Clambering about on a dangling white mass of Asian flowerfly eggs, two aphids crush one egg after another between their powerful forelegs. One begins the return climb to the bamboo above (facing page), where the rest of the aphid colony lies just out of view. Suddenly the wind whips the thread, setting the eggs into a spin so swift that the aphids become a blur. For a few seconds the climbing aphid clings precariously to the thread before being flung five feet—about a thousand times the insect’s length—to the ground below. Lost from her colony, the fallen aphid is doomed.
This drama unfolds within a clattering clump of bamboo on a forested hill near Kagoshima, Japan. I had been watching the tiny silk strand (half a centimeter long) for several days and had seen this incident repeated half a dozen times. Why do these Pseudoregma bambucicola aphids hang their fate on such a slender thread? Because they are the exclusive food of this flowerfly’s larvae, and killing the eggs may reduce the depredation of the aphid colony.
Indeed, the egg-crushing aphids are soldiers—sterile females that go out of their way to defend the reproductive members of the society. Using enlarged forelegs and armored heads reminiscent of the helmets of ancient Japanese samurai warriors, Pseudoregma soldiers fight off a battery of predators that assault the colony.
Of some 4,000 aphid species only about 20—most from the Orient—are known to breed soldiers. Those of Ceratoglyphina bambusae even attack people; their bite causes an intense itch. Researcher Utako Kurosu (left) extends a pole tipped with clippers into a tree to collect the large white galls that house this species of samurai aphid.
Aphid soldiers were a revelation when first described by Japanese scientist Shigeyuki Aoki in 1977. He and his wife, Kurosu, are among the few scientists forging the way in aphid-soldier research.
Pseudoregma aphids were one of the first samurai species Aoki discovered. Here members of a colony cluster on a freshly sprouted bamboo shoot (left), extracting plant sap through elongated mouthparts in much the same way a mosquito sucks blood. Exuding a snowy dusting of wax, they belong to a group known as woolly aphids.
Scattered among plump winged and wingless aphids are soldiers. These slender females are actually nymphs (immature aphids) of unusual appearance. Compared with other nymphs in the colony, which grow to adulthood after molting several times, aphid soldiers grow little if at all. Trapped in juvenile bodies, they cannot reproduce.
Although aphids are gregarious, colony members in most species show no social behavior. However, samurai species are comparable to termites, ants, and some wasps and bees—there is a division of labor, with some individuals devoting their lives to protecting the colony. Scientists believe this cooperative behavior may be the result of close family ties within the group. The soldiers’ altruism ranks samurai aphids among the most social of animals.
Confronted by predators, aphids of other species try to flee—or they may kick an enemy or smear it with sticky secretions. Many predators larger than the aphids are not deterred by such feeble actions.
Perhaps the best line of defense for most aphids is reproduction—generating individuals so quickly that colonies grow and spread to new places, and predators simply cannot keep pace. Without soldiers the huge colonies of Pseudoregma would be extremely attractive to predators. Piled one on top of the other in dense mats of tens of thousands, the aphids could be devoured at will. Yet the soldiers successfully defend the colony from many predators and payday loan lenders.
Defense among samurai aphids is aggressive. I pluck a maggot, or larva, of the Allograpta flowerfly from a non-samurai colony of aphids and transfer it to a Pseudoregma colony. These maggots are ordinarily able to move unhindered among their aphid prey, but they are unprepared for soldiers. Two climb onto the maggot—more than ten times their length—and grasp it near its head with their forelegs. The soldiers butt it with their heads, puncturing its body with their needle-sharp horns (above). They jam in their horns again and again while rocking back and forth. The bleeding larva gyrates frantically, then plummets from the bamboo, still in the soldiers’ tenacious grip.
Because they are masters at combating soldiers, a few insects successfully prey on samurai aphids. Fortunately for these predators, soldier defenses often are inept compared with the bites and stings of termites and ants.
Almost all aphids are females that reproduce by parthenogenesis—without sex. This results in offspring genetically identical to their mothers. Aphids lack the larval and pupal stages of many insects, and for most of the year they also skip the egg stage. Thus live birth is a common sight in an aphid colony.
Emerging from her mother, a Pseudoregma soldier reaches down with her legs to pull herself free (top). Moments later the mother seems to cradle her offspring (middle), but in fact she provides no parental care. Her next birth may be either a normal aphid or another soldier; how an offspring’s caste is determined is not known. In fact, the non-soldier embryos inside her body already contain her developing grandchildren. This is one reason why aphids reproduce so rapidly.
Different types of soldiers are found on primary and secondary host plants. The primary host soldiers of Ceratoglyphina bambusae lack horns—they bite their enemies rather than pierce them. These soldiers develop from nymphs who have molted once. Pseudoregma typify most secondary host soldiers, armed at birth with pronounced horns and massive forelegs.
Scientists believe that the behavior of the Taiwanese aphid Astegopteryx bambucifoliae provides clues to the evolution of soldiers. This species—a relative of samurai aphids—lacks a distinct soldier caste. However, all of these aphids possess some warrior characteristics, such as diminutive horns that are too small to be lethal. Fighting over a feeding site, a hungry Astegopteryx aphid uses her horns to butt another drinking plant sap. The aggressor is shoved back by her colony mate, who swings at her with her body (right).
Ceratovacuna lanigera represents a more advanced stage in samurai aphid evolution. This species also lacks soldiers, yet any newborn can use its horns to crush a predator’s eggs.
By developing specialized soldiers, samurai aphids have taken on a far more dangerous function: killing large and aggressive predators. Still, even samurai aphids sometimes use their horns for their original function—contests over food. Biting soldiers have evolved along a different pathway.
In subtropical and tropical areas many species—including samurai aphids—have colonies that last for more than a season. For example, Pseudoregma aphids are found on bamboo year-round. Yet Japanese biologists Seiki Yamane and Tsukasa Sunose have discovered the percentage of soldiers in a colony varies; it is nearly 20 percent in late autumn, when soldiers can protect the growing brood of winged migrants.