PTIntroduction
Silkworm rearing is an ancient art that began in China around 5,000 years ago. It comprises the cultivation of mulberry trees, silkworm breeding and the production of silk threads for the textile industry, and is considered to be one of the oldest agro-industrial activities practised by humankind(6). The world's four major producers of silk are China, Japan, Brazil and India.
The caterpillar of the species Bombyx mori, known as the silkworm, is an insect of great economic importance, since its cocoons are used in the manufacture of silk(2).
About 95% of the silk produced worldwide comes from this species (6). This insect is fully domesticated due to the strong genetic manipulation it has undergone in order to obtain a good silk-producing species. Silk yarn is a philament- based product of protein origin, which is produced in the glands of the silkworm caterpillars (6).
The biggest threat to silkworm breeding are the diseases that can directly attack silkworm, such as: nuclear polyhedrosis virus; cytoplasmic polyhedrosis virus; infectious sagging virus and densonucleoseis virus (3,5).
The egg
The egg measures approximately 1 to 1.3 mm, it is oval and flattened. After laying, it is yellow, and if fertilised it turns strong yellow, orange and finally grey (6). After breaking dormancy the caterpillar hatches after 7 to 21 days (1).
Caterpillar or larva
The larva cycle from egg hatching to cocoon formation lasts about 24 days (1,3).
At this stage, the caterpillar feeds exclusively on the leaves of the mulberry tree which comes from China. Botanically it belongs to the family Moraceae and the genus Morus (3).
When the caterpillar leaves the egg, it is 1 to 3 mm long, it grows a lot and needs to change its chitinous exoskeleton, this change is called moulting, with 4 skin changes, in which the caterpillar stops feeding (1,3). The time between skin changes is called age or instar, which takes the larva from hatching to cocoon formation through 5 ages(3).
Cocoon formation
At the 5th age the caterpillar is 7 cm long and the most developed organ is the silk gland, at the end of this age the caterpillar weaves a silk cocoon, consisting mainly of the proteins fibroin and sericin, which are produced by the silk gland cells and expelled through the mouth (1). The mechanism of silk thread formation in seric glands is unique, and very much studied. The seric gland is morphologically divided into 3 parts: posterior, median or central and anterior. The posterior part or secretory region synthesizes the fibroin molecules and the P25 protein, which form the insoluble thread. In the median region, there is secretion of sericin, which provides the thread with an adherent layer. The anterior part is responsible for secreting a simple silk thread ready for cocoon formation. In this region there is also mucoidin, which helps the silk thread to pass through the gland. The fibroin solidifies when the thread comes out, but the mucoidin layer remains soft for some time, allowing the caterpillar to glue the different layers of thread together to weave the cocoon (4).
The cocoon
Cocoons have different colours and different shapes, which depend on the breeds:
as to colour: white refers to Chinese and European breeds; yellow to European breeds and greenish to Indian breeds.
as to shape: rounded is a characteristic of Chinese breeds, oval-shaped of European breeds, and peanut-shaped of Japanese breeds.
There is a preference for white cocoons because they are easier to dye (3).
Pupa or Chrysalis
Inside the cocoon, the caterpillar's body transforms, forming the pupa or chrysalis, continuing the transformation process until the formation of the adult individual, the moth. This stage lasts about 10 days (1).
The Adult
To get out of the cocoon the moth releases an alkaline liquid that corrodes one of the ends of the cocoon creating an opening for its exit (1). At this stage the insect does not feed and is dedicated to the reproduction of the species. After mating, the female lays 200 to 500 eggs, and the male dies. This stage lasts about 10 to 16 days (1).
Bibliography:
(1) Brancalhão, Rose M. C. - Bicho da seda. Acesso online: http://www.seab.pr.gov.br/arquivos/File/complexo_da_seda/b_mori.pdf. 2005.
(2) Lira, Michael J. S. - Susceptibilidade do ovário de Bombyx mori L., 1758 (lepidoptera:bombycidae) ao vírus da poliedrose nuclear múltiplo. Dissertação apresentada ao curso de Pós-graduação em ciências biológicas (área de concentração – Biologia Celular), da Universidade Estadual de Maringá, para obtenção do grau de Mestre em Ciências Bio. Maringá-PR. 2007.
(3) Brasil. Coordenador: Luiz Antonio F. Cascão - Estudos para o desenvolvimento de atividades agrícolas e industriais integradas. Projectos especiais. Sericicultura. s/d.
(4) Silva, Patrícia R. e Zanetti, Ronald - Resposta técnica produzida pelo Serviço Brasileiro de respostas técnicas. 2006.
(5) Watanabe, Hitoshi - Genetic resistance of the silkworm, Bombyx mori to viral diseases. Current Science, Vol. 83, nº4, Special scction: Recent advances in silkworm biology. http://www.wormspit.com/bombyxsilkworms.htm. 2002.
(6) Zanetti, Ronald- Notas de aulas de ENT 110- Sericicultura. DEN/UFLA. s/d.
