THE movie posters for ''Godzilla'' read: ''Size does matter.'' Hey, wipe the scales from your eyes, you overstuffed iguana! Size is not the point: points are the point. Titillators and claspers are the point. So are hooks, knobs and grooves, cerci, epiprocts and paraprocts. How about something that unfolds blade by blade, like a Swiss Army knife -- or the phallus of a male oriental cockroach? If big is the best you can do, darling 'Zilla, no wonder you've been a box office snore.

Up and down the phylogenetic scale, in all creatures where a female's eggs are fertilized internally, the male's phallus must perform pretty much the same task: penetration and ejaculation. Yet, as comparative biologists have found, the penis and its accessory parts are often far more complex and embellished than utilitarian design would seem to require. The organ of the male cat is stippled with spines, that of some male monkeys is flanged as though with wings, while the human penis has its foreskin and the distinctive bulbed glans beneath.

Among insects, male reproductive organs differ so dramatically from one species to another that entomologists rely on the particulars of the cerci, epiprocts and other bits of arthropod genitalia to distinguish between species that otherwise look alike.

Scientists have long debated the reasons for such ornateness of penile shape, or morphology. By one prevailing theory, male organs are diversely proportioned for much the same reason that keys are: to keep strangers out of the house. In this scenario, the male penis serves as a barrier against cross-copulations between members of two different species, an act that would either be a sterile waste of reproductive energy, or would result in the birth of genetically weak offspring. Thus, the males of a species must all have uniquely shaped keys to match the genital locks of their corresponding females. Charles Darwin himself believed that the evolution of reproductive organs was driven by such practical concerns as the maintenance of species purity.

But now a researcher from Sweden has published results strongly suggesting that here, at least, the great man was wrong. In a detailed study of insect genitalia and its association with insect sexual practices, Goran Arnqvist of the University of Umea offers evidence supporting another theory for the divine absurdity of phallic diversity. His results, which appear in the current issue of the journal Nature, indicate that male reproductive organs are the pelvic equivalent of peacock feathers -- the fruit of sexual selection.

Sexual selection stands with natural selection as a mighty engine of evolutionary change. But while natural selection favors practical traits that help an animal survive in its habitat, like a fur coat to weather winter or a long tongue to pick out ants from an anthill, sexual selection favors traits that, for whatever reason, lend a male an advantage in the harsh competition for access to a female's eggs. The most familiar example of sexual selection in action is the peacock's gorgeous, awkward plumage, which does nothing to help the bird escape a predator, but which does plenty to attract a peahen's eye and persuade her, ''Mate with me, not with that drip over there.''

As Dr. Arnqvist demonstrates in his new report, the hand of sexual selection does not stop at molding the properties necessary for attraction and courtship. Because females of many species mate with multiple partners, he says, it is not enough that a male have the wherewithal to solicit a copulation. He must also possess features that keep his sperm in the race once mating has occurred. And what better way to fight the spermic fight than through modifications of the vehicle that delivers it?

''Charles Darwin saw sexual selection as the force whereby certain males are more attractive to females than are other males, and so they gain more matings, leading to the evolution of things like bright plumage,'' Dr. Arnqvist said in a telephone interview. ''The thing that Darwin did not realize, but that has become apparent to us in recent years, is that sexual selection can operate in different ways, not just on the level of mating, but on the level of fertilization as well. That's where the variance in genital morphology comes in.''

Dr. Arnqvist proposes that nearly every fillip, flange and blade somehow plays a role in assuring its owner's fertilization success. He proved his thesis by comparing two groups of closely related insects that differed in their mating systems. One group consisted of species that were polyandrous, in which the female copulates with many males, while the other was of species that were monandrous, in which the female mates only once in her lifetime.

Gathering the right mix of representative insects was a grueling task, not the least difficulty being that monandry is rare among insects, as it is elsewhere in the animal kingdom. ''It was a huge amount of work finding good examples,'' Dr. Arnqvist said. ''I talked with hundreds of researchers all over the world, and read I don't know how many thousands of papers.'' Eventually, he managed to find species from four different insect orders: mayflies; butterflies and moths; flies and mosquitoes, and beetles.