Today we have a biology picture story from Athayde Tonhasca Júnior. Athayde’s narrative is indented, and you can enlarge the photos by clicking on them.
To boldly go where no insect has gone before
Athayde Tonhasca Júnior
In April 2016, a birdwatcher on the Dutch coast spotted a buff-tailed bumble bee queen (Bombus terrestris) flying in from the sea. Then another bee, and another, then a wave of bees. Altogether, several hundred bees arrived at the Dutch shore (Fijen, 2020). What probably made the birdwatcher stop watching birds to count bees was the fact that as the bee flies, the nearest land eastwards is England, 160 km across the North Sea.
A buff-tailed bumble bee queen © Holger Casselmann, Wikimedia Commons:
Bumble bees have been observed flying between Estonia and Finland (80 km), England and Jersey (28.4 km), and Skye and the Outer Hebrides (24 km). So to jump from England to The Netherlands is notable, but not implausible. In fact, the seasonal arrival of bumble bee queens along the Dutch coast is not uncommon.
One giant leap for mankind, one small step for bumble bees © Visible Earth, NASA:
We know little about insect dispersal, so the occasional fortuitous observation of their long voyages amazes us. On a moonless night in 2014, a Brazilian hydrographic survey ship sailing the South Atlantic stopped over the Montague seamount to collect samples. Hull and deck lights were turned on to help the crew with their lonely task: they were 389 km from the coast and 764 km from the island of Trindade, with no other vessels nearby. Shortly after the lights were turned on, insects from all directions were flying towards the ship. Most of them collided against the hull and fell into the sea, but researchers on board captured 13 true bugs (Hemiptera), three moths and one dragonfly (Alves et al., 2019). How those insects made it that far into the sea at night and for what purpose, is anybody’s guess.
Montague seamount, anchorage site of the ship Cruzeiro do Sul (20°21′57.60″S, 36°38′46.80″W). ES: Espírito Santo State; RJ: Rio de Janeiro State © Alves et al., 2019.:
Interestingly, bumble bees flying over water bodies are often spotted because they have been following ferries, ships or sailing boats. Nobody knows why they do this: they may be using vessels or their wake as navigation aids. In any case, these observations tell us that bumble bees travel for long distances, which suggests they are migrating.
Strictly speaking, ‘animal migration’ refers to individuals traveling long distances back and forth, like many birds and mammals do. This has never been documented for insects, as no single individual completes the cycle. Instead, insects mate and reproduce along the way or at the end of the outbound journey; only their offspring travel back. But even if done in stages and by different generations, many insects migrate, sometimes in an impressive fashion. The monarch butterfly (Danaus plexippus) can fly non-stop for about 16 h over water at average speeds of 37.5 km/h, and their migratory path extends to almost 5,000 km.
Monarch butterfly migration map © U.S. National Park Service, Wikimedia Commons:
If bumble bees migrate, the consequences are profound. The ability to disperse over long distances would help solve the problem of local shortages of food or nesting sites, or unfavourable changes such as habitat degradation. It would also help bees escape parasites or other enemies. But pulling up stakes may have nothing to do with a rough neighbourhood: it could be a mechanism to increase the genetic diversity of the population. If a new queen stays around after emergence, she has a good chance of mating with a closely related male.
Whatever the triggers, migration is a powerful survival tactic. It could explain why some bumble bee species seem to persist in hostile, intensively farmed areas. These bees may not in fact survive for long, but their numbers may be replenished periodically by new migrants.
We have just started to understand the travelling plans of our furry pollinators.
Some bumble bees may go on a journey here and there, but the marmalade hover fly (Episyrphus balteatus) is the unbeatable frequent flyer.
If you have taken a stroll in a garden or local park in Europe, you must have seen lots of flies striped orange and black hovering over flowers like tiny helicopters. These are marmalade hover flies (family Syrphidae, aka syrphid flies), which are widespread throughout Europe, North Asia and North Africa.
A female marmalade hover fly © Charles J. Sharp, Wikimedia Commons:
Adults feed on pollen, nectar and honeydew from a range of plants, while the larvae feed on aphids – entomologists say they are aphidophagous. Females locate aphid colonies by their smell and lay their eggs in the middle of them. The larvae hatch immediately, and each one devours up to 300 aphids per day until pupation. So you could say these flying morsels of marmalade are important allies of gardeners and farmers.
Some people will be surprised to know that these fragile insects embark on migrations that may cover thousands of kilometres. Each autumn, marmalade hover flies and other migratory syrphids leave Britain to spend the winter in southern Europe and the Mediterranean. Their offspring move northwards in the spring, lay their eggs, and the new generation sets out the cycle again. To survive these hazardous journeys, hover flies climb to high altitudes, where strong tailwinds take them to their intended destination.
In some years, they travel in large numbers. And ‘large’ is an understatement. By using specialised radar designed for monitoring insects (Vertical-Looking Radars or VLRs), Wotton et al. (2019) estimated that up to four billion marmalade hover flies along with the aptly named migrant hover fly (Eupeodes corollae) cross the English channel to and from Great Britain every year. This represents 80 tons of biomass. If you are impressed by these figures, you should know that hover flies account for only a fraction of insects’ latitudinal migrations known as ‘bioflows’—about 3.5 trillion insects, or 3,200 tons of biomass—migrate into southern Britain annually. Insect bioflows pour vast amounts of nutrients (particularly nitrogen and phosphorus), energy, prey, predators, parasites, herbivores and pollinators into British ecosystems. But we have only a vague understanding of their impact on food webs and local species (bioflows are also hazards to aviation: migrating insects have downed aircrafts).
The marmalade hover fly does not stand out as a particularly efficient pollinator. It is small and not very hairy, a negative mark for a member of the pollinators club because pollen transport depends on abundant body hair. Even still, each marmalade and migrant hover fly carries an average of 10 pollen grains from up to three plant species on their journey into Britain. These are not impressive figures when compared to bees, which return to their nests loaded with pollen. But considering the massive number of flies and the wide range of flowers they visit, a grain of pollen deposited on a flower here and there must add up quickly. The marmalade hover fly is known to improve the yield of strawberries, but we just haven’t paid much attention to these unpretentious pilgrims.
Don’t fly with me, let’s not fly, let’s not fly away
Insects made their first appearance on this planet between 450 and 500 million years ago. But they really took off evolutionarily – and literally – some 80 million years later when they acquired the ability to fly. From then on, insects could explore a three-dimensional world to occupy every nook and cranny of a habitat, escape predators, disperse widely and search for food more efficiently. Insects soon became the dominant creatures on Earth.
A Meganeura monyi fossil, one of the largest recorded flying insects (65-70 cm wingspan) © Didier Descouens, Muséum de Toulouse. Wikipedia Creative Commons:
The ability to fly gave insects so many advantages and opportunities that it may seem inconceivable to give up flight. And yet, many species have done just that. Brachyptery (wing reduction) or aptery (loss of wings) is widespread among insects. It is easy to understand the uselessness or even disadvantage of wings for bedbugs, fleas, lice and other sedentary creatures. But winglessness seems odd for insects we commonly see flying about such as wasps, beetles, and butterflies.
For these insects, wing reduction or wing loss almost always happens to females: males usually retain fully functional wings. The large velvet ant (Mutilla europaea), is a case in point; the male is winged and a capable flier, while the female is apterous, a trait that makes her look like an ant – hence the species’ common name. But in fact this creature is a wasp that parasitizes several species of bumble bees.
A female velvet ant © Tiia Monto, Wikimedia Commons:
The reasons for the loss of flight in insects have baffled scientists for a long time, and Charles Darwin was one of the first to come up with a theory to explain it. Intrigued by the unusual number of apterous beetles on the island of Madeira, Darwin suggested that flightlessness was a survival strategy. To avoid being blown into the ocean by the strong winds that buffet the island year round, the local insect fauna adapted by losing their wings and keeping their feet firmly on the ground.
Darwin’s theory was tested recently by Leihy & Chown (2020) with data gathered from 28 Southern Ocean Islands, a collection of isolated, wind-swept specks of land in the southern regions of the Atlantic, Pacific and Indian oceans. About half of the islands’ indigenous species are unable to fly, which is nearly ten times the global incidence of flightlessness among insects.
Number of flightless (orange) and flying (blue) insect species in the Southern Ocean Islands © Leihy & Chown, 2020:
By analyzing variables such as wind speed, temperature, air pressure, habitat fragmentation, and presence of predators or competitors, the researchers validated Darwin’s hypothesis: wind speed was the main environmental contributor to insect flightlessness. But Darwin didn’t get it quite right: the risk of being blown away is not the main evolutionary driver – after all, even a tiny island is a huge mass of land for an insect. Instead, the enormous energetic cost of flying seems to be the cause.
Indeed, brachypterous or apterous insects are more common in areas where a considerable amount of energy is required for flight such as arctic regions, mountains and deserts; or in stable habitats where dispersal is not vital for survival, such as caves, termite and ant nests, and on vertebrate hosts. Flight muscles comprise 10-20% of an insect’s body weight, and sustained flights consume a great deal of resources. If flying is not significantly advantageous, energy could be spent on some other function – such as laying more eggs, for example.
Egg production explains why winglessness is much more common in females. Free of the costs of flying, a female can produce lots of eggs, which are considerably more expensive energetically than sperm. In fact, the abdomen of many flightless females is greatly enlarged to hold as many eggs as possible. Flight is retained in males probably because it is important for finding females.
In Britain, the belted beauty (Lycia zonaria), the winter moth (Operphthera brumata) and the vapourer moth (Orgyia antiqua) are three of the better-known species with wingless females. The belted beauty is a scarce species confined to coastal areas, but the other two are abundant and widespread; the winter moth is an invasive in North America
A male and a female belted beauty © Harald Süpfle, Wikimedia Commons.
Wings were the morphological feature that assured insects’ success on Earth, but many species made a U-turn in their evolutionary road. For them, flightlessness was the best life strategy. This apparent throwback is another demonstration that evolution is not teleological, that is, it has no objectives or ‘improvement goals’. It just provides the best means for a species to adapt and survive.