Question

Insects of giant size compared with today's insects are found in the fossil record. There were times in the past when the atmoepheric concentration of \(\mathrm{O}_{2}\) was \(1.5\) times today's concentration. Some investigators hypothesize that insect gigantism was permitted by high \(\mathrm{O}_{2}\) levels. Explain why this hypothesis is plausible.

Short answer

The plausibility of the hypothesis comes from the crucial role of oxygen in metabolic processes and the relation between organism size and metabolic rates. Insects would use their tracheal system to absorb more oxygen when its atmospheric concentration was higher, allowing a higher metabolic rate and supporting a larger size, hence explaining the existence of gigantic insects.

Step-by-step solution

Understand the role of oxygen in metabolic processes

Oxygen is a crucial component in many metabolic processes, including respiration which is the process by which organisms convert food into energy. In general, the greater the oxygen concentration, the more efficient these processes can be.

Explain the relationship between oxygen concentration and size

The size of an organism, in this case an insect, often correlates with its metabolic rate. Larger organisms typically need a higher metabolic rate to support their size and function. Increased oxygen concentration can support a higher metabolic rate, and thus allows for a greater size.

Link increased oxygen to insect gigantism

Insects, unlike humans, don't have lungs for gas exchange, but rather a system of tracheal branches that allow oxygen to reach their cells directly. If atmospheric oxygen concentration is higher, more oxygen will be available to be diffused into their bodies through these tracheal tubes, supporting larger body sizes and leading to gigantism.

Key concepts

Metabolic Processes in Insects

Insects are fascinating creatures that have complex metabolic processes critical for their survival and growth. The role of oxygen in these processes is pivotal, especially when considering respiration, the core method by which insects convert food into usable energy. Just like a fire needs oxygen to continue burning, insect cells require oxygen for metabolic fires that drive their energy production. The metabolic rate of an insect, which refers to the speed at which it converts food into energy, is intricately tied to oxygen availability in the environment.

With a higher concentration of oxygen, insects can theoretically boost their metabolic rates because their cells receive more of this essential gas to produce energy. This energy is not only necessary for movement and daily activities but also for growth and development. It allows insects to synthesize proteins, repair cells, and maintain bodily functions optimally. This is why a spike in oxygen levels in prehistoric times is hypothesized to have enabled insects to grow much larger than those we see today; their metabolic processes were supercharged by the abundance of oxygen, leading to gigantism.

Relationship Between Oxygen Concentration and Organism Size

The size that an organism can achieve is closely linked to the amount of oxygen available in its environment. Larger bodies have larger cells, or more cells, which in turn require more oxygen to maintain metabolic activities. Simply put, the more oxygen there is available, the larger an organism can potentially grow, because the metabolic processes that drive growth are more efficient and can operate at a greater scale.

In the context of insects, scientists believe that past atmospheric conditions with oxygen levels about 1.5 times higher than today’s supported their larger sizes. At higher oxygen concentrations, an insect's body could support a higher metabolic rate, facilitating the development of larger organs, stronger muscles, and more robust structures overall. A supporting example from modern times is how altitude affects organism development, with species at sea level often being larger than their high-altitude counterparts, due to the richer oxygen content of air at lower elevations.

Tracheal System in Insects

Unlike mammals that have lungs, insects have a unique respiratory system known as the tracheal system. This system is a network of tubes that extends throughout the insect's body, providing direct oxygen delivery to cells. The tracheal system functions without the help of a pumping heart; instead, it relies on the diffusion of oxygen from the atmosphere directly into the tracheal tubes. This method of respiration is efficient for small-bodied insects but poses limitations as body size increases.

However, if the oxygen concentration in the atmosphere is higher, the diffusion gradient would allow more oxygen to enter the tracheal tubes, saturating the insect's tissues more effectively and allowing for larger body sizes. This increased efficiency could support the growth of tissues and enable insects to achieve a size that would be impossible in today's oxygen levels. The tracheal system, while limiting in some respects, therefore, becomes a key factor in understanding how prehistoric insects could have reached gigantic proportions.