Toward the Science of Entomology

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Toward the Science of Entomology

Overview

In the eighteenth century entomology, the study of insects, developed as a separate branch of the life sciences. At the beginning of the century, it was not clear precisely what an insect was, with many organisms including spiders and worms being referred to as insects. By the beginning of the next century it was clear that the class of insects was defined as those invertebrates (animals without backbones) having three pairs legs and segmented bodies. It was primarily through the work of two men, Jan Swammerdam (1637-1680) and René Antoine Ferchault de Réaumur (1683-1757), that the study of insects blossomed. Although Swammerdam's work was done in the previous century, most of it remained unpublished until 1737, so it was only then that it influenced other researchers' views on insects. Réaumur's six-volume study of insects appeared between 1734 and 1742.

Background

Swammerdam and Réaumur were interested in different aspects of insect life, with Swammerdam focusing on development and Réaumur on behavior. Swammerdam was Dutch and also did extensive work in animal physiology. He attempted to disprove the old assumption, originating with Aristotle, that insects were less perfect than other so-called "higher" animals such as those with backbones. This assumption was based on three ideas: insects lacked internal anatomy, they could form by spontaneous generation from nonliving matter, and they developed by metamorphosis, that is, their internal structure changed completely during development.

Swammerdam brought a wealth of observations to bear in his arguments against all three of these assumptions. He supported the views of Francesco Redi (1626-1698), who had demonstrated in 1668 that flies did not arise by spontaneous generation from rotten meat, but rather developed from eggs laid on the meat by adult flies. With the aid of the microscope, Swammerdam's careful dissections of insects revealed that they did indeed have a complex internal anatomy. His was the first major study of insect microanatomy. But it was in his investigations on insect development that he made his greatest contributions to entomology.

Swammerdam studied development in order to refute the idea of metamorphosis that was then current. Though we still use this word today to describe development in some insect species, namely in those that undergo an extreme transformation in body form such as that of a caterpillar metamorphosing into a butterfly, the word had a different meaning in Swammerdam's time. Then it meant a sudden and total change from one kind of organism into another, something like the transmutation of lead into gold that alchemists sought. Through dissections of caterpillars and of cocoons, which caterpillars create from fine fibers they spin and from which the adult butterflies emerge, Swammerdam showed that the change was not all that sudden, that the butterflies' wings and other body parts developed for some time within the cocoons.

Swammerdam studied development in so many different insect species that he was able to create a classification system based on the different types of development. He divided insects into four groups, with those in the first group hatching directly from the egg into the adult form. Development in the other three categories involves a pupal or immature stage. In the second group, the insect emerges from the egg without wings, but usually with its six legs, and then gradually develops adult traits; the mayfly, which Swammerdam studied in great detail, belongs to this group. For those species in the third category, the change during development is more extreme, and this group includes butterflies and moths. In the last group, the pupa is hidden within a case called the puparium, as with most flies where the maggot is found within a puparium. To summarize his findings, Swammerdam created a table showing the stages in insect development in the different groups. This approach to the organization of knowledge was less common at the time than it is today and made his results more noteworthy and convincing. For reasons that are unclear, Swammerdam didn't publish most of his results. Fifty years later, the Dutch physician Herman Boerhaave (1668-1738) bought Swammerdam's papers and published them in two volumes as Bijbel der Natuure (The Bible of Nature) (1737-38).

Impact

At the time that Swammerdam's books appeared, René de Réaumur was publishing his work on insects. Born in the French town of La Rochelle, Réaumur spent most of the rest of his life in Paris, where he eventually became an influential member of the prestigious Academy of Sciences. His interests were broad and included technological studies, particularly on the production of iron and steel.

Réaumur's greatest contribution to natural history was his six-volume study of insects. He was fascinated by bees, to which he devoted an entire volume. He discovered that in all hives there is only one queen that produces all the eggs. He kept track of individual bees by coloring them with various dyes, a technique that is still used by entomologists. Réaumur was one of the first to do comprehensive quantitative studies on insects. He found that by dunking a beehive in cold water, he could slow the activity of the bees to the point where he could separate them into several categories and count the number of each.

Caterpillars and butterflies were the focus of the first two volumes of Réaumur's work. He repeated Swammerdam's studies on the development of the butterfly within the cocoon. Like Swammerdam, he came to the conclusion that the fact that parts of the adult butterfly are present in a smaller size within the cocoon supported the idea of preformation. This was the concept that development simply involved the enlargement of a tiny, preformed adult found in the egg. This idea received a lot of support in the eighteenth century, in part because of the observations of Swammerdam and Réaumur on insect development.

Preformation was popular because it neatly explained how the complexities of organisms arose. With preformation, all these complexities were created in the original individuals of a species, which have been compared to those toy boxes which when opened reveal a smaller version of the same figure, which when opened reveals a still smaller version, and so on. The view opposite to preformation was epigenesis, meaning that a beginning embryo was merely a formless clump of cells that slowly became more complex and ordered as it grew, with cells becoming organized into tissues and organs as they developed. Epigenesis was difficult for many to accept; it seemed mechanistic and even atheistic since the tissue appeared to develop on its own without any help from a creator. On the other hand, preformation implied the existence of a creator who formed all the adults-within-adults at the time of the original creation.

Réaumur contributed to an important observation that seemed to support the idea of preformation. Studies on aphids, small insects that live on plants, suggested they could arise from eggs that had not been fertilized by sperm. This process is called parthenogenesis, and it implied that the form of the individual was already present in the egg and that the sperm did not add anything essential for development. Réaumur attempted to test this idea by raising aphids in isolation from birth, so that there would be no sexual contact. Though this work was unsuccessful because the insects died before they became sexually mature, his student Charles Bonnet (1720-1793) was able to show that aphid eggs could indeed develop without fertilization. Réaumur did come up with another interesting finding on aphids. He calculated that, under ideal conditions, a single female aphid could produce 5 million offspring during her reproductive lifetime of about four to six weeks. Again, he used quantification to make a point about insect characteristics, in this case, about reproductive potential. It should be mentioned that aphids don't take over the world, despite being able to produce so many offspring, because not only would such numbers only be produced under ideal conditions, but most aphid offspring are eaten by predators or die from other dangers before they themselves become sexually mature.

Since Réaumur had done economically important work in technology and was interested in insect behavior, it's not surprising that he investigated the economic significance of insects. He argued that research on insects was important because of the destruction they can do attacking plants, the wood of houses and furniture, and even clothes. He himself did research on the lifecycle of the clothes moth in order to find the best way of eliminating it. But Réaumur also saw the positive benefits insects provide in the form of the products they produce—from the fibers of silkworms to the wax and honey of bees and the red dye produced from ground cochineal insects.

While Réaumur is the most noted entomologist of the eighteenth century, a number of others also made important contributions to the field. In 1760 Pierre Lyonnet (1707?-1789) published a comprehensive study of the goat-moth caterpillar, and this foreshadowed the increasing specialization and narrowing focus that was to be seen in the work of many nineteenth-century entomologists. C. de Geer (1720-1778) of Sweden continued Réaumur's work; he did further studies in insect classification that, in turn, influenced Carolus Linnaeus's (1707-1778) system of insect classification, and it was Linnaeus's work that put classification on a new, more organized footing. Réaumur's writings also influenced the studies of Georges Buffon (1707-1788), who produced a monumental 10-volume work on natural history during the second half of the eighteenth century. At the beginning of the nineteenth century two other French zoologists, Jean Baptiste de Lamarck (1744-1829) and Georges Cuvier (1769-1832), published works that built on Réaumur's entomological research, and these publications were followed by those of the English entomologist W. E. Leach. All three contributed to clarifying precisely which organisms qualified as insects and which belonged in other classes such as the Arachnida (spiders) and the Crustacea (lobsters and shrimp).

MAURA C. FLANNERY