Insects of a Lost World

The Carboniferous period is the golden age of prehistoric insects. During this roughly 60-million-year span, atmospheric oxygen rose to levels never seen before or since, swamp forests covered vast areas of the tropical continents, and insects responded by diversifying explosively and growing to sizes that seem almost impossible by modern standards. This is the era of Meganeura, the giant dragonfly-like insect, and Arthropleura, the car-length millipede. It is also the period when insects first evolved the ability to fly.

The Carboniferous World

The Carboniferous takes its name from the enormous coal deposits laid down during this period. The tropical regions of the Earth were covered in dense, swampy forests of giant lycopsid trees (scale trees), ferns, horsetails, and cordaites. These forests grew in waterlogged conditions, and when trees fell, many were buried before they could fully decompose. Over millions of years, these buried forests compressed into the coal seams that powered the Industrial Revolution.

The same conditions that created coal also created exceptional fossil preservation. Insects that fell into swamp waters were sometimes buried quickly in fine sediment, preserving their wings, bodies, and even gut contents in remarkable detail. The Mazon Creek deposit in Illinois is one of the most famous examples.

Atmospheric oxygen is estimated to have reached approximately 35% during the Late Carboniferous, compared to 21% today. This had profound effects on insect biology, as discussed in the Oxygen and Gigantism article. Higher oxygen levels allowed insects to grow far larger than their modern relatives, because insect respiratory systems (tracheal tubes) become more efficient at delivering oxygen when ambient concentrations are higher.

The First Winged Insects

One of the most important events in insect evolution occurred during the Carboniferous: the origin of flight. Insects were the first animals ever to evolve powered flight, beating birds by about 150 million years and bats by roughly 300 million years.

The earliest winged insect fossils come from the Early to Middle Carboniferous. Early winged insects belong to the Palaeoptera, a group that includes the ancestors of modern dragonflies and mayflies. Unlike most modern insects, palaeodictyopterans and other early winged forms could not fold their wings flat against their bodies — they held them outstretched at all times, similar to modern dragonflies.

The evolution of wing folding, which appeared later in the Neoptera, was a major innovation that allowed insects to exploit new habitats like bark crevices, leaf litter, and burrows. Both folding and non-folding winged insects were abundant during the Carboniferous.

Major Insect Groups

Several major insect groups flourished during the Carboniferous:

Palaeodictyoptera

These were large, often colorful insects with piercing-and-sucking mouthparts. Many had prominent beak-like structures used to feed on plant fluids. Some, like Mazothairos, had winglets on the prothorax (the first thoracic segment) in addition to two pairs of true wings, giving them a six-winged appearance. Palaeodictyoptera were among the most common large insects of the Carboniferous.

Protodonata (Meganisoptera)

This order includes the giant griffinflies, the most famous being Meganeura and Meganeuropsis. These predatory insects resembled enormous dragonflies and were the apex aerial predators of their time. With wingspans reaching 70 cm or more, they are the largest flying insects ever known.

Blattodea (Early Cockroaches)

Cockroach-like insects, sometimes called "roachoids," were extremely abundant in Carboniferous forests. Fossils like Archimylacris and Aphthoroblattina show that the basic cockroach body plan was already well established. These insects were generalist feeders and are among the most commonly found Carboniferous insect fossils.

Early Orthopteroids

Ancestors of modern crickets and grasshoppers were present, though they were not yet as diverse as they would become in later periods.

Ecosystem Role

Carboniferous insects played important roles in their ecosystems. Giant griffinflies were aerial predators, likely hunting other flying insects. Palaeodictyoptera fed on plant fluids, acting as early herbivores. Cockroach relatives were detritivores, helping to break down plant material on the forest floor. This period represents the first time that insects occupied a wide range of ecological niches, a pattern that would only expand in later eras.

End of the Carboniferous

As the Carboniferous gave way to the Permian period, conditions began to change. The vast tropical swamp forests shrank as the climate became drier and more seasonal. Oxygen levels, while still high, began to fluctuate. These changes would reshape insect communities, driving some groups to extinction while creating opportunities for new ones.