Planet of the Bugs: Evolution and the Rise of Insects (35 page)

5
. One diverse group of plant-decomposing arthropods was present in the Late Carboniferous: the oribatid mites. Primitive wingless insects and millipedes were also responsible for some of the decomposition.

6
. While the beetles (order Coleoptera) have been known from Permian fossils for quite some time, only recently has evidence for Carboniferous beetles been discovered, by Béthoux (O. Béthoux, “The Earliest Beetle Identified,”
Journal of Paleontology
83 [2009]: 931–37). While the first beetles, as well as some other kinds of insects with complex metamorphosis, may have first originated during the Late Carboniferous, they were still rare and not diverse, and consequently did not yet have a profound ecological impact on forests. My discussion of beetles will await the next chapter, on the Permian period, when beetle species diversified and became common.

7
. I do not mean to imply that coal production ceased after the Carboniferous, but only that from the period onward, the diversity of decomposing organisms, and competition for plant materials as food, increased. There are some significant coal deposits from much more recent times, such as the Paleocene open-pit mines near Gillette, Wyoming.

8
. Along with the jumping bristletails, the silverfish were formerly placed in a larger order called Thysanura, a name that is still to be found in older literature. They are separated now because of their different mandible forms, as discussed in the last chapter.

9
. Scientists have recently discovered that modern wingless insects, including silverfish and even worker ants, can manage a kind of gliding flight while falling from trees. Their bodies seem to function as an airfoil. During free fall they can turn and glide back to the tree trunk, and so avoid falling all the way to the ground.

10
. Ancient paranotal lobes share a similar venation pattern with modern fully formed wings, which implies that wings evolved from appendages resembling the paranotal lobes.

11
. This is clear because fossil paleodictyopteroid nymphs from Mason Creek are associated with
Macroneuropteris
foliage and have plant spores in their guts (Conrad Labandeira, personal communication).

12
. We don’t know the exact date of the very earliest flying insect, but we do know that winged insects were abundant and diverse by the Late Carboniferous, 320 million years ago. They may have maintained sole ownership of the airways for 100 to 150 million years or more before vertebrates finally took to the air.

13
. Evidence suggests that the Paleodictyoptera were not the only plant-feeding insects that evolved during the Carboniferous. Other feeding styles, from fossil plant damage and coprolite evidence, include boring into plant tissues and stems, preying on seed fern seeds, and feeding (hole-feeding, margin-feeding, and scratching the surface) on the external foliage of multiple Carboniferous plant species.

14
. In addition to spores, coprolites preserve microscopic fragments of plant tissues and wood which are assignable to particular plant-host species. So the study of coprolites provides insight into the herbivore consumption patterns and food webs of ancient times.

15
. We have limited knowledge of the Carboniferous plants’ chemical defenses. The common kinds of chemical defenses in modern plants, such as phenols, alkaloids, and tannins, degrade into by-products during fossilization, and cannot be recovered. However, fossils of some Carboniferous seed ferns contain resins, which preserve unique secondary chemical compounds that, according to Smithsonian paleontologist Conrad Labandeira, probably were used as insect-deterring compounds.

16
. I’m making a bold assumption here. Fossil evidence as to where immature griffenflies lived is very rare. They might have been fully aquatic, like modern dragonflies, or they might have been terrestrial or semiaquatic in the humid forest undergrowth. We do know that the forests were filled with diverse potential predators, such as centipedes, spiders, scorpions, and amphibians. It seems to me that in order to grow as enormously large as an adult griffenfly, the developing young must have lived in a somewhat sheltered habitat. At least they would have avoided spiders, centipedes, and scorpions in the fresh water ponds, so developing there seems far more likely.

17
. Because some insect paleontologists think that the Carboniferous roaches are considerably different from modern ones, they refer to these creatures as “Paleozoic roachoids” or “ovipositor-bearing cockroaches.” This terminology is cumbersome, so I’ve elected to use more familiar terms and simply call all of the ancient species “roaches” or “cockroaches.” Although they are unlike modern roaches in some important ways (for instance, they have a visible ovipositor), if we could travel back to the Carboniferous, we would readily recognize them as roaches and probably declare the place one huge roach-infested swamp.

CHAPTER SIX

1
. Fans of movie trivia may recall that Clint Eastwood made his brief debut in this film as the heroic jet fighter pilot who fires the rocket that destroys the giant mutant spider.

2
. This developing drama of the warm-bloods has one interesting side note. About this time some small insects of the now-extinct order Diaphanopterodea evolved long, slender mouthparts, and some of them resembled mosquitoes. This is the first instance of possible blood feeding by insects, perhaps not coincidentally at the same time that warm-bloodedness appeared. Maybe insects were going on the attack. I’m just speculating, but did ancient blood-sucking insects spread diseases among the herds of protomammals? Living mosquitoes are known to transmit more than two hundred kinds of blood-borne diseases, so it is certainly plausible that Permian blood-feeding insects, such as diaphanopteroids, might have transmitted fatal diseases among the herds of Tartarian protomammals.

3
. Order diversity was greater then because the Permian was a combination of lingering Carboniferous lineages, in addition to numerous later lineages, that diversified at that time and had descendants that survived the extinction. These survivors formed the core of the modern insect fauna that thrives today.

4
. As we discussed in
chapter 2
, trilobites originated and were most diverse during the Cambrian period. They become scarcer, relative to other marine groups, in Paleozoic-era sediments. Even so, I selected trilobites as my symbol of the entire era because they persisted across that time, but do not appear at all in Mesozoic or Cenozoic sediments. For the end of the Permian, fossils of brachiopods, bryozoans, and crinoids are better geological markers because they were more common then. By the Late Paleozoic, trilobite species diversity was very low, so perhaps their extinction was inevitable.

5
. Volcanic explosions across Permian Siberia spewed 1.5 million times as many airborne particles as the 1981 eruption of Mount Saint Helens.

6
. Fossils of Permian grylloblattids still show wings, but by the Late Cretaceous period they were wingless.

7
. Some authors treat the homopterans as part of the order Hemiptera (discussed in the next chapter), and divide them into three suborders: Sternorrhyn-cha, Auchenorrhncha, and Coleorrhyncha. This nomenclature is obviously a bit cumbersome, so I prefer to refer to these insects by their simpler and more familiar name: homopterans.

8
. The oldest fossil insect larva and the oldest fossils of adults from several groups known to have complex metamorphosis are from the Permian period. There are some fossil plant galls from the Late Carboniferous, and since modern galls are mostly caused by insect larvae, some scientists, such as Smithsonian paleontologist Conrad Labandiera, have speculated that complex metamorphosis first developed late in the Carboniferous. The discovery of some putative holometabolous insects from the Late Carboniferous led Nel and colleagues to refer to complex metamorphosis as “a crucial innovation with delayed success” (A. Nel et al., “The Earliest Holometabolous Insect from the Carboniferous: A ‘Crucial’ Innovation with Delayed Success [Insecta: Protomeropina: Protomeropidae],”
Annales de la Société Entomologique de France
, n.s. 43 [2007]: 349). Most entomologists credit the drier climate (Permian aridity) for stimulating the diversification of holometabolous insects during the Permian period. Whatever the reason, we do know that they diversified explosively during the Late Permian, and that the groups with complex metamorphosis survived the end Permian well.

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.