Are Insects’ Sizes Correlated With Oxygen Levels?

Research at Duke has shown that the level of oxygen in the air can affect insect size. Insects breathe through a system of air-filled tubes called tracheae, and as an insect grows, it requires more oxygen but does not grow the tracheae. Recent empirical findings support a link between oxygen and insect size, including: (i) most insects develop smaller body sizes in hypoxia, and some develop larger sizes in hyperoxia.

Previously, insect size correlates well with oxygen levels, but after the mid-Jurassic period, the correlation goes away probably because other flying animals begin to diversify. Competition and predation mean big flying. Tracheal hypermetry suggests that larger insects invest a greater fraction of their body as their respiratory system, which prevents increases in size.

Insects are small relative to vertebrates, possibly owing to limitations or costs associated with their blind-ended tracheal respiratory system. The giant insects of the late Palaeozoic occurred when oxygen demand outstrips supply, leading to the insect stopping growing. When insects are reared in a low oxygen atmosphere, they grow to a much smaller than normal size.

Higher oxygen levels could support larger body sizes by facilitating increased oxygen. However, a large body of work shows that oxygen levels influence body size in insects (12-19). Hypoxia not only reduces body size but also constrains the evolution. The leading theory attributes their large size to high oxygen concentrations in the atmosphere (over 30%), compared to 21%. Researchers have speculated that the higher oxygen concentration allowed insects to grow much bigger. Tubes carry oxygen, and the study suggests that higher oxygen concentrations allowed insects to grow much bigger.

How Big Do Insects Get?

Stick insects can grow nearly two feet long and represent a diverse group of large insects still present today. While species like the atlas moth (Attacus atlas) have impressive wingspans of 27 centimeters (10. 6 inches), they are not on par with some of the larger extinct relatives. The giant weta is the heaviest living land insect, weighing around 70 grams, and the coconut crab, which can weigh up to 4. 1 kilograms, is sometimes referred to as an insect despite being classified differently.

Researchers continue to study why modern insects do not achieve larger sizes, with hypotheses suggesting limitations in oxygen availability and exoskeleton molting processes. There are over 3, 000 stick insect species, highlighting significant variation in size within this group. Historically, during the Paleozoic era, insects thrived in larger sizes due to higher atmospheric oxygen levels. Today, the emperor moth (Saturnia pavonia) boasts a wingspan of up to 80 mm, while the non-native Unarmed Stick Insect (Acanthoxyla inermis) can reach lengths of up to 125 mm.

Typically, any arthropod over 3 inches is considered "giant," including various large insects. The largest giant water bugs can exceed 120 mm in length. In ancient times, creatures like dragonflies had wingspans reaching several feet, showcasing the evolutionary changes in insect size over millions of years. Present-day ecological factors and physiological limitations restrict insects from growing beyond certain limits, making the study of their size evolution an intriguing area of research.

Does More Oxygen Mean Bigger Insects?

En el estudio de cómo el oxígeno ambiental influencia el tamaño de los insectos, se ha observado que a mayor concentración de oxígeno en la atmósfera, mayor es la cantidad que logran absorber los insectos a través de su sistema de tubos respiratorios llamado tráqueas. A medida que los insectos crecen, requieren más oxígeno, pero las tráqueas no se expanden, lo que limita su desarrollo. Investigaciones recientes han encontrado evidencia empírica que relaciona los niveles de oxígeno con el tamaño de los insectos, destacando que muchos tienden a ser más pequeños en ambientes con menos oxígeno.

Experimentos han confirmado que los insectos como los libélulas pueden crecer más en atmósferas hiperbáricas de oxígeno. Durante el Carbonífero, hace aproximadamente 300 millones de años, los insectos gigantes prosperaban gracias a niveles más altos de oxígeno, alcanzando tamaños hasta un 15 por ciento mayores que sus contrapartes criadas en condiciones normales. Sin embargo, también hay dudas, pues algunos estudios sugieren que las concentraciones de oxígeno quizás no eran lo suficientemente elevadas para justificar el tamaño gigantesco de estos insectos.

Es posible que los insectos contemporáneos, como las mariquitas, alcanzaran tamaños sorprendentemente grandes si se incrementara la concentración de oxígeno en el aire. Aunque la teoría predominante sostiene que la abundancia de oxígeno permitió la evolución de insectos gigantes en épocas pasadas, las medidas actuales de sus tráqueas indican que la entrega de oxígeno a sus tejidos puede volverse más complicada en especies mayores. Los hallazgos sugieren que, aunque el aumento de oxígeno puede favorecer cierto crecimiento, no se traduce directamente en un aumento proporcional del tamaño de los insectos.

What Determines The Size Of An Insect?

Temperature and oxygen levels significantly influence insect growth and body size, with the "inverse temperature-size rule" indicating that higher temperatures produce smaller adults. Insect weight correlates with volume (cubic) while strength relates to cross-sectional area (squared), making larger sizes challenging to sustain without thicker legs. The determination of body size, a poorly understood aspect of development, is gaining increased research interest.

A recent study employs modeling to identify key factors in size determination. Adult insects do not grow, as size is dictated by the larval stage's secretion of ecdysteroids during metamorphosis. Larger insect sizes are indicators of fitness, as they correspond with longer lifespans and improved reproductive success. Competitive abilities, particularly in males, are enhanced by larger body sizes.

Research indicates that genetic components also contribute to the critical size ranges within insect species. Factors affecting adult size include egg size, larval nutrition, and developmental conditions. Genetic influences merit consideration alongside environmental impacts on body size across diverse insect species, particularly with regard to the translation of size into fitness metrics. Common observations show that size variation arises from genetics, nutritional differences, temperature, environmental impacts, and randomness.

Additional insights from studies suggest that atmospheric oxygen levels impact insect size, with insects relying on a tracheal system to breathe. Variations in insect size, particularly among species like dragonflies and damselflies, highlight a global pattern shaped by environmental factors. Overall, body size is a crucial characteristic affecting various ecological aspects, influencing movement and behavior, impacting trophic interaction dynamics, and being modulated by temperature and oxygen levels. Moreover, mechanisms for sensing body size depend on species-specific cues related to oxygen limitations or stretch receptors, triggering hormonal responses that regulate growth.

Where Are The Highest Oxygen Levels On Earth?

Extensive measurements indicate that the highest concentrations of oxygen are observed at high latitudes, where the ocean is cold, well-mixed, and ventilated. In contrast, mid-latitudes, particularly along the western coasts of continents, are known for oxygen-deficient zones. Notably, greater O2 levels can be found in the upper layers of biologically rich oceans due to photosynthetic activity. The relationship between gas pressures and concentrations in the ocean and atmosphere is governed by Henry's Law, which plays a role where oceanic bacterial systems interact with atmospheric conditions.

Although oxygen is the most abundant element in Earth's crust, its high reactivity means it predominantly exists in compound forms, such as water, carbon dioxide, and silicates. Before the advent of photosynthesis, Earth's atmosphere lacked free diatomic oxygen (O2), with only small amounts being released by geological and biological mechanisms without accumulating in the reducing atmosphere.

Noteworthy, Earth's atmosphere today consists of approximately 21% oxygen, a significant increase from the past. Oxygen levels have fluctuated greatly over geological time, with peak concentrations occurring during different periods, such as a rise to about 35% around 300-250 million years ago. It is estimated that 50-80% of Earth's oxygen production originates from the ocean, primarily from microscopic photosynthetic plankton, which are responsible for generating more oxygen than the largest terrestrial plants.

Additionally, factors influencing the concentration of oxygen in the oceans include biological consumption and the balance of photosynthetic output against respiration and decomposition by marine life. Overall, understanding the dynamics of oxygen production and depletion in marine environments is crucial for examining Earth's atmospheric composition and its historical fluctuations.

How Are O2 Levels Linked To Animal Size?

The evolutionary success and size of animals are closely linked to oxygen levels, particularly during significant increases like that which occurred around 300 million years ago, when oxygen levels rose to about 35%. This rise facilitated the evolution of larger animals, such as giant dragonflies. Essential to this phenomenon is a size-oxygen interaction, where larger animals benefit from increased mean and maximal body sizes due to heightened oxygen availability.

Recent empirical studies underscore this connection, revealing that insects tend to develop smaller sizes in low oxygen conditions (hypoxia) while some can evolve larger sizes in high oxygen conditions (hyperoxia). For example, experiments showed that flies grew larger when exposed to higher oxygen levels. The link between oxygen and animal size is further exemplified by "Carboniferous gigantism," suggesting variations in oxygen availability heavily influenced animal evolution.

Larger animals possess more cells, leading to increased respiration that demands more oxygen, thus linking size directly to oxygen supply capabilities. Interestingly, research indicates that growth in oxygen-rich environments prolongs development rather than accelerating growth rates. Furthermore, lower oxygen levels can limit the growth of certain species, like alligators. It appears that the constraints on insect size may stem from the diffusion limits of oxygen within their bodies. Overall, the rise in atmospheric oxygen has been crucial to the evolutionary trajectory of animal diversity and size throughout Earth's history.

Why Are Insects So Big?

The size of prehistoric insects may be attributed to higher oxygen levels in the atmosphere, which exceeded 30% compared to today’s 21%. This increase in oxygen facilitated the development of larger body sizes, as insects lack lungs and rely on trachea for oxygen diffusion directly into their tissues. The diversity of survival tactics among different insect groups leads to varying outcomes concerning their size and adaptation.

Fossil evidence reveals that giant insects, such as the dragonfly-like Meganeura, thrived approximately 300 million years ago during the Carboniferous and Permian periods, with wingspans of up to 75 cm.

The evolution of birds around 150 million years ago seems to correlate with a decrease in insect size, despite the ongoing rise in oxygen levels. It is suggested that the oxygen demand of growing insect bodies increases faster than their respiratory capacity, limiting size. Empirical findings support a connection between oxygen levels and insect size, as insects in low-oxygen environments tend to be smaller. Furthermore, while certain species like the atlas moth exhibit impressive sizes today, they are dwarfed by their ancient relatives.

The prevailing theory points to a combination of atmospheric changes and oxygen availability, with some newer studies proposing that there could be more factors involved, including genetic preconditions. In summary, the ancient giants of the insect world thrived due to a surplus of oxygen, which is no longer available, resulting in a much smaller modern insect population.

Why Does Oxygen Affect Size?

This study explores how the fixed tracheal system in insects limits their size due to oxygen supply challenges as they grow. As insect body mass increases, so does their oxygen demand; however, tracheae do not expand to meet this demand, leading to decreased growth when oxygen supply becomes inadequate. Evidence indicates a direct relationship between oxygen levels and insect size, with smaller body sizes observed in low-oxygen (hypoxic) environments and larger sizes in high-oxygen (hyperoxic) ones.

Historical variations in atmospheric oxygen levels likely contributed to the larger sizes seen in ancient species like Odonates. The research suggests that low oxygen levels can induce molting at smaller sizes, supporting the notion that body size is regulated by oxygen availability. In terrestrial ectotherms, size sensitivity to low oxygen is linked to other factors, although tracheal expansion limits maximum sizes. Influences such as temperature interactions affect oxygen diffusion rates in bodily tissues, complicating size development.

In a related study of the galaxiid fish, oxygen consumption in relation to body mass was examined, indicating significant effects of environmental oxygen on growth. Notably, when insects are raised in low-oxygen conditions, they manifest considerably reduced sizes. The interplay of oxygen and size is crucial, suggesting that in current and historical contexts, oxygen levels have fundamentally shaped animal size distribution and growth dynamics. This relationship underscores the essential role of oxygen in evolutionary patterns and physical constraints on size within and among species.

How Are Bugs Proportional To Oxygen Size?

As insects grow, their oxygen requirements increase, yet their tracheal systems do not expand correspondingly. This leads to a situation where the demand for oxygen surpasses the available supply, causing growth to cease. In environments with low oxygen levels, insects like Drosophila flies exhibit significantly reduced sizes compared to normal. Research has shown that Drosophila raised under varying oxygen concentrations (high at 40 kPa, normal at 21 kPa, and low at 10 kPa) and selectively bred for larger size over 11 generations show that those cultivated in high oxygen achieve larger sizes, though their growth remains genetically explicable.

Numerous empirical studies point to a relationship between oxygen availability and insect size, with key observations: (i) Insects tend to develop smaller body sizes in low-oxygen (hypoxic) environments, while some may evolve larger sizes under high-oxygen (hyperoxic) conditions. Fluctuations in atmospheric oxygen levels throughout the Phanerozoic era correlate with insect size, indicating that elevated oxygen likely promotes the evolution of larger insects.

During periods of high oxygen, such as the Carboniferous and Permian eras, insects may have reached larger sizes due to enhanced tracheal system scaling. However, larger insects tend to allocate a greater proportion of their body mass to their respiratory systems (tracheal hypermetry), which complicates size increases. Despite this, most insects remain smaller than potential, primarily determined by genetics rather than atmospheric oxygen alone.

Recent findings also imply that enhancements in size due to high oxygen do not uniquely apply to insects but could encompass other arthropods as well. Ultimately, the interplay between oxygen concentration and insect body size reflects complex evolutionary and developmental adaptations within this diverse group.