The Evolution of Insects From Prehistory Onward

The Evolution of Insects From Prehistory Onward Goliath beetles and sphinx moths would be described as large by just about anyone living today, but some prehistoric insects would dwarf these evolutionary descendants. During the Paleozoic era, the Earth teemed with giant insects, from dragonflies with wingspans measured in feet, to mayflies nearly 18 inches in breadth. While over a million insect species live today, truly giant insects no longer exist. Why did giant insects live in prehistoric times, but disappear from the Earth over time? When Were Insects the Biggest? The Paleozoic era occurred 542 to 250 million years ago. It is divided into six periods of time and the last two saw the development of the largest insects. These were known as the Carboniferous period (360 to 300 million years ago) and the Permian period (300 to 250 million years ago). Atmospheric oxygen is the single most limiting factor on insect size. During the Carboniferous and Permian periods, atmospheric oxygen concentrations were significantly higher than they are today. Prehistoric insects breathed air that was 31 to 35 percent oxygen, as compared to just 21 percent oxygen in the air youre breathing right now.   The largest insects lived during the Carboniferous period. It was the time of the dragonfly with over a two-foot wingspan and a millipede that could reach ten feet. As conditions changed in the Permian period, the bugs diminished in size. Yet, this period did have its share of giant cockroaches and other insects we would certainly classify as giants. How Did the Bugs Get So Big? The cells in your body get the oxygen they need to survive via your circulatory system. Oxygen is carried by the blood through your arteries and capillaries  to each and every cell in your body. In insects, on the other hand, respiration occurs by simple diffusion through the cell walls. Insects take in atmospheric oxygen through spiracles, openings in the cuticle through which gasses enter and exit the body. Oxygen molecules travel via the tracheal system. Each tracheal tube ends with a tracheole, where the oxygen dissolves into the tracheole fluid. The O2 then diffuses into the cells. When oxygen levels were higher as in the prehistoric era of giant insects this diffusion-limited respiratory system could supply sufficient oxygen to meet the metabolic needs of a larger insect. Oxygen could reach cells deep within the insects body, even when that insect measured several feet long. As atmospheric oxygen decreased over evolutionary time, these innermost cells could not be adequately supplied with oxygen. Smaller insects were better equipped to function in a hypoxic environment. And so, insects evolved into smaller versions of their prehistoric ancestors. The Biggest Insect That Ever Lived The current record holder for the largest insect that ever lived is an ancient griffenfly.  Meganeuropsis permiana  measured an impressive 71 cm from wing tip to wing tip, a full 28-inch wing span. This giant invertebrate predator inhabited what is now the central U.S. during the Permian period. Fossils of the species were discovered in Elmo, Kansas and Midco, Oklahoma. In some references, it is called  Meganeuropsis americana. Meganeuropsis permiana  is one of the prehistoric insects referred to as giant dragonflies. David Grimaldi, in his hefty volume  Evolution of the Insects, notes this is a misnomer. Modern day odonates are only distantly related to the giants known as prodonata. Other Giant, Ancient Arthropods An ancient sea scorpion,  Jaekelopterus rhenaniae, grew to 8 feet in length. Imagine a scorpion bigger than man! In 2007, Markus Poschmann unearthed a fossilized claw from this massive specimen in a German quarry. The claw measured 46 centimeters, and from this measurement, scientists were able to extrapolate the size of the prehistoric eurypterid (sea scorpion).  Jaekelopterus rhenaniae  lived between 460 and 255 million years ago. A millipede-like creature known as an  Arthropleura  reached equally impressive sizes.  Arthropleura  measured as long as 6 feet, and 18 inches wide. While paleontologists have yet to find a complete fossil of  Arthropluera, trace fossils found in Nova Scotia, Scotland, and the United States suggest the ancient millipede would rival an adult human being in size. Which Living Insects Are the Biggest? With well over one million insect species on Earth, the title of Biggest Living Insect would be an extraordinary achievement for any bug. Before we can confer such an award to a single insect, however, we need to determine how we measure bigness. What makes a bug big? Is it sheer bulk that defines a creature as large? Or something we measure with a ruler or tape measure, determined by centimeters? In truth, which insect wins the title depends on how you measure an insect, and who you ask. Measure an insect from the front of the head to the tip of the abdomen, and you can determine its body length. That might be one way to choose the biggest living insect. If thats your criteria, your newest world champion was crowned in 2008, when entomologists discovered a new stick insect species in Borneo. Chans megastick,  Phobaeticus chain, measures a full 14 inches from head to abdomen, and a full 22 inches if you stretch the tape measure to include its extended legs. Stick insects dominate the competition in the longest insect category. Prior to the discovery of Chans megastick, another walkingstick,  Pharnacia serratipes, held the title. For many insects, its wings spread far wider than the size of its body. Would wing span be a good measure of an insects size? If so, youre looking for a champion among the  Lepidoptera. Of all the living insects, butterflies and moths have the largest wing spans. The Queen Alexandras birdwing,  Ornithoptera alexandrae, first earned the title of the worlds largest butterfly in 1906, and in over a century, no larger butterfly has been discovered. This rare species, which lives only in a small area of Papua New Guinea, can measure over 25 cm from wing tip to wing tip. While thats impressive, a moth would hold the biggest living insect title if wing span was the sole criteria. The white witch moth,  Thysania agrippina, outstretches any other Lepidoptera with a wing span of up to 28 cm (or 11 inches). If youre looking for a bulky bug to anoint as the biggest living insect, look to the  Coleoptera. Among the  beetles, youll find several species with a body mass that is the stuff of science fiction movies. Giant  scarabs  are known for their impressive size, and among this group, four species remain deadlocked in the competition for biggest:  Goliathus goliatus,  Goliathus regius,  Megasoma actaeon, and  Megasoma elephas. A lone cerambycid, the aptly named  Titanus giganteus, is equally massive. According to the Book of Insect Records, researched and compiled by the University of Florida, there is no credible way to break the  tie between these five species  for the title of bulkiest bug. Finally, theres one last way to think of bigness when it comes to insects – weight. We could put insects on a scale, one by one, and determine which is biggest by grams alone. In that case, theres a clear winner. The giant weta,  Deinacrida heteracantha, hails from New Zealand. An individual of this species weighed in at 71 grams, though its important to note the female specimen was carrying a full load of eggs at the time she stepped on the scale. So which of these insects should be called the biggest living insect? It all depends on how you define big. Sources University of Bristol (2007, November 21). Giant Fossil Sea Scorpion Bigger Than Man. ScienceDaily. Retrieved March 22, 2011, from  ScienceDaily.Sues, Hans-Dieter (2011, January 15).  Largest Land-Dwelling Bug of All Time. National Geographic News Watch. Retrieved March 22, 2011.Evolution of the Insects, by David Grimaldi.Dudley, Robert. (1998). Atmospheric Oxygen, Giant Paleozoic Insects and the Evolution of Aerial Locomotor Performance. The Journal of Experimental Biology 201, 1043–1050.Dudley, Robert. (2000). The Evolutionary Physiology of Animal Flight: Paleobiological and Present Perspectives. Annual Review of Physiology, 62, 135–55.