The Biology of Bombyx mori, circa 1840

Deirdre Deegan

Throughout history, scientists have struggled to understand the biology and origins of the silk worm, Bombyx mori. However, the focus of these attempts have changed over time. In the early and mid 1800s, the main focus of interest was disease and evolution of the Lepidopteran insect. In their quest to quantify the biological processes that determined how the silkworm lived, reproduced, and the origins of diseases that beset them, scientists were bound by the application of theories prevalent at the time. By the 1990s, scientists had become increasingly more sophisticated about the molecular nature of matter, and the focus shifted to the chemistry of the silk that the insect produces. This paper will concentrate on the biology of Bombyx mori circa 1840, the underlying theories that were the basis for the understanding of the biology of the silk worm, and how the scientific breakthroughs of the mid and later 1800s forever changed the nature of research into the biology of the silk worm and the diseases which plague it.

There is a general congruity of facts on the physical biology of the insect in sources from the 1840s, 1890s and 1990s. Each of the sources I examined report that the insect we commonly refer to as the silkworm is classified in phylum Arthropoda, class Insecta, order Lepidoptera (Gr. lepidos, scale, and petron, wing) subfamily Cerura, and is known by its genus and species names, Bombyx mori. Bombyx mori is characterized by membranous wings coupled at base and covered with overlapping scales. Each possesses mouth parts characterized by a sucking tube which is coiled when not in use, larvae (caterpillars) with chewing mandibles for plant eating, stubby prolegs on the abdomen, silk glands for spinning cocoons, and plumed antennae.

The scientific or biological theories prevalent in the sources from the 1800s relied on theories that the scientific community now considers to be either seriously flawed or completely disproven. These outdated theories served as the foundation upon which biological investigations of Bombyx mori rested. An 1895 monograph on the bombycine moths by Alpheus Packard begins with a description of "rough grouping of [bombycine moth] features [into] three stages." The grouping attempts to discriminate between "the congenital and the acquired characters" of the insect. The theory of acquired characteristics, developed by Jean Baptiste de Lamarck (1744 to 1829) was the first theory to attempt to present a complete explanation of evolution.

Lamarck's theory, published in 1809, the year of Darwin's birth, posited that organisms striving to meet the demand of their environment acquire adaptations and pass them genetically to their offspring. According to Lamarck, the giraffe evolved its long neck because its ancestors lengthened their necks by stretching to obtain food and then passed the lengthened neck to their offspring. Over many generations, these changes accumulated to produce the long neck of the modern giraffe. As Hickman explains, Lamarck's concept of evolution is now known as a transformational theory, since according to it, the individual organisms change or transform their appearance to produce evolution. Transformational theories are now rejected because genetic studies show that traits acquired by an organism during its lifetime, for example, weight-strengthened muscles, are not inherited by offspring. Darwin's evolutionary theory differs from Lamarck's; it is a variational theory, since it posits that evolutionary change is caused by the differential survival and reproduction of genetically variable organisms, not by genetic transmission of acquired characteristics. The 1895 monograph relies heavily on Lamarck's theory, and states that "[p]erhaps in no other group or order of animals may we study the subject of the inheritance of acquired characters with more success than in the Lepidoptera. In these insects the four stages of existence -- the egg, larva, pupa and imago -- are definite and fixed, and during each of the three last periods the organism is, so to speak, a different creature, with distinct and separate shape and structure, external and internal, and during each leads a different life."

A grouping of the three stages of Bombyx mori's life span begins with the larval state, where, the monograph states, there is the "[i]nheritance of what were originally acquired characters, the results of attacks of enemies. Examples are the tuber class armed with spines and sometimes with calrops (Empretica, etc.) and stripes, all apparently inherited at different periods of larval life, the least important specific and varietal characters probably having been acquired during the life of an individual.". According to the monograph, the pupa state, when the cocoon is spun by Bombyx mori, is the result of the change of a larva's habits which were passed genetically to the next generation of Bombyx mori, and is, "with little doubt, an acquired habit, originally formed by a single individual.". The monograph also attributes characteristics of the silk thread of the pupa, such as whether it is the continuous and readily reeled silk of Bombyx mori, or whether the thread is often interrupted at the anterior end, as in Platysamia ceropia, to a "slight change of circumstances [that] may have been inaugurated as the result of variation in a single individual during a single lifetime, afterwards in succeeding generations becoming fixed by homochronic inheritance." Packard goes on to note that "[o]f course acquired characters are most marked in the parts which are most used, as the maxillae, wings, and external genital armature." He further describes how the variations in the markings of the wings may often arise during an individual's lifetime and become a matter of inheritance, "as the result of sudden changes in temperature, moisture or dryness, and in changes in food of the larva."

Later, the monograph describes how the different stages of the Bombyx life span represent one of the adult forms that appeared in its evolutionary history. This notion, known as the biogenetic law or simply as recapitulation, was developed by the German zoologist Ernst Haeckel, a contemporary of Darwin. Haeckel believed, for example, that the human embryo with gill depressions in the neck signified a fish-like ancestor, basing his theory of recapitulation on Lamarck's flawed concept of acquired characteristics. Packard explores the theory of recapitulation in the monograph by positing that the evolutionary history Bombyx mori can be understood by simply observing its life stages: larva, pupa and imago. By this Packard meant that Bombyx mori at one point in its evolution was only a larva in its adult form, during its next evolutionary stage it was larva and then pupa as an adult, and most recently is in its present evolutionary stage goes from the larva to the pupa to the imago form.

From the early 1800s to the early 1900s, a major concern of silkworm breeders was the onset of a variety of diseases which could kill large masses of the caterpillar. There were several diseases which afflicted the insect. One major disease of concern was the so-called "mark" disease (also known as muscardine) of silkworms. As early as 1833 Agostino Bassi, in Italy, had documented some of his experiments and discoveries concerning "mark" disease. However, he was unable to perform the appropriate experiments in the presence of the Commission of members of Faculties of Medicine and Philosophy, which was necessary to get official recognition of his theory that year. In 1834 he repeated his request, this time successfully.

In 1835, Bassi published "On the Mark Disease, Calcinaccio or Muscardine, a Disease that Affects Silk Worms and on the Means of Freeing Even the Most Devastated Breeding Establishments." He had spent several years researching the spontaneous generation of the disease and dedicates one chapter of the book to the description of his attempts to observe the spontaneous production of the disease. The theory of spontaneous generation, or biogenesis, had been believed since ancient times. The theory purports to explain how it is that life can arise from non-living material or by reproduction of parental organisms. In the absence of sophisticated microscopes and other modern scientific equipment, mice appeared to arise from putrefied matter, insects from dew, frogs from damp earth. The first attempt to debunk the theory of spontaneous generation occurred in 1668 when Francesco Redi exposed meat in jars, some uncovered and some covered with parchment or wire gauze. The meat in all the vessels spoiled. However, only the open vessels had maggots, and Redi observed that flies were constantly entering and leaving these vessels. He concluded that no worms would be found if flies did not have access to the meat. Although Redi's refutation of spontaneous generation was widely known, the doctrine was too firmly entrenched to be abandoned. However, in 1767 Abbe Lazzaro Spallanzani conducted experiments that dealt another blow against the theory of spontaneous generation. Spallanzani boiled extracts of meat and vegetables, placed them in clean containers, and sealed the necks of the containers. He immersed the sealed flasks in boiling water for several minutes to make sure that all germs were destroyed and left some tubes open to the air, as controls. After two days, he found the open flasks swimming with organisms; the others contained none. However, this experiment did not end the doctrine of spontaneous generation. The advocates of the theory maintained that air, which Spallanzani had excluded, was needed for the production of new organisms and that his method destroyed the vegetative power of the medium. When oxygen was discovered in 1774, Spallanzani's opponents cited it as the vital element that he had excluded from his experiments. Finally, in 1861, after an elegant experiment with air-borne organisms and yeast, Louis Pasteur was able to prove otherwise, and proclaimed that the doctrine of spontaneous generation would "[n]ever ... arise from this mortal blow." Pasteur overcame the objection to the lack of air by introducing yeast into a flask with a long S-shaped neck that was open to the air. The flask and its contents were then boiled for a long time, cooled and left undisturbed. No fermentation occurred in the flasks because all organisms that entered the open end were deposited on the floor of the neck and thus could not reach the yeast contents of the flask. When the neck of the flask was cut off, the organisms in the air could fall directly on the fermentable mass and fermentation quickly ensued. Pasteur concluded that since suitable precautions had been taken to keep the germs and their reproductive elements out, no fermentation or putrefaction could take place.

Even before Pasteur delivered this empirical death blow to the ancient theory, many scientists had questioned it and some of them had abandoned it. When Bassi's attempts to spontaneously generate the mark disease in silkworms failed, he wrote that "this ... cast me into the deepest dejection. I looked on the cause of the dreadful calcinaccio as covered by an impenetrable veil, and I despaired of removing it from the darkness to the light of day ... Humiliated in the extreme ... I wept over my lost laurels and bitterly lamented the adverse fate which had put me to so much study, expense, and labor in vain. It was the year 1816: oppressed by a terrible melancholy ... I found the courage to one day shake off its yoke, and, challenging adverse fortune once more, I began to interrogate nature afresh invarious ways, with the firm resolve never to abandon her until she had been tamed and answered my questions honestly ... Having failed by so many different processes to produce the Muscardine in the silk worm except by the use of the true calcified worm, it occurred to me that it did not originate spontaneously in the insect, and that it needed an extraneous germ which entered the insect from outside and caused the nature and habits and all the ways by which it enters silk worm establishments and makes its way into the worms, infecting then with the dreadful disease, and, what is more important, to find, if possible, a means of preventing the disease from developing by stopping the contagious germ from entering both breeding establishments and silk worms ... "