One of the many mutant opponents that the movie monster Godzilla contends with is Mothra, a giant mothlike creature with a wingspan of 7–8 m. Science fiction creatures like these can be critiqued on the grounds of biomechanical and physiological principles. Focusing on the principles of gas exchange that you learned about in this chapter, what problems would Mothra face? Why do you think truly giant insects are improbable?

Verified step by step guidance

1. Understand the principles of gas exchange in insects: Insects breathe through a system of tubes called tracheae, which directly deliver oxygen to tissues and remove carbon dioxide. This system works well for small organisms but becomes inefficient as body size increases.
2. Consider the scaling of the tracheal system: As the size of an insect increases, the volume of its body grows faster than the surface area of the tracheae. This means that larger insects would need disproportionately larger tracheae to supply enough oxygen to their tissues, which could be structurally impractical.
3. Analyze the impact of increased body size on oxygen diffusion: The diffusion of gases through the tracheal system is effective over small distances but becomes less efficient over longer distances. In a giant insect like Mothra, the distance for oxygen to diffuse would be much greater, potentially leading to inadequate oxygen supply to cells far from the tracheal openings.
4. Evaluate the mechanical limitations: The exoskeleton of insects must be shed and regrown through a process called molting. Supporting a massive exoskeleton and the physical process of molting would be challenging for a giant insect due to the increased weight and the strength of materials required.
5. Reflect on evolutionary feasibility: From an evolutionary perspective, the constraints on respiratory efficiency, structural support, and other physiological factors make it improbable for insects to evolve into very large sizes under current Earth-like conditions.

Key Concepts

Gas Exchange in Insects

Insects primarily rely on a tracheal system for gas exchange, where air enters through spiracles and travels through a network of tubes directly to tissues. This system is efficient for small body sizes but becomes less effective as size increases, limiting oxygen delivery and carbon dioxide removal. For a giant insect like Mothra, the increased distance for diffusion would pose significant challenges in meeting metabolic demands.

Surface Area to Volume Ratio

The surface area to volume ratio is a critical factor in biological systems, influencing how organisms exchange materials with their environment. As an organism grows larger, its volume increases faster than its surface area, leading to decreased efficiency in processes like gas exchange. For Mothra, this means that the larger body would struggle to obtain sufficient oxygen relative to its size, making survival as a giant insect improbable.

Metabolic Rate and Size

Metabolic rate refers to the rate at which an organism converts food into energy, which is influenced by its size. Larger animals generally have lower metabolic rates per unit mass compared to smaller ones, but they require more energy overall. Mothra's massive size would necessitate an extraordinarily high metabolic rate to sustain its activities, which is not feasible given the limitations of its gas exchange system and the physical constraints of being a giant insect.