parasites (Smallgange et al., 2013) and their fitness is often reduced compared to uninfected individuals (Ewald, 1983; Anderson et al., 2000). Though the above insects are parasites, drinking the nectareous blood from us conscious human-beings, the real damage they cause results from their transportation and injection of microparasites into our bodies. Though many die from these diseases, many more suffer increased morbidity throughout their lives if treatment is unavailable. In addition, these diseases are almost exclusively associated with the tropics[§], where poverty is more common and treatment and prevention schemes insufficient (The World Bank, 2016; WHO, 2017). Whilst this likely plays a role in global disease incidence figures, the currently accepted hypothesis explaining the high diversity and incidence of tropical disease is a combination of several factors. The increased temperature, rainfall, and humidity allow dipteran vectors of disease to develop quickly from larvae to adults in abundant breeding grounds. This is paired with the lack of a winter season that, in temperate zones, limits insect populations by forcing them to hibernate. The tropics have a far more diverse array of species across the board and this includes reservoir wildlife species that carry diseases transmissible to humans (zoonoses), as well as the winged insect vectors themselves. These factors accumulate to create a perfect storm of disease around the equator, causing morbidity and mortality to people on a global scale.
In essence, winged insects have affected almost every facet of our lives. Be it through the relationship they have with flowering plants, forming the diverse biological communities throughout the world, their pollination in wild and agricultural environments, or their transmission of debilitating or fatal diseases to people and other animals, they have an astonishing influence on terrestrial life, including the lives of people. Whilst many important evolutionary events have occurred since the Cambrian explosion, insect flight must be in the running for one of the most important in the post-Cambrian era, perhaps only surpassed by the production of lignin in trees or the evolution of the human mind.
Until next time.
Blog written my Max Tercel (twitter: @MaximumInsect; email: firstname.lastname@example.org)
* There is some debate over certain specimens found. Garrouste et al. (2012) found an insect fossil dated back to the late Devonian that shares several characteristics with early winged insects, most notably dicondylic mandibles, but does not have wings. They argue it may be a juvenile form, indicating that adult forms may be winged. However, Blanke et al. (2015) showed that dicondylic mandibles are likely the primitive condition for all insects, including bristletails, suggesting that the presence of the mandibles is not consequential.
† These figures include all animals, not just winged insects.
‡ That said, our close ancestors (like us) were primates, likely brachiating and omnivorous, somewhat reliant on fruits and other plant matter (roots, flowers, leaves etc.) living in arboreal environments in the tropics where angiosperms dominate.
§ Some of these insect transmitted diseases were common in the Northern Hemisphere. For example, malaria, then called “marsh fever”, was widespread in Kent and Essex between the sixteenth to the nineteenth centuries (Dobson, 1980).
Firefly gif: http://www.vincentbrady.com/fireflies
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