Two male harlequins butt heads in a battle over prime 

 mating territory on a fallen fig tree. When the 

 combatants are equally matched in size, as these are, 

 the fight may last as long as half an hour. 



Dauid W. Zeh 



irritated by the probing claws, the female 

 harlequin flexed her abdomen and the 

 pseudoscorpion crawled aboard, disap- 

 pearing beneath her wing covers. 



Just from the size and bulbous appear- 

 ance of its claws, we could tell that this 

 pseudoscorpion was a big male. This 

 marked external difference between the 

 sexes — known as sexual dimorphism — 

 suggested that strong sexual selection (ei- 

 ther through female choice or male com- 

 petition) had exerted its force on this 

 species. Darwin was the first to recognize 

 that sexual selection might exaggerate and 

 perpetuate certain male traits, but more 

 than a century after he first drew attention 

 to this phenomenon, an unresolved prob- 

 lem still puzzles evolutionary biologists: 

 If, over long spans of evolutionary time, 

 champions of male combat or the flam- 

 boyant beaus preferred by females consis- 

 tently sire more offspring than do their 



smaller or less showy rivals, the "lesser" 

 males should eventually disappear from 

 populations. But they don't. Indeed, the 

 enormous variability in the size of C. scor- 

 pioides males in museum collections 

 prompted Austrian taxonomist Max Beier 

 to describe it as the most variable pseu- 

 doscorpion known. 



We realized that the beetle-riding pseu- 

 doscorpion was an ideal species for study- 

 ing how male variability is maintained, but 

 finding pseudoscorpions and the beetle 

 hosts in their natural habitat had always 

 been difficult. 



When we first began our research in 

 1987, we searched the Panamanian forests 

 for two months without finding a single 

 dead fig tree. Then, one morning in early 

 December, we decided to combine field- 

 work with sightseeing and hiked Las 

 Cruces Trail. Cut through the forest by 

 slaves, this pathway was once the conquis- 

 tadors' major route across the isthmus. 

 Our only companion was a giant Morpho 

 butterfly fluttering erratically down the 

 path ahead of us, its metallic blue wings 

 flashing against a background of lush 

 green. Following our lepidopteran scout 

 around a bend, we came upon a dead, but 

 still standing, fig tree, a mere twenty yards 

 from the infamous trail. 



The roots that buttressed the 130-foot- 

 tall tree were surrounded by fallen bark 

 and mounds of pale yellow sawdust, con- 

 spicuous evidence of harlequin beetle lar- 

 vae tunneling within its trunk. The tree 

 was pockmarked with dozens of elliptical 

 holes, tunnel entrances leading deep into 

 the heartwood. Most striking was the rip- 

 pled appearance of the exposed outer sap- 

 wood, where the beetle larvae had gouged 

 large, curving tunnels just beneath the sur- 

 face. The decaying tree was an oasis in an 

 otherwise hostile environment. The wood 

 of fig trees is very soft compared with 

 most other tropical species, and the copi- 

 ous, nutrient-laden sap supports thriving 

 colonies of bacteria and yeast, the basis of 

 the rotting tree's food web. 



The dead tree itself seemed strangely 

 alive, with loud gurgling noises emanating 

 from the trunk. (These sounds, we learned 



later, were produced by the wood-boring 

 larvae of pantophthalmid flies, feeding 

 ravenously as they cut perfecdy cylindri- 

 cal holes. One of the largest flies in the 

 world, it has its own species of pseu- 

 doscorpion hitchhiker) The rotting wood 

 was an entomologist's paradise, buzzing 

 with anvil-headed fruit flies; stilt-legged 

 flies; blue-bodied, yellow-headed stra- 

 tiomyid flies; weevils; giant orange click 

 beetles; rove and bark beetles; and four- 

 inch-long cockroaches. And there were 

 predators: female parasitic wasps, tailless 

 whipscorpions, ambush bugs disguised as 

 miniature garbage heaps, and raiding 

 hordes of ants. All were feeding, fighting, 

 mating, or depositing their eggs. 



In the sawdust and under the bark, we 

 found C. scorpioides by the dozen — large 

 males, small males, females carrying 

 brood sacs, nymphs. This was the primary 

 habitat of the beetle-riding pseudoscor- 

 pion. (The trees provide an ideal nursery 

 for developing young, and fly and beede 

 larvae growing in the wood provide the 

 adult pseudoscorpions with an abundant 

 food supply.) To exploit such a rich, but 

 ephemeral, resource, a small, flightless 

 arthropod first faces the daunting chal- 

 lenge of dispersal. Traveling between 

 these patchily distributed habitats is well 

 beyond its own abilities. While other pseu- 

 doscorpions hitch rides by hanging on to 

 the legs of various flying insects, C. scor- 

 pioides has evolved behaviors that allow it 

 to travel in relative luxury aboard the ab- 

 domens of harlequin beetles, a far less 

 hazardous method of dispersal. 



Four to twelve months after the female 

 harlequin deposits her eggs, her offspring 

 develop into five-inch-long larvae and are 

 ready to pupate. But first the larvae pre- 

 pare for their emergence as adults by cut- 

 ting a disk eight inches in diameter in the 

 bark covering their tunnel entrances. By 

 the time the adult beetles begin to emerge 

 from their pupal chambers, the resources 

 of the decaying fig tree have become se- 

 verely depleted, and its population of sev- 

 eral hundred pseudoscorpions is ready to 

 disperse. Attracted by chemical cues and 

 surface vibrations, the pseudoscorpions 



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