The Spider’s Charade


By Ximena Nelson X. ネルソン
English 日本語 日本語
Imposters abound in the animal kingdom. Peruse any textbook description of mimicry—in which one species evolves to resemble another—and you will encounter various classic ex­­amples, such as the king snake, which copies the coral snake, or the hoverfly, which masquerades as a bee. Less familiar, but in many ways even more fascinating, are the mimics in a genus of jumping spider known as Myrmarachne, which look for all the world like ants.  動物界はペテン師であふれている。生物が別種の生物に似た形態に進化する「擬態」について述べた教科書をざっと見れば,実に様々な例が載っているはずだ。サンゴヘビをまねたキングヘビ。ハチに扮装したハナアブ。そして,それほど知られてはいないものの多くの点でさらに魅力的なのがアリグモ属(Myrmarachne)として知られるハエトリグモの仲間で,どこから見てもアリなのだ。
Unlike other jumping spiders, with their furry, round bodies, Myrmarachne species have smooth, elongate bodies that give the appearance of having the three distinct parts—head, thorax and abdomen—of ants, despite having just two. To complete the charade, the spiders walk on their three rear pairs of legs and raise the fourth pair overhead, waving them around to simulate ant antennae. They even adopt ants’ characteristically fast, erratic, nonstop mode of locomotion in place of the stop-and-go movements other jumping spiders make. It is an Oscar-worthy performance and the secret of this group’s success: more than 200 species of Myrmarachne thrive in the tropical forests of Africa, Asia, Australia and the Americas. This rich diversity makes ant mimicry the most common form of mimicry. Yet it is the least known.  通常のハエトリグモが丸っこくて毛の生えた体をしているのと異なり,アリグモ属のクモの体は長細くてツルツルで,アリと同じく体が3つの部分(頭部と胸部,腹部)に分かれているように見える。実際は2つなのだが。そしてお芝居の仕上げとして,後ろの3対の脚で歩き,残る前の1対を頭の上に掲げて揺り動かし,アリの触角に見せかける。さらに,他のハエトリグモは少し進んでは止まりまた進むという歩き方なのに,アリグモはあちこち不規則に素早く常に動き続けるアリそっくりの歩き方だ。
New research is exposing the mind-boggling complexity of the ant mimics’ charade, however. Like the king snake and hoverfly, Myrmarachne species gain a survival advantage by looking like other species—in this case, lethal ant species, because predators of spiders steer clear of both the ants and their look-alikes. But, it turns out, the spiders pay for that advantage: to give a convincing performance, they must expose themselves to considerable risk. The evolutionary forces that led to their fakery have left the ant-mimicking spiders living on the knife’s edge, walking a fine line between avoiding one enemy and falling prey to another. In revealing the unexpected perils of mimicry, studies of these remarkable arachnids show the phenomenon of mimicry in a new light.  それでも近年の研究によって,アリをまねるこの擬態の想像を絶する複雑さが明らかになってきた。キングヘビやハナアブと同様,アリグモ属は他の生物種に似ることで生存上の利点を得ている。クモを捕食する動物は攻撃的なアリやそれに似たものを避けるので,アリグモはアリをまねることで敵から逃れている。