Planet of the Bugs: Evolution and the Rise of Insects

by Scott Richard Shaw

  9. Nel and colleagues have suggested that scorpionflies may have originated during the Late Carboniferous, but they become common as fossils during the Permian (Nel et al., “The Earliest Holometabolous Insect from the Carboniferous”).

  10. Sometime during the middle Mesozoic era, a lineage of scorpionflies diverged and evolved into blood-feeding ectoparasitic parasites of birds, mammals, and possibly dinosaurs. The order Siphonaptera, commonly called fleas, are now known to be most closely related to the snow scorpionflies, family Boreidae. The fleas evolved rapidly along with mammals during the Cenozoic era, and now comprise at least 2,500 living species, so the modern Mecoptera are substantially more species-rich if the fleas are reclassified and considered as part of the scorpionflies.

  11. R. J. Mackay and Glenn B. Wiggins, “Ecological Diversity in Trichoptera,” Annual Review of Entomology 24 (1979): 185.

  12. Nel and colleagues have suggested that the stem group of these orders may have originated as far back as the Pennsylvanian subperiod of the Carboniferous. But the caddisflies did not diversify until the Permian, and moths only much later, during the Late Mesozoic.

  13. Douglas H. Chadwick and Mark W. Moffatt, “Planet of the Beetles,” National Geographic 193 (no. 3): 100.

  14. Although inconspicuous, the Psocoptera have diversified greatly and there are at least 4,400 described species. Since they are tiny and live in concealed places, there probably are many bark lice species still undiscovered.

  15. This isn’t the first time we have considered continental drift’s profound impact on the history of life. Back in chapter 2 we discussed how late Precambrian continental drift aligned the continents in a way that led to the global Varanger ice ages, which probably caused massive extinctions among ancient microbial life. We also considered how ongoing continental drift brought the planet out of the Varanger ice ages, possibly triggering the Cambrian explosion of life.

  CHAPTER SEVEN

  1. Along with our Thanksgiving turkey, all modern birds and certain kinds of dinosaurs, including the feathered ones, have a wishbone. This bone is an example of what evolutionary biologists call a synapomorphy—a uniquely shared characteristic that provides evidence of common ancestry. You can think of it as another time message if you wish.

  2. Although Apatosaurus is now the correct scientific name for Brontosaurus, I take the artistic liberty of using “brontosaur” as the common name, as it is more easily recognized. This is not the first time that a disused scientific name was adopted for a common one: you have probably heard of the duck-billed Platypus, but you may not know that Platypus is no longer standard; instead, this animal is now called Ornithorhynchus anatinus.

  3. Until the end of the Cretaceous, our mammalian ancestors were little more than tiny and furry shrewlike insectivores that scampered through the forest leaf litter, hid under logs, and no doubt lived in constant fear of the massive predatory dinosaurs. It was a dreadful time in our history for sure, but the tasty and nutritious insects sustained us mammals until the K-T asteroid finished off the last of the big, nasty brutes. But now with the discovery that birds are dinosaurs, too, we have to acknowledge that the dinosaurian dynasty that ruled the Mesozoic has also done well for itself in recent times.

  4. Our particular culture doesn’t care to eat insects all that much, but we still can’t avoid them. Most of us consume about a pound or more each year, ground up in flour or cereals or mixed in with fruits and vegetables, and some studies have suggested that our cultural aversion to eating bugs may be contributing to B vitamin deficiencies. Some people would like to exclude all insect parts from our diets, but there’s a couple of good reasons why the Food and Drug Administration has been unable (or unwilling) to do that. First, it’s virtually impossible to screen all the insects from plant food sources. Second, there’s no real good reason to keep them out because, in most cases, including insects with plant materials actually improves the nutritional value of our food.

  5. I’ll have much more to say about wasp diversity in the following chapters. For now, it should suffice to tell you that wasps are one of the hyperdiverse insect orders, with species diversity perhaps comparable to the beetles, and that the most diverse lineages of wasps have evolved parasitic behaviors.

  CHAPTER EIGHT

  1. Goliath beetle adults may be among the heaviest insects that ever lived, but we have only recently discovered that the immature forms of some South American rhinoceros beetles are even heavier. The larva of one of these, the Hercules beetle, is known to weigh as much as 120 grams. Many insects reach their peak body weight not as adults but in their last larval stage, just prior to pupating; since the immature forms of large tropical beetles feed deep in wood, it is very possible that we have not yet determined the heaviest living insect.

  2. Holland’s description of the new dinosaur is included in a longer paper about the characteristics of its head: W. J. Holland, “The Skull of Diplodocus,” Memoirs of the Carnegie Museum 9 (1927): 379–403. Although from 1900 to 1931 Holland published fifty-five papers about dinosaurs and named and described two new species, he is much better known for his entomological works. Over the course of his professional career he published some five hundred scientific papers, mostly about insects. His best known works certainly were The Butterfly Book, published in 1898, and The Moth Book, published in 1903. No other books of the time were so influential in introducing so many people to the study of butterflies and moths.

  3. However, the Utahraptor, which was found in Utah after the book and first movie appeared, was about the size of the Velociraptor depicted in the movies.

  4. In the end, we don’t know for sure if the allosaurs hunted in packs, but we do know that they fed on meat, and that it was a dangerous business. On display in the University of Wyoming Geology Museum is a cast of one of the most complete juvenile Allosaurus skeletons ever unearthed. Affectionately dubbed Big Al, the skeleton is notable not so much for its completeness but more for the imperfections of some bones. The gnarly outgrowths on several ribs and one foot provide solid evidence of past wounds partly healed. Allosaurs fought hard for their meals, and they sustained serious injuries in the process.

  5. We don’t know the actual number of living wasp species, but it is enormous. The reason is simply that many of them are microscopic, undiscovered, and unnamed. Hymenopterists, the entomologists who study wasps, have already named around a hundred thousand species, and most believe that there are millions. Many think that the species diversity of microscopic parasitic wasps is comparable to that of the hyperdiverse beetles. Some (myself included) think that wasp diversity may actually exceed that of beetles.

  6. Primitively the hymenopteran ovipositor was composed of four shafts; however, in most living wasps the upper two are fused into one inverted U-shaped upper valve. Exceptions include some sawflies where the ovipositor tip is divided into four shafts, possibly a remnant of the primitive condition, and some ophio-niform Ichneumonidae, which have the upper shafts divided except at the tip (thought to be a secondarily evolved condition). In most modern wasps there are three flexible shafts: a broader upper shaft, and two narrower lower shafts.

  7. These wasp-associated microorganisms are what we call the polyDNAviruses, and they appear to be one of the key reasons for the endoparasitic insects’ vast success.

  8. These extreme specializations of body form across the larval lifespan are an example of hypermetamorphic adaptations, or hypermetamorphosis.

  9. The sudden development of silk glands in emerging parasitic wasp larvae is pretty impressive when you consider that the vast majority of other young silk-spinning insects, like caterpillars, develop these glands over their entire larval lifetime. Wasp larvae don’t need silk at all while they are feeding inside a host insect, so they repress their gland’s development until the final molt.

  10. Historically, termites have been classified as a distinct insect order, Isoptera, but recent studies treat them as a lineage within the order Blattaria, or Blattodea
, the cockroaches.

  11. The idea that termites are social feeding colonies bound together by their common need to exchange gut symbionts is known as the symbiont hypothesis of social evolution. To be fair, this is another controversial topic, and several other viable hypotheses may explain the origin of the termite’s social behavior. One popular idea is that since termites develop slowly and live in hidden habitats rich in concentrated food, they enjoy several advantages to staying within their multi-generational family groups. They mutually benefit from sharing food, more easily defending the group, and jointly caring for the young. It is very risky for individuals to disperse and find suitable locations for new successful nests; most attempts at founding new colonies certainly end in failure. Ultimately, individuals that stay inside an established colony have a much better chance of surviving.

  12. Archaeopteryx has long been regarded as the oldest bird until the recent discovery of the Chinese feathered dinosaur Microraptor, which had apparent flight feathers on both its front and hind legs. The discovery of Microraptor has rekindled the hot debates about the origin of birds and bird flight. I don’t expect to resolve them here. Rather, I’d just like to observe that even if Microraptor was fully arboreal, it must have descended from formerly ground-dwelling ancestors. All of the little raptors were toothy carnivores, so they certainly included insects in their broad diet. Whether Archaeopteryx or Microraptor is the subject of our discussion, it is worth noting that chasing insects into the trees and air likely drove the origin of bird flight.

  13. Robert Nudds at the University of Manchester in England has recently studied the load-carrying capacities of fossil feathers from Archaeopteryx and the ancient Chinese bird Confuciusornis. He found that these early birds had flimsy feathers and probably did not fly very well. They may have just glided from branch to branch, or used their wings to slow their descent when falling. Nudds’s research was profiled in Science News, June 5, 2010.

  14. Archaeopteryx is usually pictured as nibbling on a little lizard or a fish. I don’t doubt that they ate lizards, salamanders, and such, but certainly, like their ancestors, they were highly omnivorous and widely insectivorous.

  15. This is just the total for the bird lice. The Phthiraptera subsequently radiated extensively on mammalian hosts during the Cenozoic era, so the total of living bird and mammal lice is nearly five thousand species.

  CHAPTER NINE

  1. The Andes Mountains of western South America were formed over the last 140 million years as the continent was pushed westward and into the Pacific crust by the widening of the South Atlantic’s ocean floor. This was a long, slow process, and the western parts of South America did not become mountainous too quickly. The main uplifting of the Andes occurred more recently, between 23 million and 5 million years ago. The Andes were not tall enough to direct the Amazon River eastward across its present pathway to the Atlantic until about 10 million years ago. Likewise, the Himalayas did not form until comparatively recently, during the last 60 million years, as India collided with the Asian continent.

  2. The Gnetales are examples of dioecious plants: their sexual organs are located on separate individuals.

  3. The Lepidoptera share with their sister group, Trichoptera (caddisflies), the same silk-producing mechanism that we discussed in chapter 6. Even as tiny leaf miners, primitive caterpillars were lining their tunnels with silk secreted from modified salivary glands. Silk—even those fine silkworm strands that we cherish as ties, shirts, scarves, stockings, and underwear—is little more than dried insect spittle.

  4. These kinds of defensive chemicals are restricted to particular plants, and occur in varying amounts among them. Since they are not required for the primary metabolism of plant growth, scientists call them secondary chemicals. Although some of these chemicals may have originated from developmental waste products, many seemingly evolved strictly to defend against the insects’ plant-feeding behaviors.

  5. Perhaps this doesn’t do enough justice to the tunneling abilities of some bees, which nest in a wide variety of soil types, including both soft soils and heavy clays. Impressively, some Wyoming bees are known to tunnel nests into rock—into the soft sandstone of the Laramie Formation.

  6. You may recall that when we first considered the parasitoid wasps’ early evolution, we discussed the importance of the larvae’s closed hind gut, which prevents the young from fouling their local feeding area with their own feces. This adaptation was equally helpful to the nest-provisioning solitary wasps, and ultimately to social bees, ants, and paper wasps. Like termites, these groups needed to solve the problem of sewage control before becoming abundantly social. While the termites consume their feces, the wasps hold it until after their larval feeding is complete.

  7. One exception is the weaver ants, which stimulate their larvae to spin silk by squeezing them. These ants then tie together leaves with the silk strands and form nests in trees.

  8. While the kin selection hypothesis has dominated the literature for several decades, a recent controversial paper by E. O. Wilson and colleagues challenges it and refocuses attention on other ideas, such as the mutual benefits of nest sharing for food gathering and defense. For more information, see Martin A. Nowak, Corina E. Tarnita, and Edward O. Wilson, “The Evolution of Eusociality,” Nature 466 (2010): 1057–62.

  9. If you still doubt whether or not big dinosaurs could feel a wasp’s sting, please consider this. An insect sting’s painfulness is categorized on a scale of 0 to 4 by the Schmidt Sting Pain Index, which was created by the eminent entomologist and wasp specialist, Dr. Justin Schmidt. The sting of the most painful insect, the South American bullet ant, is described as “pure, intense, brilliant pain, like fire-walking over flaming charcoal with a 3-inch rusty nail grinding into your heel.” Justin O. Schmidt, “Hymenoptera Venoms: Striving toward the Ultimate Defense against Vertebrates,” in Insect Defenses: Adaptive Mechanisms and Strategies of Prey and Predators, ed. D. L. Evans and J. O. Schmidt, 387–419 (Albany: State University of New York Press, 1990).

  10. For a good synopsis of the multiple dinosaur extinction theories, see Robert Bakker’s book The Dinosaur Heresies. Bakker himself favors the hypothesis that, during the late Cretaceous, dinosaurs were naturally declining because the Asian and North American populations mixed after these areas were connected by the Bering Land Bridge. Many species may have gone extinct due to competition over limited resources or because of new invading predators well before the asteroid finished of the last of the big dinosaurs.

  11. Recent studies of the Deccan Traps lava beds in India have brought volcanoes back into the limelight. These volcanic eruptions have been dated near to the end-Cretaceous mass extinction, and were massive enough to have caused global climate change and possibly some extinction, especially in marine ecosystems. This doesn’t refute the importance of an end-Cretaceous asteroid impact, but it suggests that many species may have already been declining before then.

  12. Nevertheless, the asteroid-impact hypothesis draws attention to the fact that big rocks occasionally collide with this planet and may affect patterns of life over the long haul. Some paleontologists plotted the data for species diversity over time and noticed that the extinctions at the end of the Cretaceous and the end of the Permian are not the only major ones—they are just two of the largest. Mass extinctions appear to be periodic and repeat on a somewhat cyclical basis. On average, it looks as if an extinction event of varying intensity occurs about once every twenty-six million years. Granted, this could just be a coincidence, but it prompted scientists to speculate about how and why such events might regularly repeat. The dominant notion is that the earth is getting hit by asteroids, and that something out there with gravitational pull is affecting their stable orbits, as well as the orbits of comets, on a cyclic basis, about once every twenty-six million years. Perhaps either a remote small planet or dark star with a broad twenty-six-million-year orbit perturbs them whenever it passes near to our solar system.

  CHAPTER TE
N

  1. The oldest fossil hominid is Ardipethecus ramidus, found in the Afar Rift of Ethiopia and estimated to be 4.4 million years old. Its skeletal anatomy suggests that it lived in trees, had a largely plant-based diet, and possibly had bipedal capability. There were definitely bipedal hominids in eastern Africa by 3.6 million years ago, demonstrated by the Laetoli fossil footprints in Tanzania, left in volcanic ash by two individuals. Australopithecus afarensis, a short, 3.5-foot-tall hominid known from the famous Lucy skeleton, evolved by 3.2 million years ago. By 2.5 million years ago there were abundant East African hominids in the form of Australopithecus africanus, followed by Australopithecus boisei around 2 million years ago. The later species had flat molar teeth assumed to be well adapted for grinding plant foods. Shortly after this time, Homo rudolfensis evolved, which is considered to be the most ancient species in our genus Homo. By 1.5 million years ago, Homo erectus had arrived, and the migration out of Africa is thought to have begun. By about half a million years ago, Homo sapiens and Neanderthals evolved and were coexisting.

  2. Our philosophy of science relies on parsimony, or simplicity, for selecting a preferred hypothesis. If chimps and humans use tools, the most parsimonious explanation is that both inherited the behavior from our common ancestor. This makes plenty of sense when you consider that chimps and homonids evolved from insectivorous tree-dwelling primates. A more complicated—and less likely—explanation is that tool use evolved independently in both groups.