Why Did Giant Insects Go Extinct?

Insects, including giant insects, have evolved significantly over the past 300 million years. These extinct insects, part of the extinct order Meganisoptera, were predatory and ruled the prehistoric skies during periods when Earth’s atmosphere was rich in oxygen. After the evolution of birds around 150 million years ago, insects got smaller despite rising oxygen levels.

Giant insects dominated the prehistoric skies during periods when Earth’s atmosphere was rich in oxygen. They reached their biggest sizes about 300 million years ago during the late Carboniferous and early Permian periods. In the late Paleozoic Era, some insects evolved into giants due to high atmospheric oxygen levels. When oxygen levels returned to lower levels, the insects grew smaller.

The decline in atmospheric oxygen and the rise of birds contributed to the demise of giant insects. Insects have inefficient oxygenation mechanisms, which limits their size. The larvae of many ancient insects likely passively absorbed oxygen from water and were not able to regulate their intake well. Various theories can explain the supersizing of insects.

First, higher oxygen levels in the atmosphere (30% then versus 20% now) allowed the insects to grow larger. Predation and competition from birds also shrunk prehistoric giant insects to their modern size about 150 million years ago. The leading theory is that the higher proportion of oxygen in the air meant that passive spiracle-based respiration could work for bigger insects.

In conclusion, the evolution of insects has led to a shift in their size and behavior over time. The decline in atmospheric oxygen and the rise of birds have contributed to the decline of giant insects on Earth.

Why Did Insects Get Smaller After 150 Million Years Ago?

Giant insects once dominated the skies during periods of elevated atmospheric oxygen levels, particularly around 300 million years ago during the late Carboniferous and early Permian. Notable species, like griffinflies, had impressive wingspans comparable to modern hawks. However, the emergence of birds about 150 million years ago marked a pivotal change in insect size. Despite a continued rise in oxygen levels, scientists at the University of California, Santa Cruz, discovered that insects began to shrink following this evolutionary leap.

Birds introduced new predation challenges and competition, which significantly impacted the size of insects. As the atmospheric oxygen levels gradually declined over millions of years, the maximum size potential for these invertebrates also decreased. The study highlighted that for the first 150 million years of insect evolution, wing size fluctuated in response to oxygen levels. The size reduction was also attributed to ecological factors, where being smaller became advantageous due to energy efficiency and resource scarcity as climate conditions changed.

In essence, the decline of giant insects can be traced back to the interplay between environmental changes and the rise of birds, leading to a dramatic shift in the maximum sizes achievable by these creatures. Consequently, modern insects are significantly smaller than their prehistoric ancestors, with current oxygen levels being notably lower than those seen 70 million years ago, reinforcing the trend toward smaller sizes in the insect population.

Are Giant Insects Still Alive?

There are no living giant insects or fossils of their tracheae, compelling biologists to study related modern insects to test the oxygen pulse hypothesis—the idea that high atmospheric oxygen enabled insects to grow larger. Jon Harrison, a physiologist and biology professor at Arizona State University, is fascinated by ancient giant insects and investigates why they evolved and subsequently disappeared, likely due to their breathing mechanisms.

While large insects still exist in intertropical regions, such as beetles over 18 cm long, prehistoric giants like Meganeuropsis permiana, a griffinfly of the extinct Meganisoptera order, had wingspans far exceeding those of today’s largest insects like the atlas moth.

In the Triassic period, insects were larger than in the Jurassic, following the appearance of pterosaurs. However, a 20-million-year gap in the insect fossil record and a concurrent drop in oxygen levels make it difficult to determine when and why insect sizes decreased.

Today, over a million insect species live, including sizeable ones like stick insects and the atlas moth, but none reach the immense sizes of their ancient counterparts. Meganeurids had the largest wingspans known, yet their bodies were lighter than some modern Coleoptera. Giant insects thrived when Earth’s atmosphere had higher oxygen levels, but after the evolution of birds about 150 million years ago, insects became smaller despite rising oxygen levels.

Structural limitations in insects' respiratory systems, specifically their tracheae, prevent them from attaining the massive sizes seen in the past. Additionally, studies have revealed that species once thought extinct, such as the Lord Howe Island stick insect (Dryococelus australis), are still alive. Overall, while millions of insect species exist today, truly giant insects no longer roam the earth due to evolutionary pressures and physiological constraints that limit their size.

Did High Oxygen Levels Cause Extinction Of Giant Insects?

The fluctuations of atmospheric oxygen levels are closely linked to the evolution and extinction of giant insects, as proposed by biologists like Harrison. They suggest that elevated oxygen concentrations allowed for the existence of larger species, while declining rainforest habitats and oxygen depletion resulted in their extinction. This environmental shift favored smaller insects better adapted to the changing conditions.

Recent experiments reveal that modern dragonflies grow larger in oxygen-rich atmospheres, supporting historical evidence of giant dragonflies from 300 million years ago, which thrived when atmospheric oxygen exceeded 30%.

However, oxygen levels proved more harmful to juvenile stoneflies, indicating that larvae had difficulty managing gas exchange. The first 150 million years of insect evolution demonstrated a direct correlation between insect wingspans and oxygen availability. Skepticism exists regarding the oxygen-size hypothesis, with some research suggesting that high oxygen levels correlate with the emergence of giant Palaeozoic animals, including insects, during the Permo-Carboniferous period.

This phenomenon is attributed to atmospheric hyperoxia, allowing giant insects to acquire adequate oxygen through their small spiracles. Despite rising oxygen levels post-bird evolution, insects experienced a decline in size. While higher oxygen has been associated with the potential for larger insects, it does not automatically induce growth in smaller species. The giant insects of the Carboniferous era thrived due to increased oxygen from evolving plant compounds like lignin and suberin. Overall, the prehistoric environment favored the evolution of giant insects, shaped largely by significant oxygen concentrations.

Why Did Giant Insects Disappear?

Giant insects once thrived on Earth, especially during the late Carboniferous and early Permian periods, when atmospheric oxygen levels were significantly higher, reaching 31 to 35 percent. The largest known insects, such as Meganeura, dominated the skies at that time. However, their reign came to an end around 150 million years ago with the emergence of birds, notably the early avian species Archaeopteryx.

As birds evolved the ability to fly, they became formidable predators, leading to increased competition and predation on insects. Despite rising oxygen levels during this period, insects began to decrease in size.

Researchers, including Matthew Clapham and Jered Karr from the University of California, noted that the unique respiratory makeup of insects limits their ability to grow large. Insects breathe through small openings called spiracles, which restrict their oxygen intake, especially compared to the efficient respiratory systems of birds. This mechanical limitation became evident as birds developed faster and more agile flight capabilities, solidifying their predatory advantage.

Although atmospheric conditions favored large body sizes for insects, their decline correlates with shifts in evolutionary dynamics brought about by the arrival of birds. The ancient larvae of many insects passively absorbed oxygen from water, which they could no longer do effectively as environmental changes unfolded. Consequently, even as oxygen levels remained relatively high, giant insects were unable to sustain their massive sizes, leading to their eventual extinction. Ultimately, the rise of birds and the limitations of insect respiration played key roles in the disappearance of prehistoric giant insects.

How Many Years Ago Were Giant Insects Common On Earth?

Hundreds of millions of years ago, giant insects dominated Earth. A notable example is Meganeura, a genus of extinct insects related to modern dragonflies, existing around 300 million years ago. One specific member, M. permiana, was first documented in Kansas in 1937, boasting a wingspan exceeding 2 feet (0. 6 meters). The fossil record for insects dates back roughly 400 million years, with winged insects known as Pterygotes undergoing significant diversification during the Carboniferous period.

This era, lasting from approximately 359 to 299 million years ago, is famously termed the "Age of Giant Insects." The environment, characterized by high oxygen levels and diverse ecosystems, fostered the growth of insects that reached animal-like sizes, with dragonflies comparable to crows and millipedes extending as long as cars.

During this period, fossils reveal the prevalence of enormous dragonflies and sizable cockroaches. However, as atmospheric oxygen levels declined and birds emerged as predators, these giant insects faced extinction. It is estimated that insects originated about 480 million years ago, during the Ordovician, coinciding with the advent of terrestrial plants. By the late Carboniferous and early Permian, around 300 million years ago, insects like Meganeuropsis permiana thrived.

Despite the eventual reduction in size due to evolutionary pressures, the legacy of these prehistoric giants offers fascinating insights into Earth’s distant ecological landscapes. Thus, while millions of insect species exist today, none reach the immense sizes of their ancient forbears, marking a significant evolutionary transition over geological time.