“Insects are incredibly cool and charismatic. There are more identified insect species than all the other taxonomic groups put together. They have found novel ways to make their living in the world,” said Jon Harrison of the School of Life Sciences, Arizona State University.
“Insects are the most successful group of animals on the planet,” he continued. “They pollinate all fruits, nuts and vegetables (including coffee and chocolate!); they pollinate most flowering plants; they recycle biosphere waste; they are a major food source for fish, birds and mammals; and, mosquitoes are the second most dangerous animal on the planet.”
“Other than plants, small organisms dominate the earth’s biomass − their mass is 200 times more than humans. They are small but doing a lot,” he said.
Harrison with John Terblanche of the Centre for Invasion Biology at Stellenbosch University and Arthur Woods of the Division of Biological Sciences at the University of Montana, is at STIAS to write the definitive book on how insects breathe.
They described the astonishing systems for gas exchange that insects use. They explained that unlike the cardiovascular systems used by mammals (breathing air into lungs, with gas transport coupled to blood flow), insects breathe using tracheal systems, air-filled tubes that divide finely throughout the body, delivering oxygen and removing carbon dioxide almost directly from individual cells and mitochondria. Tracheal systems are highly flexible to support both the high metabolic rates needed for flight and the very low metabolic rates necessary for surviving long periods of unfavourable conditions.
“We believe that understanding the tracheal system is central to understanding the massive ecological success of insects and to controlling important grain pests and disease vectors,” said Woods.
Harrison explained some of the historical highlights in understanding insect breathing. “Aristotle thought they didn’t breathe,” he said. “Because they didn’t appear to have blood, lungs or hearts or drown. These opinions were held for more than a thousand years.”
Dutch microscopists Antonie Van Leewenhoek and Jan Swammerdam were the first to use microscopes to look at the dissected tracheal system in the late 1600s – giving a first anatomy of how insects breathe. The work of English chemist Joseph Priestly in the 1700s and later French chemists Antoine Lavoisier and his wife Marie-Anne Paulze and others made it clear that, like humans, insect cells contain mitochondria which gives them the ability to take sugars and carbohydrates and combine them with oxygen to create ATP (adenosine triphosphate, the energy-carrying molecule in cells). This requires a constant supply of oxygen to the mitochondria – so insects do breathe!
Insect evolution and breathing mechanics
Terblanche explained that looking at the evolution of the insect tracheal system allows us to draw inferences about ancestral traits.
“Insects emerged about 460 to 480 million years ago,” he said. “We can map out the different respiratory structures present in deepest evolutionary time. Skin respiration with some gills is the deepest evolutionary trait and associated with the aquatic environment.”
Once arthropods like myriapods and hexapods moved on to land, the respiratory structures had to be internalised to prevent excessive water loss. “Enormous pressure to survive the much drier conditions on land was the driving force for insects to evolve a tracheal system,” explained Terblanche.
Recently, scientists developed the abiity to track the development of the tracheal system. The entire tracheal system of a larval fruitfly develops from less than 2000 cells. Developmental biologists can track individual cells through time to see how each primordial cell forms a piece of the tube-like respiratory system.
Woods explained in more detail the mechanics of insect respiration. “Insects actually do it better than humans,” he said. “The heart and blood are not important for moving the oxygen. They don’t have lungs; instead they use an air-filled system of tracheal tubes. It’s a high-capacity system for rapid gas exchange. The tracheal tubes run throughout the body. Gases move in and out of the system with valved spiracles controlling the traffic. Abdominal pumping drives unidirectional flow through the insect, with air sacs within the abdomen acting as a bellows. During flight, wing motions cause changes in the volume of the thorax that automatically create ventilation to support flight.
“This tracheal system also helps make bugs stress resistant,” he continued. “The average insect can recover from 36 hours without oxygen – self paralysis supresses their metabolism. When re-exposed to air, oxygen diffuses in to restart their systems.”
“If we compared the world’s best athlete to a locust − the maximum ventilation rate is ten times higher in locusts.” he added. “It’s a super-performance system that can sustain a metabolic rate up to 150 times higher than for a resting insect.” This very high metabolic rate occurs in part because the body temperatures of insects can rise to 30-40°C when flying.
He also pointed to aquatic insects which make up a huge fraction of freshwater animal diversity. “Dragonflies are terrifying predators if you happen to be a tiny bug in the water,” he said. “Insects reinvaded the water about 50 times during their evolution resulting in a dazzling array of adaptations to breathe under water.”
“An insect’s respiratory system develops over their lifetime and is replaced during moulting,” added Terblanche. “They can become more finetuned for high-performance flight ability. They are capable of ready and rapid change with both conserved processes and finetuning.”
And what can all of this teach humans?
Understanding better how insects ‘work’ and adapt can lead to substantial applications for human benefit – ranging from biomedical to flying and defence applications and even inspiration for the home of the future including soundproof insulation inspired by moth wings; UV filtering by dragonfly wings; water recycling inspired by Malphigian tubules (which are like insect kidneys); and, air flow and carbon-dioxide capture inspired by tracheae.
Studying tracheoles has led to anticancer therapies. “The way low oxygen triggers proliferation and migration of insect tracheoles is very similar to the mechanism that promotes capillary development in response to hypoxia in humans,” explained Terblanche. “Tracheal growth in insects and capillary growth in humans both use fibroblast-like growth factor and, to grow, tumours need FGF. Drugs that block the FGF action are now successful anti-cancer drugs.”
“Researchers are looking at insect eyes − specifically fruit flies which have no eyelids but are coated in a nanoscale coating of proteins and wax that self-cleans – for potential ideas for contact lenses,” said Harrison.
Terblanche also explained how we can control insect pests with non-chemical methods while not harming the product they are infesting. “High carbon dioxide and low oxygen combined with a temperature ramp rapidly kills pests while preserving the commodity.”
“Insects are evolving resistance to every type of pesticide,” said Harrison. “There is strong selection pressure for them to evolve resistance. It’s a huge problem. We must get smarter at handling it.”
And how intelligent are insects? “We are still working to understand their cognitive abilities, but they are remarkably intelligent for their brain sizes,” said Harrison. “They can learn and teach each other, make mental maps of the world, and find their way home through complex terrain. We know they can perceive danger and try to escape. They show behaviour that indicates fear and anxiety. The high performance of their tiny brains seems to arise from their much smaller, densely packed neurons and higher per-gram energy use.”
