Planet of the Bugs: Evolution and the Rise of Insects (29 page)

FIGURE 9.3. Face and tusk-like mandibles of a soldier caste of a New World army ant,
Eciton hamatum
, from Costa Rica. Ants such as these are formidable predators in Neotropical forests.

The social structure of the bees, paper wasps, and ants is very much like the one we considered earlier for their distant relatives, the termites: each of these lineages has evolved caste systems with workers, soldiers, and queens; each has overlapping generations of long-lived adults; and each cooperatively cares for their young. By convergent
evolution, the termites and social Hymenoptera have also developed similar methods of chemical communication, complex nest structures, behaviors for exchanging liquid foods between nest mates, abilities to create and use chemical trails, and territorial behavior.
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On the other hand, termite and wasp societies have some major differences. Among termites, caste is determined by exposure to chemical pheromones, while among Hymenoptera, it is determined by nutrition and the type of food fed to the larva. Termites employ child labor, while bees, wasps, and ants typically do not. Because termites have gradual metamorphosis—their children are smaller models of the adults—the young are capable of working in the colony as they grow larger, and the adult workers take full advantage of the youth labor camps. The bees, ants, and wasps, by contrast, have complete metamorphosis: their young larvae differ enormously from the adults in form and possible functions. Social Hymenoptera larvae are essentially helpless grubs that exist as growing and feeding machines. For the most part, they can contribute nothing useful to society and do not begin working until the adult stage.
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Also, termites have more equality of the sexes—there are both male and female workers—while the bees, ants, and wasps have female-dominated (Amazon) societies. There are many potential reasons for this, but an important consideration is that female wasps have a defensive stinger, while female termites do not. Female wasps also have a sperm-storage organ (the spermatheca) and the ability to fertilize—or not—each egg as it is laid. Since, as discussed earlier, in the Hymenoptera an individual’s sex is determined by whether or not an egg is fertilized, bee, ant, and hornet queens mostly choose to produce female children. Presumably this is because female wasps are much more useful for hunting food or defending the colony nest with their stingers.

Termites evolved from a single common ancestor that became social, while social behavior among the Hymenoptera originated independently in multiple groups. Evolving the habit of building and provisioning nests certainly played an important role in the Hymenoptera’s transition to sociality. A solitary wasp mother must leave her nest unattended while looking for food, thereby opening it and her offspring to predators and parasites. Even a very small social group benefits from allowing some individuals to guard the nest entrance while others search for food or building materials. Another important factor
must have been the evolution of the Hymenoptera’s defensive stinger. The stinger is a powerful tool not only for paralyzing and subduing prey but also for defending a growing nest against raids by larger animals, which in the Cretaceous years might have been dinosaurs, birds, or small insectivorous mammals. But perhaps the most fascinating aspect of social wasp evolution is the intriguing possibility that the wasps may have been genetically predisposed to becoming social because of their haplodiploid sex determining method.

For many years, one of the great questions about the social insects was why they evolved sterile workers. Charles Darwin pondered this puzzle of natural selection, but he was unable to solve it. If worker bees, ants, and wasps are sterile, they do not contribute any of their own offspring to the next generation. Why would any organism take care of someone else’s children and not leave any of their own? A potential answer to this seeming dilemma, known as the kin selection hypothesis, lies in the curious genetic implications of the Hymenoptera’s haplodiploid method of making males or females, whereby unfertilized eggs develop into haploid males, and fertilized eggs develop into diploid females. In the ants, bees, and wasps, sisters share more genes with each other (75 percent) than they share with their own daughters (50 percent). If, for example, a female worker ant were to develop her own ovaries, mate, and produce her own children, she would on average share 50 percent of her genetic makeup with each of her daughters. On the other hand, when female workers help their mother raise more of her daughters (more sister workers), they are helping to raise more individuals that share on average 75 percent of their genetic makeup, since these workers have the same mother and father. As strange as that sounds, it may help explain why the Hymenoptera have produced so many successful societies.
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The proliferation of social wasp societies during the Cretaceous years, but not earlier, is likely due to their tandem coevolution with the flowering plants. Bees totally depend on flowers for pollen and nectar, and in turn, they have promoted the flowering plants’ success by impelling them to evolve and maintain successful pollination systems. The paper wasps might have gained their paper-producing ability somewhat earlier, but they needed to provision their nests with meaty food. Social evolution flowered figuratively while flowers bloomed literally, precisely because the angiosperms bred a cornucopia of edible
insects, providing not only the wasps but also the ants with abundant provisions. The ants might well have evolved sooner, but their rise was advanced by the flowering plants’ wide-ranging nesting opportunities—and by their nutritious seeds, which the ants, in turn, began moving and dispersing, thereby shaping Cretaceous plant communities. The angiosperms also stimulated a surge in homopterous insects—treehoppers, aphids, mealybugs, scales, and other relatives—whose sugary excretions supplemented the ants’ diets.

Everybody Must Get Stung

The demise of
Tyrannosaurus rex
,
Triceratops
, and the other big dinosaurs is another of those lingering murder mysteries. While less catastrophic than the end-Permian mass destruction, the end-Cretaceous extinction has spawned even more solutions and public attention. The reason, perhaps, for this scrutiny lies in the charisma of the great dinosaurs and the implied importance of their extinction to our own species: it presumably opened the pathway toward the diversification of the mammals and the ultimate evolution of human consciousness. And yet all our big-brainy attentiveness to this event has not solved the mystery.

Among the many explanations for the dinosaurs’ decline and fall is the idea that Cretaceous plant toxins poisoned them. However, it seems unlikely that these toxins would have been so pervasive as to drive the dinosaurs to extinction. Just as in the modern world, there would have been plenty of nontoxic plants and plenty with variable toxin concentrations. Herbivorous dinosaurs should have been capable of nibbling leaves and rejecting bitter or poisonous plants, just as herbivorous mammals are apt to do. It’s certainly possible that as Cretaceous plant communities developed greater flowering plant diversity with more toxins, the dinosaur communities became more specialized and possibly less diverse and abundant. The angiosperms may have contributed to the decline of big dinosaur herbivores over a long period (millions of years, perhaps), but it is very unlikely that they were poisoned to extinction in a short time.

Some scientists have suggested that new kinds of insects, which might have spread deadly diseases, evolved. We have no clear evidence of this, but we can be fairly certain that the Cretaceous dinosaurs had
to contend with small blood-sucking flies, and that the feathered dinosaurs, including some of the raptors, very likely had lice, probably parasites similar to the bird lice. Modern birds, reptiles, and mammals are attacked by various kinds of blood-feeding insects, which carry a number of different illnesses, so it is difficult to reason why dinosaurs would be immune to such assaults. It seems likely that they would have been bitten by these insects since at least the Jurassic years, and that epidemics would have soon followed, causing dinosaur population numbers to rise and fall. Maybe disastrous epidemics annihilated some species; however, there is no good reason to imagine that a single disease would wipe out all the dinosaurs at the end of the Cretaceous period, and yet somehow spare the birds and mammals.

The stinging social wasps, ants, and bees surely disturbed the dinosaurs during the Late Cretaceous. These increasingly vicious insects would have been crawling all over the forests. The vegetarian dinosaurs would have gathered their leafy salads ever more carefully, trying to avoid both plant toxins and, more serious, getting stung, which could have been lethal. The long-throated ones might have been more vulnerable, perhaps, but any browsing animal would be at serious risk of suffocating if they accidentally ate a wasp and were stung inside their throat. The stinging insects would have posed a choking hazard to even the gently nibbling duck-bills (hadrosaurs). And none of the stinging insects were happy about having their nests disturbed. Modern species are capable of holding their own against invaders, whether bird or mammal; because they first evolved alongside Cretaceous dinosaurs, some of which were likely to be at least partly insectivorous, it seems reasonable to suppose that stinging social wasps perfected their venoms in large part to defend themselves against nest-marauding smaller dinosaurs. Gone were the Triassic insect happy meals. The raptors, and perhaps even
Tyrannosaurus rex
, felt the wrath of the social wasps and the ants; they would have had to finish their meals quickly or risk getting stung by these opportunistic insects quick to scavenge bits of meat from animal carcasses. Has a hornet ever visited your picnic table?

It’s possible that the venomous insect societies contributed to the dinosaurs’ decline. But again, it’s difficult to imagine more than that, unless the dinosaurs had wasp venom allergies, which is something we can never know. As entertaining as it may be to picture them gasp
ing, swelling, and choking from wasp stings, I think it’s far more likely that the stinging insects simply made the dinosaurs’ final years really miserable.
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When Worlds Collide

It’s not like we need any more possible explanations for the dinosaurs’ decline.
¹⁰
Many reasonable hypotheses exist, but they all boil down to terrestrial and extraterrestrial causes. On the terrestrial side, the main ideas involve either biological interactions (with plants, insects, mammals, or other dinosaurs) or physical factors (climate change, continental drift, or volcanic eruptions). On the extraterrestrial side, which includes assorted ideas regarding meteors, supernovas, and cosmic rays, unless you have been living in a cave with the wolves for a few decades, you are probably aware of the widely popular asteroid impact hypothesis. This idea was championed by Luis Alvarez during the 1980s mainly because of the discovery that the rock boundary layer between the Late Cretaceous and the Early Tertiary years, the so-called K-T boundary, is loaded with unusually high concentrations of the relatively rare element iridium. High levels of iridium have indeed been found in the K-T boundary layer at many sites around the planet. As far as we know, they could come from only two possible sources: either the element was released from an impact with an extraterrestrial object, such as a large asteroid or a comet, or from deep in the earth’s crust by massive volcanic eruptions. The asteroid-impact hypothesis has gained additional support with the discovery of a potential impact site in the Gulf of Mexico, off the Yucatan coast. Although some good geological evidence suggests that Late Cretaceous volcanic events caused the dinosaurs to go extinct, when you get right down to it, the asteroid-impact hypothesis is just plain sexy.
¹¹
The idea that a huge rock slammed into the earth, resulting in global mass extinctions and general chaos . . . that’s a real attention-getter. And while there really is good evidence that the earth did get slammed by a big rock at the end of the Cretaceous, let’s not forget that we still can’t clearly see the actual extent of its damage. Truthfully, we will likely never know for sure if the last
Tyrannosaurus
died from an asteroid impact or by inhaling toxic volcanic fumes, or if it ate a poison flower, was stung
by a bee, caught the bird flu, or simply died peacefully of old age in a meadow full of flowers.
¹²

The actual effects of an asteroid impact would vary according to the size of the object hitting the earth, but the potential effects could be devastating. Locally there would be an explosion, debris, heat, and toxic gasses. A really large impact would create a massive plume of airborne particles that could block sunlight and produce global climate change. If there is a lesson to be learned, perhaps it is this: as with the end-Permian extinctions, once again the effects of global disaster seem to be hardest on the big land creatures, as well as on marine organisms. Some insects (and plants) disappeared at the end of the Cretaceous, but unlike the end-Permian extinctions, no insect orders were lost. Some species decline occurred, but diversity increased again over the following millions of years. We need only look around our modern world to see that the insects, along with the flowering plants, the birds, and even the small shrewlike mammals (from which we fortuitously evolved), all managed to survive that asteroid impact. Even during the most catastrophic times, life is remarkably tenacious and resilient, especially if you are very small and have six legs.