There are good reasons why invertebrates are so small; ecology and the environment keep them in check. However, there was a time when insects were as large as crows. What happened to the vanishing giants?
A thunderous sound, resembling the beating of 100 snare drums, can be heard from above the canopy. Startled by the intruder, the creatures hiding in the shadows between the trees scatter across the forest floor. The assailant dives for a wood-eating beetle, snatching it in its massive jaws and soaring away with the flash of an iridescent wing.
Meganeuropsis permiana is a griffinfly named after the Permian period, which lasted 299-252 million years ago. It has a wingspan of 71cm (28in), which is twice the size of today’s largest dragonflies and is possibly the largest insect to have ever lived (certainly the largest flying insect in the fossil records) (and probably would have been about the same size and weight of a modern-day crow). The Permian period was teeming with life, and much of it was familiar to us. There were amphibians, reptiles, fish, and insects, but dinosaurs had yet to be discovered. Sharks, rather than prehistoric marine reptiles like ichthyosaur and liopleurodon, were the apex predators of the Permian seas.
Therapsids, four-legged carnivorous creatures with long jaws and razor-sharp teeth, were the Permian’s top land predators. While some, like gorgonopsia, resembled large sabre-toothed rodents or small, stocky dogs, they were not mammals.
This was a time when giant insects ruled the earth. M. permiana is just one of many Permian insects that outweighed their contemporaries. Arthropleura, relatives of modern-day millipedes, reached more than 2m (6.6ft) in length, weighed around 50kg (110lbs), and roamed the forests of the Carboniferous period 358-298 million years ago before becoming extinct in the early Permian.
But what if there were no restrictions on animal size and everything grew to be the same size? To properly answer this question, it is necessary to consider what limits the size of various animals. (But you can learn what might happen if those restrictions did not exist in the video below). One theory for why Permian insects grew so large is that atmospheric oxygen levels were higher than they are today (as high as 30% in the Permian, compared to 21% today). Insects breathe through spiracles, which are openings in the sides of their bodies that lead to a network of fluid-filled tubes into which oxygen diffuses and is then taken up by the muscles.
“This is an inefficient way of breathing in comparison to the way we breathe,” says Tim Cockerill, a broadcaster and entomologist at Falmouth University in the United Kingdom, holding aloft a giant tropical fruit beetle from West Africa in his palm. “Most insects [today] have a size limit of about this size. Insects would not be able to breathe in the same way if they were larger than they are now (in 21% atmospheric oxygen).”
Could the environmental conditions have been ideal for massive Permian insects? “If we look at that wiggly graph of oxygen levels going up and down over time, it almost exactly corresponds to the size of the largest insects around that time,” Cockerill says. However, the difference in insect sizes may not be due solely to atmospheric oxygen levels. The abundance of small prey and the absence of birds (who had yet to appear in the tree of life) may have also allowed invertebrates to thrive.
Other prehistoric animals that dwarf their modern relatives can be found in the fossil record. Another example is Jaekelopterus rhenaniae, a giant arthropod that lived in the Devonian period (about 100 million years before the Permian) and grew to be 2.5m (8.2 ft) long (imagine an oversized woodlouse or pill bug with some vicious-looking claws).
J. rhenaniae is sometimes referred to as a “sea scorpion” because of its long, segmented body with large claws at one end and a thin tail at the other, which resembles crabs, lobsters, spiders, millipedes, bees, and ants. Unlike today’s scorpions, which can grow to be 20cm (8in) long, J. rhenaniae and its giant relatives were aquatic, which could explain how they grew so large.
The largest modern arthropods are all marine, with the Japanese spider crab reaching 3m (10ft) from toe to spindly toe. Exoskeletons are hard outer shells that protect arthropods from predators. They develop by shedding their exoskeletons, revealing a new, soft shell beneath. For larger species, the new exoskeleton can take hours or days to harden, leaving the animal floppy and vulnerable.
On land, this limits the size of an invertebrate. If the new shell is too large, it will deform due to gravity. However, in water, the soft body is supported, allowing invertebrates to grow larger before becoming unwieldy.
Physical forces also limit the size of vertebrates, as they do mammals and dinosaurs. It takes more energy to generate the blood pressure required to circulate blood through their systems because they are large. Even taking a breath becomes more difficult as the effort required to inflate lungs under the strain of a larger body increases.
While African elephants are the largest land animals, aquatic mammals are many times larger. An adult blue whale, the world’s largest animal, weighs around 180,000kg, or the equivalent of 40 elephants. That size is only possible in water where movement is allowed. Their organs can be crushed by their own body weight if they lack the buoyancy of water.
Herbivorous modern and prehistoric giants include whales, elephants, and long-necked sauropod dinosaurs. This is due, in part, to body heat and metabolism. Larger animals have a higher surface area to volume ratio, which means that an elephant, for example, has more volume than its skin’s surface area. A mouse, on the other hand, has far less volume in comparison to its surface area.
Larger animals retain body heat far more effectively than smaller ones, which is why mammals with a lot of volume, such as elephants, rhinos, and hippos, are relatively hairless and must cool themselves through other means, such as mud baths.
Another reason is that their plant-based diet expends a lot of energy during digestion as heat. “Elephants are essentially giant walking fermentation vats, and dinosaurs are essentially walking breweries, so they generate a lot of heat,” says Kate Lyons, assistant professor of biology at the University of Nebraska in the United States.