Katydids and Bush-Crickets: Reproductive Behavior and Evolution of the Tettigoniidae

  • Darryl T. Gwynne
Cornell University Press: 2001. 344 pp. $42.50

With great reluctance, I have decided against my initial impulse to give this book to everyone I know for Christmas. Some friends might appreciate it, but I can't quite see my Texas in-laws expressing much enthusiasm. This caveat notwithstanding, Katydids and Bush-Crickets is a fascinating and enjoyable read, full of both charming trivia and scholarly insight. The book is brimming with 'did-you-knows?': for example, the term diapause (the temporary cessation of development in insects) was coined by the renowned Harvard expert on ants, William Morton Wheeler, in his 1893 thesis on katydids. The book is also beautifully illustrated, with many line drawings, breathtaking colour plates, and figures that are all redrawn from their original source — a welcome touch of clarity and uniformity that is absent in many such books. Virtually all of the chapters begin with pertinent and amusing quotations from sources ranging from Oliver Wendell Holmes to Dean Martin.

Why is this group of insects — called katydids in North America and bush-crickets in Europe — so interesting? Although Darryl Gwynne provides information on diversity, evolution and the economic importance of the family, the focus of the book, and the answer to the question, is the katydid sex life. Unlike most insects, and indeed most animals, katydid males contribute more than just sperm when mating. They generally provide females with a nutritious parcel, the spermatophylax, which is attached to the sperm-containing ampulla. The female consumes the spermatophylax after mating, while the sperm move into her reproductive tract.

This nuptial gift can represent up to 40% of the male's body mass, and hence is not easy to produce. Males may mate only once every few days, garnering the resources to manufacture one of these massive meals between mating. The hefty donation has important implications for sexual selection, because the general rule is that the reproductive success of males is limited only by the number of females they can inseminate, rather than by the investment they make in producing offspring. This is likely to be far more variable than the factor that generally limits female reproductive success — the number of offspring they can produce and (where appropriate) rear. Because females in a species usually make the greater investment, sexual-selection theory predicts that females will benefit by maximizing the quality of their offspring, but males will benefit more from maximizing the quantity.

All that is reversed in katydids and in several other organisms, including sea-horses and their relatives the pipe-fish, as well as a handful of birds and insects. If males do not merely produce a dab of semen, but must proffer a costly gift to females before mating, they would be expected not only to be choosy about their mate, but also to be sensitive to the availability of resources in the environment in a way that males in other species are not. Such a reversal of sex roles provides a crucial test of current theory on how the sexes are expected to differ.

How did the spermatophylax evolve? Gwynne evaluates two hypotheses. The paternal-investment hypothesis suggests that males provide females with a meal because it increases the fecundity of the female or the quality of the offspring she produces, and hence increases the male's fitness along with that of the female. According to the ejaculate-protection hypothesis, in contrast, the spermatophylax serves to keep females busy eating it while sperm are transferred. Males providing larger gifts therefore gain a larger proportion of fertilizations.

In a careful dissection of experiments on a variety of species, Gwynne concludes that the paternal-investment hypothesis has received the more support; in many katydids, the ampulla is completely empty long before a female has finished her gelatinous meal.

Nevertheless, much remains to be discovered about katydids. Indeed, one of the book's strengths is the prevalence of ideas for further research — there's a possible dissertation project on virtually every page. Why, for example, have so few katydid species lost acoustical function, compared with their equally musical relatives the crickets? What favoured the occasional evolution of parthenogenesis in this unlikely-seeming group? Why do some katydids mate only once? The section on risks and survivorship is particularly dramatic: "in nature, death usually comes from a plethora of hostile forces", including parasitoid flies that home in on the male's calling song, horsehair worms that burst from the body cavity like lethal spaghetti, as well as bats, frogs, birds and fungi.

One can quibble with any work of this magnitude, and I did my share. The references on a few topics, such as the role of sexual selection in speciation, were curiously outdated. Sexual-conflict theory — the conflict between males and females over mating — is a relatively new, hot topic in sexual selection, but is mentioned rather briefly; its explicit incorporation into ideas about spermatophylax evolution might have been fruitful. Gwynne draws a distinction between so-called passive and active female choice, a dichotomy I have always disliked. These bits of carping aside, Katydids and Bush-Crickets is a book to share with all students and professionals in ecology and evolutionary biology — and, who knows, maybe also with a few relatives at Christmas.

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