Planet of the Bugs: Evolution and the Rise of Insects (28 page)

The benefit of pollination systems to both plants and insects is clearly imprinted in the fossil record since the Middle Cretaceous. Fossils from this time are rich not just with diverse, new flowering plant species but with new species of flower-associated insects. Of particular note is the appearance and diversification of new flies with long, hollow, tubelike mouths modified for dipping into and sucking nectar from deep inside flowers. These long-beaked flies—the early bee flies, flower-loving flies, and tangle-winged flies (families Bombyliidae, Mydidae, and Nemestrinidae, respectively)—are among the earliest of the highly specialized flower-feeding insects. Moreover, by looking at remnant living Gnetales species such as
Ephydra
, as well as still-living primitive flowering plants like water lilies and magnolias, we can see that these ancient kinds of flowers achieved their success by attracting communities of assorted generalized insects—all with a taste for flowers. Close codependencies, like those between orchids and certain bee species, are the result of coevolution over the past hundred million years or more.

The key groups of plant-eating insects—stick insects, katydids, plant bugs, leafhoppers, thrips, beetles, sawflies, flies, and moths—were already in place before the Cretaceous began. All enjoyed intense bursts of speciation that paralleled the plants’ rapid diversification. But of all these evolutionary explosions, none can rival the rise of the moths and butterflies during the Late Cretaceous. No other animal group since has more successfully colonized the flowering plants. What propelled them to evolutionary greatness was the feeding habits of their immature larval stages—caterpillars.

Lepidoptera Domine

Like the flowering plants they chew upon, leaf-feeding caterpillars are so common that it is hard to imagine earth without them. These externally feeding insects burst upon the Cretaceous scene only a mere ninety million years ago, along with the flowering plants’ early radiation. They evolved from more ancient, microscopic larvae that tunneled their way through plant tissues, which protected these larvae
from predators, wind, and the sun’s drying effects. External-feeding caterpillars coped with their harsher environment by evolving thicker cuticles to prevent water loss, but a more serious challenge of life on the outside was simply staying on the plant. Unlike a leaf-tunneling insect, a caterpillar needs to cope with motion. Plants may not get up and run around like animals, but their leaves frequently rustle in the wind and their branches often sway in storms. Holding onto the plant is really crucial. If a caterpillar falls off, it will likely die before it can find the plant again, or it will waste great amounts of energy trying to regain its former position.

The Lepidoptera in particular had a wonderful preadaptation for success in the outer world: silk.
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While most of us will recall the value of silk cocoons in protecting the transforming moth pupa, we tend to forget the importance of this material to the feeding caterpillar, for which it is a matter of utmost survival. That silk strand is literally a lifeline in the exposed world. As a caterpillar moves about a plant, it is constantly spinning a sticky silk thread, which adheres to the surface. If a caterpillar falls, it will release more silk, with which it can safely descend to another part of the plant, or up which it can return to its starting point. Upon the tips of its abdominal legs are microscopic spines and hooks, called crochets, which grab onto the silk threads while it walks, and keep the caterpillar firmly attached to the plant, even on windy days. The original inventors—and the most successful users—of Velcro, caterpillars also tie together leafs with silk and employ the bundles as feeding shelters to avoid the harsher outer environment and hide from predators and parasitoids.

As the Cretaceous progressed, moth caterpillars and the very first butterflies became increasingly efficient at chewing on plant parts. If there was an edible portion, it seems that caterpillars managed to find it. They eat leaves whole, scrape leaf surfaces, and skeletonize leaf veins, and they also eat buds, flowers, fruits, seeds, stems, and even roots. All of this nibbling and crunching may sound like a total catastrophe for the early flowering plants, but they did not take the assault lightly. As insect feeding increased, the plants responded by evolving more efficient defenses. Some developed thicker cuticles or microscopic spines that are difficult to digest, others thick or gummy saps that bind to insect mouthparts and are impossible to chew. Many other plants evolved chemical defenses that make their leaves bitter, or even
toxic.
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More than a hundred thousand such defensive compounds have been recognized so far, and new ones are constantly being discovered with the exploration of tropical plants. They seem to rival the diversity of the plant-feeding insect armies and include tannins, alkaloids, cyanogenic glycosides, coumarins, flavonoids, steroids, and terpenoids, to name only a few. While these names may seem foreign, the alkaloid compounds, for instance, include caffeine, nicotine, morphine, atropine, cocaine, strychnine, quinine, and curare. When we sip coffee or smoke a cigar, how many of us pause to reflect on the hundred million years of insect–plant coevolution that made these things possible?

FIGURE 9. 1. This well-preserved fossil butterfly,
Prodryas persephone
(order Lepidoptera, family Nymphalidae), is from Eocene-age rocks of Florissant, Colorado, estimated to be at least thirty-four million years old. Butterflies first evolved during the Cretaceous but survived the end-Cretaceous extinctions to become important herbivores in the modern world. (Photo by Frank Carpenter. Museum of Comparative Zoology, Harvard University. © President and Fellows of Harvard College.)

The angiosperms’ chemical arsenals did not doom the plant-feeding insects. If anything, they stimulated the evolution of even more creative feeding strategies. If an angiosperm evolves a totally new defensive chemical, this may prevent many generalist herbivores from continuing to eat that particular species. But there are always bound to be
a few specialist insects that, because of their unique abilities, discover ways to keep eating the plant. Some insects simply avoid the chemicals. For example, a chewing insect might cut the large leaf veins, thus preventing defensive compounds from flowing into the area where it is feeding, or it might feed selectively on new growth with fewer toxins. Or an insect with piercing mouthparts might feed selectively by puncturing areas of the plant which lack the chemicals. Many insects evolve mechanisms for detoxifying plant poisons or, even better, find ways to incorporate them into their own body’s metabolism—thereby turning plant chemicals into their own defense against predators.

Attack of the Bee Girls

The Cretaceous surge of flowering plants and their associated insects also stimulated a parallel explosion of parasitic and predatory groups, since every new insect species that evolved to exploit a flowering plant was itself potential food for insect-eating species. The stinging wasps (Aculeata), for instance, descended from a group of Late Jurassic parasitic wasps that shifted their egg-laying duct from the ovipositor to just above the base of the ovipositor, which was itself modified into a hypodermic syringe used only to inject venoms. This allowed the sting to specialize as a venom-delivery device and ultimately as a purely defensive weapon—a transformation that in the Late Jurassic quickened the conquest of a new wasp group, the nest-provisioning wasps, and in the Cretaceous gave rise to several other new social insect groups: social wasps, bees, and ants.

Probably not much changed for the Jurassic wasps with the new egg-laying duct. The females continued to sting and paralyze insect hosts and to lay eggs on them. However, once the sting evolved, females no longer had the option of drilling and injecting eggs deep into their hosts. They had to lay their eggs directly on easily exposed insects. But the same exposure that allowed the female wasps to easily find these insects also allowed other predators and parasites to discover them, which is why some females started moving their paralyzed hosts to a more secluded spot. With this move, the first nest-provisioning solitary wasps made their debut in the Late Jurassic.

More often than not, a female nest-provisioning solitary wasp will dig a hole or tunnel in the ground or find a hollow plant stem. Then
she goes hunting. She finds a suitable food item, such as a caterpillar, stings it, and injects her paralyzing venom, just as her parasitoid wasp ancestors did for millions of years. Next she grasps the paralyzed insect with her mandibles or legs. Maybe she flies away with it, or maybe she just drags it along the ground. In any case, with determination and hard work, she takes that insect back to her nest, stuffs it into the hole, and lays a single egg upon it. Then she deftly seals up the nest entrance, and starts the process over again in another location.

FIGURE 9.2. A female dryinid wasp (order Hymenoptera, family Dryinidae) fossilized in Dominican amber, estimated to be twenty to thirty million years old, with a visible sting. Modern dryinids are all ectoparasitoids of Homoptera adults or nymphs, which they catch with their chelate forelegs and sting to cause paralysis; then they lay a single egg between the host insect’s overlapping thoracic or abdominal segments. (Photo © George Poinar Jr.)

As female wasps became better at searching for exposed insects during the Early Cretaceous, they refined their stinger into the supreme hunting tool, and many new nest-provisioning species evolved. As their hunting skills diversified, wasps also grew more creative at making nests. They began to tunnel into wood, hollow out the pithy centers of plant stems, and sculpt nests on vertical rock and cliff faces
out of clay or mud. To further hide their nests, some modern wasps close it, fly over it, and scatter sand to erase all traces of the entrance. Other mother wasps pick up a small stone with their jaws and tamp down the soil over the nest entrance. Thus, wasps were the first stone tool users, probably tens of millions of years before any primate or human ever picked up a rock.

High Society

The invention of nest provisioning once again set the stage for the transition from solitary to social behavior. During the Cretaceous period the three major lines of social Hymenoptera all evolved from nest provisioning ancestors. These are the groups known to us as the bees, the paper wasps (including hornets), and the ants.

Bees are, simply, very hairy social wasps that have evolved into plant-feeders rather than carnivores. Primitive bees lay eggs in tunnels in the ground just like many nest-provisioning solitary wasps, and their method of provisioning cells is similar.
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They just pack the chamber with pollen instead of paralyzed prey. As bee societies further diversified some evolved arboreal nest-making habits and began occupying pithy or hollow tree branch stems. Eventually some were able to sculpt cells and nests from pieces of wax secreted from wax glands, allowing them to occupy larger cavities in trees or cliffs.

Paper wasps evolved the capacity to produce tough paper by mixing macerated wood pulp with their own saliva. They sculpt cells from this “paper,” into which they stockpile paralyzed and chewed food items, usually other kinds of insects, for their larvae. Paper wasps have become expert architects, shaping an assortment of sophisticated multi-celled and multitiered enclosed shelters; their unique ability to make nest paper and attach it to almost anything has allowed them to move from soil shelters and hollow logs to a variety of more exposed nesting places, such as on leaves, on branches, and on rock faces. Since many paper wasps harvest and feed on caterpillars of various sorts, their ability to build nests directly on leafy plants allows them to live close to their food sources.

Ants are wingless social wasps that, unlike the bees and paper wasps, have more successfully abandoned the ancestral behavior of stocking single cells with food, and raise their young in larger com
munal chambers. In this regard, the nests of many ants are more similar to those of termites. Ants also moved out of the soils to occupy a variety of plant habitats; many nest in hollow stems, inside nuts or large seeds, and even on open branches by tying together leaves with silk. Some ants, such as the infamous army ants of South America, have abandoned natural shelter nests entirely and, when needed, form temporary camps or bivouacs from the intertwined bodies of their own workers. The ants have diversified their diets more than any other group of social insects, and eat other bugs, plant materials, seeds, fungi, and sugary honeydew from a number of insects, including aphids, treehoppers, mealybugs, and caterpillars.