WHAT DO you say about a male scorpion-fly (H. apicalis) who impersonates female flies in order to steal food and females from other males? That he's a thief - a scoundrel, if you will - who hoodwinks "straight" males out of their food for his own lustful and gluttonous ends? Or that he's simply a streetwise survivor, to be admired for his knack of living the good life in the treacherous jungle of insect society?

Whatever one's opinion, it now seems clear that this roguish brand of transvestite fly eats better, has sex more often and probably lives longer than most other males of his species. But he does so at the expense of those unsuspecting males, just when they themselves are primed for sexual intercourse with a female.

According to University of New Mexico biologist Randy Thornhill, the "sting" of the transvestite fly works this way:

A "normal" male scorpionfly, after having gone through much time and danger capturing an arthropod, displays his prey as an enticement to passing females. If a female is impressed with the prey, she feeds off it and consents to intercourse, feeding throughout copulation.

But male and female scorpionflies look very much alike - apparently even to other male scorpionflies. So, when a male H. apicalis comes fluttering up, lowers its wings and moves its abdomen in a way peculiar to a receptive female, the interested male can hardly be blamed for offering his prey in hopes of a relationship. It is at this point - before any actual genital contact - that the posing transvestite attempts to snatch the prey away from his surprised victim and abscond in search of a female of his own.

Transvestites are successful in this venture 22 percent of the time, according to Thornhill, who has studied such behavior meticulously among marked scorpionflies. In the rest of the attempts, the would-be victims either do not fall for the ruse or manage to hold on to their prey during the struggle. Rather than turn to female impersonation, another type of pirating male attempts to forcefully swipe the prey from an exhibiting male, sometimes even while he is copulating with a female; this tactic is successful 14 percent of the time .

"Transvestism...is clearly adaptive," Thornhill reports in Science magazine. It not only provides the successful thief with more eating and copulation opportunities, but it reduces by 42 percent the time spent hunting his own prey - an activity that exposes scorpionflies to dangerous predators, primarily web-building spiders.