There is a bit of old gossip that continues to be retold amongst biologists. According to this tale, the eminent British biologist, J. B. S. Haldane, was attending a cocktail party when he was confronted by a prelate with a rather unusual question. Repeated ever since the late 1930s, this brief encounter has become a staple when discussing the diversity of life on our planet. The religious gentleman's question was: “As a student of biology, Dr. Haldane, what can you tell us about the nature of God?” Unfazed by the gravity of the question, the eminent biologist is said to have responded simply and directly: “An inordinate fondness for beetles!”
Yes indeed! The Creator seems to have a fondness for beetles. Why else would there be so many of them? At latest count, beetles number around 380,000 described species.1 What this means is that a student of insects has gone to the trouble of providing the description for a new species, given it a Latin name, designated the specimens to which the name applies, and published all this in a scientific text or periodical. Nowadays, students of insects (entomologists) like to have an illustration published along with the description of a new species, and they insist that a designated specimen—the type specimen—be deposited in a public museum or a university collection.2 All this effort, all over the world, has given us around 380,000 scientific names for beetles! Compare that with about 5,500 species of described living mammals, or a bit more than 10,000 species of birds. Shifting to the plant world, there are over 310,000 described species of land plants on our resplendent planet. This is a conservative number, including mosses, liverworts, ferns and their allies, all the conifers, as well as more than a quarter million flowering plants. Add all these green plants together and they still don't reach a number of species that can match the beetles. Returning to animals, there seem to be more species of beetles than there are species of all the non-microscopic animals living in all the oceans. This aquatic crowd includes jellyfish, many kinds of worms, mollusks of all sorts, lots of crabs, and thousands of fish species. Three-hundred and eighty thousand described species are a whole lot of species! And, as if that weren't trouble enough, their numbers keep growing as scientists discover new and undescribed species living high in rainforest trees, and even little ones hiding in the soil not far from where many of us live.
WHY ARE THERE SO MANY KINDS OF BEETLES?
Beyond God's pleasure, what might be the reasons for beetles being so numerous? Like so many puzzles in biology, this question elicits many answers. Surely, the first of many factors is the most obvious: beetles are little. The largest weigh about as much as a mouse, the smallest less than a fly. In strong contrast: there are fewer than a dozen species of rhinos and elephants alive in the world today. These are the largest and heaviest land animals, each requiring a range of many square kilometers in which to live and prosper. Beetles, on the other hand, can live their short lives within only a few square meters. Here we have a pattern seen throughout the living world: numbers of both individuals and species decline as the sizes of animals increase. Clearly, being small makes it possible to pack a lot of beetles into the same small area.3
Small size is just one factor helping us understand the diversity of beetles. Beetles are found in many diverse habitats, carrying on a grand variety of life activities. Huge numbers live in both temperate and tropical regions; a few can be found in the tundra and on high mountains, but none are resident in Antarctica. We know how flour beetles can invade our kitchens, carpet beetles damage wool rugs, and Japanese beetles chew on garden plants. In a warm June evening, june bugs (really beetles) fly into our screens, while fireflies (also beetles) flash signals in the dark of early evening. In addition to a grand number of species, some beetle species have populations made up of millions of individuals.
In the American West, ladybird beetles (also called ladybugs) move high into the mountainsides and congregate together to spend the winter. During the early 1900s, collectors would go into the Sierra Madre highlands of California to collect these slumbering beetles, selling them to farmers in their battle against aphids. (Ladybird beetles, both larvae and adults, feed on aphids.) This collecting activity, in the early part of the last century, saw the gathering of several tons—that's right tons—of beetles each winter. When you calculate that there may be 44,344,000 ladybird beetles in a ton of ’em, you've got a real lot of beetles.4 The take-home message is clear: by combining 380,000 species of beetles, with populations that may reach millions of individuals, it is no exaggeration to claim that our planet does indeed support billions of beetles. But what is it about beetles that have made them such a grand success?
Extinction is a constant erosive force in the history of life. A long fossil record tells us that perhaps 98 percent of all the species that ever lived are already long gone. Surviving changing climates, droughts, disease pandemics, and the occasional geological upheaval is a challenge a great majority of plants and animals have failed. Thus, when we see a lineage as successful as the beetles, we know they've got to be a well-adapted crowd. In fact, a simple glance at most any beetle will reveal their primary survival attribute: they are tough. Trekking over the leafy forest floor you can step on nearly any beetle, only to have it scurry away as you lift your boot. A hard exoskeleton amply protects a delicate interior! The key innovation here is that what was once a forward pair of wings became modified into hard wing covers (elytra) protecting the top of the entire abdomen.5 Their front and bottom sides are also armor encased. In fact, many species look like miniature tanks. The “Volkswagen beetle” got its moniker because this automobile really did resemble some beetles. Best of all, the tough wing covers of beetles can be raised up, allowing the actual wings to unfold and fly into the distance. Being rather solid means that, while most beetles can fly, they are neither fast nor agile. And that's why the larger ones fly only at night, avoiding the pursuit of predatory birds in the light of day. Being well protected, and with the ability to fly, gives them two significant advantages in the struggle for survival. But beetles have another trait that is even more significant.
A DIVIDED LIFE-HISTORY
Beetles belong to a large assemblage of insects with a four-stage life cycle; they undergo a complete metamorphosis (and are called holometabolous). Such insects have their lives divided into four separate and distinct stages. The first is the egg, which may be a short-lived stage, or it may be a means of surviving a harsh winter or a long dry season. At the appropriate time, the egg is broken open by its little occupant, a small larva. The larval stage is a significant part of the life cycle, being devoted entirely to eating and growing. Beetle larvae range from minute worm-like beings to the larger grubs we often find in soil and rotten wood, while others lead an active predatory life. Whether sedentary or active, this part of the life cycle is devoted to accumulating the energy needed to build the adult insect.
Becoming transformed from a fat grub to an elegant adult requires a separate stage: the pupa. While you may be more familiar with the pupae (plural of pupa) of butterflies and moths, all insects with a four-stage life cycle have this stage. It is within the pupal skin that much of the insect dissolves! That's correct: almost everything within their outer skin becomes liquefied. This fluid, reorganized by several special regions within the pupa, then develops into the fully adult beetle. A miraculous transformation, when you think about it, this is a significant factor in the success of many insect species. The adult can develop into a form completely distinct from its larva, able to fly off to find a mate and lead an entirely new and different life. In effect, beetles have two lives: a larval eating phase, and a travelling/reproductive mature phase. That's the good news. The bad news is that this life trajectory comes with a serious limitation: the adult can grow no more! The new adult is as big as it will ever become.
SCARABS AND BURYING BEETLES
Another significant aspect of beetle diversity is their many differing lifestyles. This allows different species of beetles to live in the same environment without getting in each other's way. For example, scarab or dung beetles spend much of their time looking for fresh dung. When they find it, a pair of beetles—mom and dad—will energetically create a spherical dung ball. The ball is rolled to a convenient site for burial, where the ball will house and nourish one of their larvae. Scarab beetles aren't likely to get in the way of burying beetles who busy themselves looking for the bodies of dead mice and birds. Burying beetles, like their scarab relatives, have complex behavioral traits to live their special lifestyles—all geared to rearing another generation of beetles.
As a boy and a collector of almost anything biological, I chanced upon a dead mouse in the woods bordering our country cabin. Here, I figured, was a chance to get a little mouse skeleton for my collections. So I tied the mouse carcass to a nearby stem with thin wire, making sure that no scavenging animal would cart away my prize. I then covered the body with leaves to further protect it. Busy ants, I thought, would clean the carcass and leave me with a fine little skeleton. Coming back only a day later to check on the deceased, I found that my mouse had disappeared. It was no longer under the covering of leaves. But the metal wire was in place, and it led into the ground. My mouse had been buried! Sure enough, after watching carefully, I noticed a pair of burying beetles digging to lower the dead mouse even deeper into the soil. As in the case of the scarab beetles, the buried mouse would provide nourishment for a new generation of beetles.
The burying beetle pair will continue feeding on their carcass and watching their young develop. They even use antibiotics to reduce bacterial activity on the carcasses they feed upon.6 In contrast, scarabs lay only one egg within the dung ball, bury it, and then go on to fashion more balls for more offspring. That's why the African savanna isn't covered with dung, despite herds of zebras and many species of antelope. In fact, Africa is home to over two thousand species of scarab beetles. By burying huge amounts of dung, these beetles perform important ecological services. Dung (cow patties are an example) can depress the growth of plants beneath them, serve as nourishment for hordes of flies, and help spread parasites and disease. In addition to removing dung from the soil surface, busy scarabs effectively fertilize the soil and help aerate compacted soil.
The importance of dung beetles became clearly apparent in Australia. Cattle were brought to Australia to develop a beef industry—great for producing hamburgers, but not so good for the landscape. It turned out there were no larger dung beetles in Australia to deal with the cow pies produced by our large domestic cattle. Soon pasture lands were bedecked with bone-dry, rock-hard cow pies. This new groundcover just sat there, like so many concrete platters, reducing the absorption of sparse rain into the soil, suppressing the growth of grass and forbs, and dramatically reducing the productivity of the land. Clearly, bigger species of scarab beetles were the missing element in the Australian outback.
Beetle variety is awesome, and not just in numbers. Diving beetles are voracious little predators in lakes and ponds, both in their larval and adult phases. Since they devour aphids, ladybird beetles are welcome in our gardens. Fireflies, or lightning bugs, fascinate us with their ability to produce flashes of yellow-green light. But there are multitudes of other kinds of insects as well. Dangerous wasps with potent poison and the stingers to deliver it; elegant dragonflies along the shores of lakes and streams; smelly stink bugs; bumblebees working one flower after another; the pesky flies, and lots more. Let's move beyond beetles and consider all the insects.
THE NUMBERS OF INSECTS
As we search for additional lineages with huge numbers of species, we find that these are also insects. The flies, order Diptera, number about 150,000 species, while butterflies and moths (Lepidoptera) have over 120,000 species. Put these all together, add in the beetles and lesser groups, and we probably have around one million species of described insects. And that's just at our current state of knowledge. Ants, bees, and wasps (the order Hymenoptera) currently number around 115,000 named species. However, one expert thinks the real number inhabiting our planet is closer to a million!7
These are huge species numbers in the world of living things. In addition, some species include large numbers of individuals. Surely one of the most spectacular expressions of insect numbers is that of locust swarms. Not only impressive in their vast numbers, these far-flying grasshoppers can devastate agricultural crops, threatening people with starvation. The most notorious of these is the desert locust (Schistocera gregaria), ranging from northern Africa to India. These are the creatures mentioned with foreboding in the Bible. My students and I witnessed a swarm of these migrating pests in eastern Ethiopia during a February field trip. Having stopped along a high mountain road, we looked into a distant valley and noticed an odd “cloud.” It was morning, and I assumed it was persisting early fog. But it did look strange; the moisture of early morning should have been long gone. The next day that very same cloud—a horde of migratory locusts—flew over our college campus. Probably about two kilometers long and half a kilometer wide (1.2 by 0.3 miles), this cloud took almost an hour to stream by. Standing under the passing horde, I couldn't begin to comprehend the numbers passing overhead. Fortunately, they weren't stopping to eat; they were in full travel mode. A few days later we saw this same swarm flying over the region's highest mountain, Gara Mulatta. To get across that mountain, these locusts were flying at an altitude of 3,400 meters (11,000 ft.)! Estimating the numbers of a particular swarm in East Africa, researchers came up with a total figure of about fifty billion individuals. And with each locust weighing around 2.7 grams (a tenth of an ounce), that swarm added up to 115,100 tons of locusts. That's a lot of grasshoppers! Nowadays, with global weather satellites showing rainfall patterns, locust outbreaks can be predicted, and measures can be taken to arrest their reproduction.
Another insect outbreak of extraordinary numbers occurs exclusively in the eastern United States. Only, in this case, the explosion of numbers is both predictable and long-delayed. Once every seventeen years (thirteen years in southern broods), our eastern woodlands are assaulted by a cacophony of deafening decibels. These are the calls of periodical cicadas that lived their long lives underground, sucking sap from the roots of trees and shrubs. Lacking the four-stage life cycle of holometabolous insects, cicadas—like grasshoppers—grow through a series of nymphal stages. Emerging together in the late spring of their seventeenth year, the nymphs crawl out of the ground and up stems and trunks. Once securely positioned, their skin splits open along the back, and the soft-bodied adult emerges. Hanging quietly for a few hours, wings expand and their exoskeleton hardens; they are now ready to fly. Soon the males begin their ear-splitting choruses; hoping to attract the ladies. Indeed, hiking through a forest harboring thousands of these tree-top choristers can be very uncomfortable! (Fortunately, they stop calling at nightfall.) Three different species of cicadas make up most of these singing swarms, and they are divided into a number of distinct geographical broods, each emerging on its own seventeen-year or thirteen-year schedule. While cicada species are found throughout the tropical and temperate world, only these particular species in the eastern United States display a uniquely synchronous emergence.
A DISTINCTIVE BODY-PLAN
Visually, the most obvious characteristic of insects is their legs. Virtually all mature insects have six legs. It makes no difference if you are looking at a fly, a bee, a grasshopper, an ant, a wasp, a termite, a roach; they've all got six legs (and are called hexapods). Here we're talking about adults; some caterpillars have six smaller legs up front and pairs of fat legs along the back segments. And then there are many insect larvae that have no legs at all (think of maggots), but for adults the picture is pretty uniform: six legs. In some, like the praying mantis, the front legs are adapted for grabbing prey, but it's pretty clear these are modified legs. Insect legs are jointed with three main sections, allowing for a wide range of movements. Lastly, claws at the end of the legs allow for grasping. Jointed legs give insects flexibility in posture, in movement, and six attachment points for hanging tight.
Another distinctive character is having an adult body divided into three sections: head, thorax, and abdomen. In fact the Latin word insectum means division. The head, up front where heads are usually found, holds the primary sensory organs (antennae and eyes), as well as the feeding apparatus. Mouthparts differ widely in different insect lineages, from those with plier-like pincers to those with piercing-and-sucking mouthparts. Having a grand variety of feeding styles allows different lineages to chow down a great variety of food sources. The second part of the body is called the thorax and provides a solid central section where legs and wings are attached. Here is where the muscles for flight and locomotion are located. The last part of the body is the abdomen, housing most of the intestines, breathing apparatus, and reproductive organs. All of these sections have a covering of rigid chitin, giving the body both rigidity and protection. But what might have been the primary key to insect success?
A GRAND INNOVATION: FLIGHT
Flight is perhaps the most significant of insect attributes. The fossil record indicates that insects were the first animals to propel themselves through thin air. Giant dragonflies flew through coal-age forests around 300 million years ago, and all the larger lineages of insects are capable of flying. The wings are not modified legs, and it isn't clear how insect ancestors managed to evolve two pairs of wings. By being attached to the top of the thorax these wings remain clear of the legs below and allow for mobility in flight.
A further advance was the ability to fold their wings against the body, great for hiding oneself in a safe niche. Two ancient lineages, the mayflies and dragonflies, cannot fold their wings against their bodies, and cannot tuck themselves into tight corners. Flight, in fact, marks the full maturity of insects. When fully developed, the wings cannot be replaced; once damaged they stay damaged. Whether having a gradual development (like grasshoppers, cicadas, and true bugs) or with a four-stage life cycle (like beetles, flies, and wasps) flight marks the end of further growth.
Have you tried to swat a fly? Not an easy task—unless you've got a flyswatter. Highly sensitive, house flies can feel an increase in air pressure as your hand moves toward them—and off they go. Overall, flies (order Diptera) are characterized by having only a single pair of wings. Behind the wings, flies have two short, slender, club-like devices called halteres. These vibrate during flight, helping maintain stability and orientation. Unfortunately, the flies include some of the nastiest organisms on the planet: mosquitoes, biting flies, and a variety of winged parasites.
The advantage of flight, coupled with a four-stage life cycle, is epitomized by nasty flies. Think of a female mosquito flying far and wide, seeking a blood meal that will allow her to create a raft of eggs. Placed on stagnant water, the eggs will give rise to predatory aquatic larvae (no lazy grubs these). The pupae float at the water's surface to release an adult mosquito capable of flying off to seek a mate, get a blood meal (in the case of females), and then finding another body of water to continue the life cycle. There are even more intricate strategies among flies.
While on a collecting trip in Costa Rica's wet and soggy Tortuguero National Park, I had forgotten my insect repellent and was pounced upon by a good number of mosquitoes. Not to worry, malaria wasn't prevalent in this area, and I figured the itchy mosquito bites would subside in a few hours. Not so! Two weeks later and back in Chicago, I still had three itchy bumps that kept right on itching. Worse yet, they were getting bigger! These things looked like growing tumors, and that was scary. So, off to a dermatologist for a closer look. Not recognizing what he was looking at, he scheduled a biopsy for the following week. But before the scheduled biopsy, while waiting to go to sleep, I was making myself miserable imagining cancer growing under my epidermis. The three enlarging tumors were located on my right thigh, my left arm, and at the top edge of my forehead. Then, quite suddenly, while lying there in bed, the “tumor” on my forehead went skritch, skritch, skritch!
This “tumor” was making a noise! Now, I knew almost nothing about cancerous tumors, but I did know they do not make audible sounds. Remembering a similar incident in Ethiopia, I realized there were three botfly larvae under my skin! Next morning, I gleefully phoned the dermatologist, and he quickly scheduled simple outpatient surgery to remove these beasts. Arriving at the hospital's surgery room, I was greeted by a crowd: doctors, nurses, and a bunch of medical students. In Chicago, there weren't a lot of botfly maggots to be dug out from under people's skin. Nevertheless, this was a show I didn't want to see, so—lying on the operating table—I stared quietly at the ceiling. Surgery proceeded and, as the second maggot was being excavated, a tall surgeon rushed into the room still wearing all his operating-room attire. Looking over the shoulder of the operating surgeon as the last maggot was removed, he blurted out, “This is really gross!”
Not just gross, but fascinating. My body had built tissue around these nasty maggots, so they were confined to my skin. That meant they grew slowly by scrapping away the surrounding tissue: skritch, skritch. I still carry a dent in my forehead where the little beast did its skritching. But here's the really extraordinary part: I had gotten these botfly larvae from mosquitoes! It seems that botflies aren't very good at finding warm mammalian bodies; but mosquitoes are! What the female botfly does is to grab hold of a female mosquito and attach an egg to one of the mosquito's legs; the egg then drops off the mosquito when the latter makes landfall on a warm mammal. This sounds more like science fiction than natural history, but it is a great example of how insects have been able to forge unique life histories, coupling flight with complex behaviors. A Field Museum zoologist recalled having collected a puma specimen many decades earlier in southern Mexico and finding many botfly larvae as they skinned the big cat.
Many genera of wasps are also flying parasites. Check out your tomato plants in late summer and you can often find a tomato hornworm festooned with little white cocoons over its body. These are made by minute parasitic wasp larvae that have finished feeding and are pupating on the doomed caterpillar. Here, natural selection has fashioned a very clever strategy. Such maggots feed first on non-vital parts of their host, before devouring the vital organs. In fact, the world's smallest insects are parasitic wasps. Inserting one of their eggs into the egg of another insect species, the little larva feeds within the solitary egg, finally emerging as a brand new little wasp.
All told, with their ability to fly, a multistage lifestyle for many lineages, and their small size, insects have proven to be one of Mother Nature's most successful life forms.8 Leaf miners, the caterpillars of very small moths and flies, do all their feeding between the top and bottom surfaces of a single green leaf. The world's smallest beetles live in decaying leaf litter on the forest floor. About a millimeter (1/24th of an inch) in length, these featherwing beetles have gotten as small as is possible for a beetle to be. The female featherwing can produce only one egg at a time; a beetle can't get any smaller than that!
Returning to the huge numbers of beetle species, I should mention the work of Terry Erwin in Panama. By fogging trees of the same species (Luehea seemannii) with insecticide, Erwin collected all the fallen insects on stretched canvas beneath the trees. In this way he attempted to survey the numbers of canopy insects in a single species of tree. Having fogged nineteen trees, he collected a huge number of little creatures, including almost 8,000 beetle specimens, representing about 1,200 species. That's just one tree species, but Panama is home to around 2,000 different species of trees! If some of these beetle species are found in only one or two tree species, you can imagine the numbers of canopy beetles in Panama.9 And surely, without the ability to fly, these many beetles could not maintain their high-canopy lifestyle. But despite their many forms and huge numbers, this six-legged alliance has not been able to get really big or really smart.
WHY HAVEN'T INSECTS GOTTEN BIGGER OR BRAINIER?
Each larger grouping of animals is tightly confined within an invariant body plan. Insects have six legs, spiders have eight legs, and crabs have ten. Our own lineage is similarly constrained. The land vertebrates, from frogs and toads, through reptiles, birds, and mammals all have four legs. Birds changed their forelegs into wings, and we changed ours into arms, but that's just a variation on an underlying and unchanging theme. Our lineage arose from early four-legged amphibians, and we are stuck in that format as tetrapods. It seems that once a new developmental protocol, such as a three-parted body plan in insects, becomes established, it is virtually impossible to change. Loss, of course is another matter; loss is easy and has occurred often. Many insects have lost their wings. Snakes have lost their legs, while dolphins transformed theirs into flippers; but the basic body plan, what German-speaking zoologists called Bauplan, remains invariant. And that's what makes it so easy to recognize crabs, spiders, and insects as crabs, spiders, and insects. But mammals range from little shrews, weighing less than a goliath beetle, to elephants and whales. Why haven't the insects done the same?
The reasons may be several. For one thing, a hard exoskeleton needs to be shed to expand body size. Grasshoppers (in the order Odonata) and true bugs (in the order Hemiptera) go through a series of molts before they are mature. These insects do not have the four-stage life cycle we've talked about in beetles and flies. Their eggs produce a little nymph, which proceeds through larger stages, molting at least four times to become the mature reproductive adult. Young grasshoppers, in fact, look like wingless adults. In contrast, the holometabolous insects with four life stages, confine all their growth to the larval stage and follow this with a major transformation within the pupa. Think about the tomato hornworm caterpillar as its mouthparts devour the leaves of your tomato plant. Fully grown, the larva will pupate and will give rise to a hawk moth, with a curled-up “tongue” that can be extended to four inches long (10 cm) and gather nectar from deep-throated flowers, while hovering with rapid wingbeats (like a hummingbird). From tough chewing mouthparts in the caterpillar to a long slender tongue in the moth, the same species lives two very different lives.
Pupation has given the holometabolous insects a platform from which to explore many different lifestyles. However, all insects remain constricted when reaching full maturity. Whether grasshopper, beetle, wasp, or butterfly, the mature insect cannot become larger because it can no longer shed its skin nor replace its wings. Really big insects, like the Goliath beetles of African rain forests and the stag beetles of Europe, become so large because they have big grubs. And since most grubs are incapable of repelling attackers, they need to be hidden away in safe surroundings. The larvae of these large beetles feed deep within rotting wood, where they are hard to find. These same large grubs may feed for four to eight years before they reach full maturity and are ready to pupate. That's a long time, and a good reason for staying small, but there's more.
A larger active animal, like a bigger more powerful car, requires more fuel. Not only more fuel, but fuel that has to be burned faster. For animals, that means bringing in a lot of oxygen to keep the fires of respiration going. Surely the best set of lungs among land animals are those of birds. In birds, air passes through the lungs and out in one continuous path; a necessity for fast continuous flight.10 And this is where insects have a major problem: they don't have anything equivalent to lungs. Air diffuses into the insect through apertures in their sides (spiracles); oxygen is absorbed into their bloodstream, and then pumped around the body. There is nothing comparable to the elaborate alveolae in our lungs, providing large surfaces over which oxygen can be absorbed by the bloodstream. Nor do they have a diaphragm that pumps air in and out of their bodies. Paleontologists conjecture that the giant insects of the Carboniferous period lived during a short period when oxygen became as much as 30 percent of the atmosphere, in contrast to the 21 percent we live with. When oxygen levels returned to lower levels, the giant insects vanished.11 Because brain tissue requires lots of oxygen to do its work, limited oxygen uptake results in little brains for little insects. The combination of a tight exoskeleton and an inefficient oxygen-uptake system has doomed insects to remaining forever small but nimble.
This presents us with a larger question: why are the insects stuck in the body plan that defines them? Similarly, why couldn't land vertebrates, over more than 300 million years, grow themselves another pair of legs? (Imagine being able to scratch your back, while your two arms and two legs are otherwise engaged). It seems that all the larger groupings of animals are tightly constrained within the confines of a stereotypical Bauplan. Thanks to recent DNA studies, we now understand why each lineage is confined to a specific body type. Research has shown how a careful program of gene activity transforms a little mass of undifferentiated cells into a much larger and more complex animal. The little ball of cells—the proembryo—comes directly from the fertilized egg, through a series of simple cell divisions. But then developmental protocols kick in and begin to chart a more complex course. They instruct the little group of cells to form a hollow ball, then have part of the wall bend inward, resulting in two cell layers. With expansion and elongation, a head end and a tail end, the bottom and the top, become clearly defined. That's how all us animals begin, whether a worm, an insect, or a vertebrate. These developmental protocols are the genesis of a different kind of diversity: the diversity of cells and tissues within a single complex animal. Once these protocols were established within a lineage, changing them has been all but impossible.12
UNDERSTANDING ANIMAL DEVELOPMENT
Robert Macarthur, late ecologist, suggested that biological scientists come in two varieties. The engineers, he proposed, want to know how things work, while the second group, historians, want to find out how things got the way they are. Animal diversity, whether billions of beetles or hundreds of cell types, can be viewed from the same two points of view. First is the here and now of how things work. So many beetles living today; so many different tissues making up the body of a single fly; so many complex processes in each living being. In contrast, there are the historical questions: How is it that our planet got to have so many beetles? And how were the developmental protocols that produce such a variety of plants and animals initiated?
Both the development of a human baby from simple embryonic beginnings and the emergence of a butterfly from its pupal skin are awesome transformations. Each is a grand advance in structural complexity, and both are driven by similar genetic protocols—though each is unique to its particular species. The fact that such developmental trajectories have produced billions of beetles should not diminish our astonishment. Some things we see around us are truly wondrous, even when there are a lot of them.
Today, with a rapidly expanding understanding of the genetic code, and the realization that fruit flies, mice, and people all use similar genes to build their bodies, the glory of morphological development is yielding its secrets. No longer the product of mysterious miracles, the similarity of genetic instructions in a wide array of animals has made clear how Mother Nature has used common tools, over and over again, to build the huge array of living beings that call our planet home.
WHY ARE THERE SO FEW INSECTS IN THE SEA?
Our planet is the watery one—the only blue and white planet in our solar system. More than three quarters of the Earth's surface is covered by either water or ice. Seen from afar, large swirls of water droplets form white clouds sweeping across our blue planet. There really is a lot of water on this planet of ours, and that's where life began: within the sea. Walk along a seashore and take a good look at the diversity of what's washed up along the beach. Don't look at the numbers of species; instead, look for the number of phyla. Phyla are the really big divisions of the animal world; they range from fishes, crabs, starfish, jellyfish, and seashells to a goodly number of worm-like creatures. All the animal phyla have representatives living in the sea. But where are the insects?
Insects belong to a large phylum of animals called the arthropods. Having jointed legs, they include the crabs, lobsters, horseshoe crabs, spiders, scorpions, centipedes, and millipedes—and the insects. Many arthropods live in the sea, but only a very few of them are insects. There are a few spiders, but no millipedes or scorpions are marine. Water beetles and mosquito larvae spend a good part of their lives under water, but they do this in fresh water. Insects live almost entirely on the land or in land-based aquatic ecosystems—why is this so?
Insects are a terrestrial lineage, and if they did evolve in water it was in fresh water. Based on the most primitive insects living today, it appears that they originated in ponds and streams—not from along the ocean shore. The richness and complexity of a land biota allowed insects to diversify more abundantly than any other living lineage. A simple reason for insects not having ventured into the sea is the vast array of predators waiting beneath the waves.
In similar fashion, vertebrates pioneered the land from streams and estuaries, not at the ocean shore. Further along in the history of life, mammals originated from terrestrial mammal-like reptiles, and since that early time only a few mammalian lineages have become aquatic. Among these are otters, beavers, and hippos in freshwater, and whales, dolphins, seals, and sea lions in marine habitats. Regardless of how much time these mammals spend in the water, all need to fill their lungs with air from above the water's surface. Unable to breath underwater, they betray their terrestrial origin. Next let's leave the Animal Kingdom and look at another lineage of terrestrial life-forms with high species numbers—the flowering plants (what botanists call the Angiosperms).
ANOTHER BIG CROWD: THE FLOWERING PLANTS
Angiosperms—flowering plants to most people—are not only numerous in species, they come in a grand variety of sizes. In the case of most beetle species, you can stuff a thousand of them in a gallon jug and still have space left over. Most flowering plants, in contrast, come in larger sizes. The smallest flowering plants are the floating freshwater duckweeds (Lemnaceae family), ranging from about the size of an O in this particular font to an inch or two in length. With only about thirty species, they are the bottom end of the size scale in flowering plants. At the other extreme, Angiosperms include eucalyptus trees up to three hundred feet (97 m) in height, baobab trees with trunks as wide as a truck, and the double coconut palm with leaves up to thirty-five feet (11 m) long. Simply stated, flowering plants are the most diverse group of larger organisms that ever lived on planet Earth! Whether small grasses or tall trees, flowering plants are major players in terrestrial ecosystems. Fish also come in a wide variety of sizes, with whale sharks reaching forty tons, but fish species number around 30,000 species. Mammals include the largest creature ever—the blue whale—reaching one hundred feet (30 m) in length and weighing over one hundred tons. But living mammals number around 5,500 species.13 In contrast, recent estimates place the number of described flowering plants at around 282,000 species. And with around a thousand new flowering plant species described every year, they are bound to get to 300,000.14
Clearly, when you combine size and species numbers with the food production of photosynthesis, the flowering plants are the biggest game in town! They include the grass family (10,550 species), the legume family (19,500 species), the exquisite orchids (22,500 species), and the sunflower family (23,600 species).15 But keep in mind, high species numbers may not add up to a major environmental presence. Orchids tend to have relatively few individuals in most plant communities. When discussing biomass, orchids are an insignificant part of natural landscapes—even in cloud forests where many sit as epiphytes on trunks and branches. Grasses are different, often dominating broad landscapes with billions of individuals. Many parts of the world are called grasslands for this very reason, including prairies, steppes, savannas, and mountain-top alpine communities.
With their generous numbers, their varied sizes, and a complex architecture, flowering plants are the primary determinants of vegetation structure in a majority of today's ecosystems. Not only are they significant for their three-dimensional forms, but also as sources of nutrition. Since all but a few flowering plants are green and photosynthetic, they stand at the base of local food chains. It is these—and other green organisms—that convert solar energy into nourishment for all of us. In fact, as flowering plants expanded their numbers over the last 100 million years, other lineages have been able to expand their numbers as well. A recent study of beetles clearly showed that those beetle lineages feeding on flowering plants have achieved greater numbers than beetle lineages using other food sources.16
Conifers are not flowering plants, and these trees dominate many colder and seasonally stressful habitats. However, while pines, firs, redwoods, junipers, podocarps, araucarias, and other conifers can have very high numbers of individuals in a great variety of forests and woodlands, they total only around one thousand species worldwide. And, though they dominate cooler northern and many montane forests, they are all trees; only their juveniles are important in the understory. More significantly, conifers defend themselves with a strong chemical arsenal. That's why conifer wood is such great building material, why very few animals feed on pine needles, and why the understory of conifer forests is so poor in species. There's simply not as much to eat in a conifer forest. In contrast, flowering plants invest less energy in self-protection and more in growth. In a sense, they “live fast and die young,” powering a more nutritious vegetation. Beyond flowering plants and conifers, ferns are another important and species-rich lineage of land plants. Ferns and the fern allies (such as lycopods and horsetails) number around 12,000 species. Though only a very few are tree-like, many species are common in moist forest understories, and some are an important component of the epiphytic community in moist forests. Because they reproduce with swimming sperm, ferns, mosses, and their allies are restricted to evergreen or seasonally moist vegetation. All told, flowering plants are both the most numerous of land plants, the most nutritious, and the most colorful.17 We will return to the subject of land vegetation and species numbers in chapter 4 and discuss the role of the flowering plants in making terrestrial ecosystems ever more complex in chapters 7 and 8.
As we've noted, plants and animals have many more species in terrestrial environments than in the oceans. But this generalization may not hold true for that kingdom of living things, which includes the smallest forms of life: the bacteria. No matter whether we are counting the numbers of individuals, numbers of lineages, variety of biochemical pathways, or varied abilities to survive in extreme environments, the bacteria rank first on planet Earth. Next, let's take a closer look at life's most successful crowd: the bacteria.