Of Killer Caterpillars and Vampire Bugs
Hawaii's weird and wacky insects evolved in splendid isolation. But now there's trouble in paradise
A VORACIOUS predator hides somewhere in this lush Hawaiian rain forest, and photographer Bill Mull has offered to lead me to it. We walk beneath lichen-crusted ohi'a trees until Mull waves me to a stop near a clump of wild ginger. He bends down for a closer look, fishes a hand lens from his pocket . . . and there it is, the killer I've come to see: an inch-long caterpillar, straddling the edge of a leaf.
At first glance it looks like any inchworm: green, soft-bodied, nonthreatening. Then Mull lets me peer through his lens at the three pairs of wicked-looking tarsal claws.
Later I see those claws in action when Steve Montgomery, the biologist who discovered these killer caterpillars, gives me a tour of his collection at the University of Hawaii. Montgomery, a member of the National Wildlife Federation's board of directors, picks up a frozen fly with tweezers and brushes it lightly against the sensory hairs on one caterpillar's hind end. Instantly, the inchworm snaps its upper body backward and sinks its claws into the fly.
FLIES ON STEROIDS:
The ancestors of Hawaii's insects arrived by wind or sea, and began to fill the vacant ecological niches--with remarkable results. One or two species of fruit flies appeared on Kauai about five million years ago and have since evolved into 1,000 of kinds of flies, including this picture-wing Drosophila (above, top) that dwarfs its mainland cousin.
Almost everywhere else in the world, caterpillars are gentle plant eaters. Only Hawaii has carnivorous inchworms that ambush their prey. And not just one kind: Here, in mid-ocean isolation, at least 18 species of killer caterpillars have evolved to fill the ecological niches that preying mantises occupy elsewhere. Carnivorous caterpillars are not Hawaii's only insect oddballs. There are flightless flies, a terrestrial water treader, underground tree crickets and summit vampire bugs that suck the juice from insect corpses, just to name a few. Many of these wacky Hawaiians are endemics--found nowhere else on earth.
What I learned this past summer, on a month-long trip to investigate Hawaiian environmental issues, is that increasingly rare native insects face many problems. Their native host plants are being overrun by exotic vegetation. Their habitats are being bulldozed for development, or chewed into oblivion by introduced goats, sheep or pigs. And they are threatened by alien competitors and predators: insects brought here deliberately to control crop pests, and animals that arrived as accidental stowaways, such as the insect-eating frogs hidden on potted plants.
Photo: © WILLIAM P. MULL
To understand why Hawaiian insects are in trouble, it helps to look at why the state has so many offbeat insects to begin with. The answer lies in the island chain's extreme isolation. Ancestral insects got here by accident: blown by the wind, floating on a raft of debris or as hitchhikers on migrating seabirds. Not many creatures survived the grueling transoceanic trip. Indeed, many insect pests, including ants, cockroaches, termites and mosquitoes, were unknown in Hawaii until humans brought them here.
The hardy insect pioneers found the evolutionary playing field wide open. Studies suggest a mere 400 founders gave rise to an estimated 10,000 insect species, in a process that scientists call adaptive radiation. Insect populations radiated out into the islands' many vacant niches--from shoreline to alpine summit and from desert to rain forest. As they adapted to new sites, new species resulted.
Of course creatures other than insects also have experienced adaptive radiation in Hawaii. The colorful and rare birds called honeycreepers get the most attention. Now some scientists are saying Hawaiian insects deserve more press. In his address to the Society for Conservation Biology, which met in Hilo last summer, Sir Robert May, chief scientific adviser to the British government, chided his audience. "It's much more glamorous to look at beautiful birds than at grotty things through a microscope," he said. "But if you are interested in how ecosystems evolved--and how they function--you can learn much more from microorganisms."
And Hawaiian insects are hardly "grotty." Dan Polhemus of the Smithsonian Institution and the Bishop Museum in Honolulu calls damselflies "the honeycreepers of the insect world-- because they fly around, they're brightly colored and they've experienced an equally spectacular radiation." For charisma, the signature bird of Hawaii--the flame-colored i'iwi--has its counterpart in the candy-apple-colored crimson Hawaiian damselfly. As for adaptive radiation, the 24 species of damselflies found in Hawaii most likely evolved from a single species, adopting new breeding habits along the way. Juvenile damselflies, called naiads, usually live underwater, but some Hawaiian naiads live on wet gravel or moss, or even in tiny pools of water at the base of clustered leaves. "Hawaii also has the only documented case of a terrestrial-breeding damselfly," says Polhemus. Naiads of Megalagrion oahuense live in damp leaf litter on the forest floor.
Hoping to see some tree-dwelling damsels, I join a field trip to Hawaii Volcanoes National Park. From the roadside, a muddy trail snakes into the forest of native ohi'a and tree fern. Not far from the trailhead Polhemus points out a tree supporting a native ‘ie'ie (climbing screw pine). Its clusters of straplike leaves resemble overgrown spider plants; spreading one leaf cluster apart, he reveals clots of mud at the base--ideal habitat for the thick-bodied naiads of M. koelense. We find none, but Polhemus later shows us one he's taken from the leaves of an arboreal lily. It has prominent gills, suited for life in an in-between world.
Photo: © WILLIAM P. MULL
After arriving in Hawaii, a few bugs found their ways into lava tubes and evolved into blind, flightless cave denizens such as this water treader, a scavenger that wanders around on dark, slime-coated walls. Some cave creatures have evolved in as few as 500,000 years--a mere blink of an eye in geologic time.
One other thing damselflies have in common with honeycreepers: most species face grave threats to survival. Two damselfly species are presumed extinct, and six more are candidates for listing. In addition to habitat loss, damselflies have to worry about the 40 fish species introduced to the islands' streams. Many--especially top minnows, brought here to control mosquitoes in the 1930s--dine on damselfly naiads. Having evolved in the absence of predators, native naiads are clueless about self-defense, says Polhemus: "They cruise around in the daytime and just get chowdered."
While we discuss damsels in distress, Dave Foote huddles over a bucket farther up the trail. A biologist with the U.S. Geological Survey (USGS), Foote is studying Hawaii's picture-wing Drosophila. He dips sponges into a smelly mass of fermented mushrooms and props the soggy wads in trees. Within minutes, the sunlight illuminates the transparent wings of flies attracted to the bait.
Picture-wings are named for the delicate black patterns on their wings--a unique pattern for each species. Like killer caterpillars, they are quirky compared to their mainland cousins. Some are ten times larger than conventional fruit flies. And picture-wings have shifted their feeding habits from fruit to decayed tree bark, fallen leaves, rotting flowers. They have bizarrely shaped legs and mouthparts, and--more like birds or mammals than flies--defend small mating territories where they engage in elaborate courtship dances. Males of D. heteroneura defend their feeding sites by butting heads like rutting bighorn sheep.
The 110 known species of picture-wings (as many as 1,000 native Drosophila may exist) appear to have evolved from one (or perhaps two) species that made it to the island of Kauai about five million years ago, according to research by Hampton Carson of the University of Hawaii. Each species prefers a particular habitat on a single island. The flies feed and lay their eggs on decaying plant matter, and some rely on a single native plant species. Given that alien plants have almost entirely replaced natives in most of Hawaii's lowlands, it's no surprise that 15 picture-wing species are proposed for listing as endangered.
As we continue along the trail I notice what look like miniature plastic birdhouses hanging in some of the trees. These, Foote explains, contain bait for introduced yellow jackets. Aggressive predators that eat other insects, these yellow jackets arrived in Hawaii in the fall of 1977, most likely in a shipment of Christmas trees. Declines in several native insects, including killer caterpillars and picture-wing Drosophila, closely parallel the expansion of wasp populations. Other exotics, such as the big-headed ant from Africa and the Argentine ant, are also implicated in the decline of native insects.
Photo: © WILLIAM P. MULL
LEAF ME ALONE:
The koa bug, one of the largest and most colorful of Hawaii's insects, feeds on plants. But a parasitic fly introduced to the archipelago to help control agricultural pests related to this insect has reduced koa bug numbers.
Scientists have been studying Hawaiian Drosophila and damselflies for more than a century. But other insect oddballs were discovered only recently. Most researchers had assumed the many lava-tube caves on the islands were devoid of insect life. Frank Howarth, an entomologist at Honolulu's Bishop Museum, proved them wrong in 1971 when he discovered a whole community of cave creatures: tree crickets that have never known a tree, water treaders that walk on slime-coated cave walls and plant hoppers that drum on tree roots to contact potential mates.
Cave insects didn't get here from the mainland. The critters you find in continental limestone caves tend to be blind, soft-bodied and flightless--hardly likely to survive a transoceanic trip. And until Howarth's discovery, experts doubted that cave creatures could evolve so quickly here. Most lava tubes persist for just a few thousand years--not long enough for speciation to occur. Even though tubes don't persist, it seems an underground network of narrow cracks and crevices does, and this network provides avenues for cave creatures to migrate from older to younger lava tubes--and to evolve in as few as a half-million years, light-speed in evolutionary terms.
Montgomery and Howarth have discovered another insect oddball--the wekiu or summit vampire bug--in yet another place previously presumed to be barren: the summit of Mauna Kea, elevation 13,796 feet. Peter Oboyski, an entomologist with USGS, let me have a look at some preserved specimens. Hardly bigger than rice grains, the bugs didn't seem too exciting till Oboyski listed their unique characteristics: unusually long legs (handy for scrabbling over loose volcanic rubble) and short, stubby wings (wekiu bugs have lost the ability to fly, a valuable adaptation in a place where gale-force wings could loft a speck-sized bug into the stratosphere).
Then there are the wekiu bug's soda-straw-shaped mouthparts. This design feature is also seen in the creature's closest cousins--bugs that pierce seeds and suck out the contents. But no seed-bearing plants grow on Mauna Kea's cold and windy summit. So wekiu bugs have adapted to pierce and suck the blood from aeolian waifs--dead and dying insects that the wind carries up from lowland valleys. Meanwhile the bugs' own blood contains antifreeze that protects them from sub-zero temperatures.
Hardy wekiu bugs can cope with extreme conditions, but not, apparently, with habitat disruption, as when a new astronomical observatory gets constructed on the already crowded summit. "The bugs live amid loose rubble," says Bill Stormont, who manages the Mauna Kea Science Reserve. "During construction, dust and runoff fill up the spaces between the rocks." Surveys made in 1982 and 1997 suggest wekiu bug numbers may have declined significantly.
Reflecting on the plight of Hawaiian insects, Polhemus says, "There's no single impact, and no single solution. We're still trying to figure out what steps to take, and in what order." For the past decade, he has spearheaded an effort to locate and monitor Hawaiian damselfly populations, gathering data essential for recovery efforts. Meanwhile Foote has experimented with exclosures that prevent feral pigs from rooting around in native forests; results suggest that when the plants they rely on are protected, endemic Drosophila can recover.
If steps on the road to recovery are small, the journey has definitely begun. Polhemus reports that the damselfly M. williamsoni, long presumed extinct, was recently found in a remote area of Kauai. That's why Montgomery keeps hiking to the top of Mount Tantalus, which overlooks Honolulu: He's looking for the Koolau flightless fly, which was first discovered on this site, but hasn't been seen alive in a century.
No luck on the day I hiked up with him. But Montgomery plans to keep on looking.
Pennsylvania writer Cynthia Berger traveled to Hawaii with funding from The William and Flora Hewlett Foundation.
Within the genetic code of Hawaii's honeycreepers, scientists may find clues to help save the world's most imperiled group of birds. To learn more, read "Searching for Hope in the Family Tree" (National Wildlife, April/May 1998).