Insects by the Numbers

By Flannery, Maura C. | The American Biology Teacher, September 2008 | Go to article overview

Insects by the Numbers

Flannery, Maura C., The American Biology Teacher

There are a lot of insects--any way you want to count them. In one of the classics of the entomological literature, Howard Ensign Evans (1984) cites a calculation of "the number of flies that might result from a single initial fertilized female in the course of an average summer in Washington, DC, assuming that all survived" (p. 160). That number is over 5.5 trillion, yes trillion. If you are having a hard time getting your mind around this quantity, another researcher estimates that the offspring of a single pair of flies would create a mound 47 feet thick over the entire surface of Germany. Thankfully, Malthus was right, at least as far as flies are concerned: limited resources and other vagaries of life keep the numbers down considerably. Still, there are more than enough insect infestations as far as most of us are concerned.

Insect numbers are also spectacular in other ways. Nigel Stork (2007) begins an article on insect biodiversity with the rather startling statement that: "To a first approximation, all multicellular species on Earth are insects" (p. 657). He bases this assertion on the fact that over half of known species belong to the class Insecta, and these more than 850,000 species probably represent only 5-20% of those actually crawling the earth today. Stork mentions these numbers and then adds that despite the speciesdominance of insects, most explanations of terrestrial biodiversity are primarily based on birds, large mammals, and plants. He then describes two recent studies that attempt to redress this imbalance. One surveyed three groups of rainforest insects in the lowland rainforests of New Guinea: those that eat wood, fruit, and foliage. This study found a low rate of change in species over the territory monitored, an area about the size of South Carolina. In a related study, the researchers found similar host specificity in these rainforests and in temperate regions of Europe.

However, a study of caterpillars in the Western Hemisphere found less specificity in temperate forests. There may be several reasons for this divergence in results, including the varying methodologies employed. Also, the data for the New World study were much more extensive and covered three quarters of a century while the New Guinea data were all recent. In addition, there's the possibility that these two regions are indeed different from each other, that species diversity is influenced by evolutionary history. What both studies did have in common is that they focused on herbivores and paid no attention to insects that feed on dead material, bacteria or fungi--and parasitic insects were also ignored. What portion of forest populations these other species represent is also debatable, with some estimates running as low as 16% and others as high as 50-70%. That's quite a range, and it suggests that there is an awful lot we just don't know about these ubiquitous creatures.

Drosophila Genetics

Even when it comes to Drosophila, there's still a great deal to learn, though they've been the object of intense study for a hundred years (Kohler, 1994). They were passe as research subjects for awhile when molecular biology fell in love with bacteria. But as molecular techniques became more sophisticated, there was renewed interest in Drosophila in the 1980s (Weiner, 1999). One of the "new" leaders in the field was Edward Lewis (1918-2004) who had been educated during the first wave of interest in fruit flies and had never abandoned them. He was rewarded for his perseverance with the Nobel Prize in 1995 for his work on the homeotic genes that control segmentation in early development.

There are hundreds of species in the Drosophila genus and because it has already been so extensively studied it is the focus of much research attention at the moment. Just as the rich seem to get richer, the well-studied seem to get even better studied because there's such a sound foundation on which to build. It's much easier to do genomic work on species for which many genes, their locations, and products are already known. …

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