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Correlations between histological changes of larval integument and hemolymph ecdysteroid levels during the last larval instar of the silkworm bombyx mori

Mori, T.; Kiguchi, K.

Japanese Journal of Applied Entomology and Zoology 29(4): 284-297

1985


ISSN/ISBN: 0021-4914
Accession: 005062061

Morphological changes of the larval integument in the last larval instar of Bombyx mori were investigated by light and electron microscopy, and correlated with known ecdysteroid levels in the haemolymph. About 15 fibrous layers were observed in the larval cuticle of 5th instar larva which had just exhibited ecdysis. During the feeding stage, the epidermis secreted 7-12 fibrous layers day by day to reach a total of about 70 layers when the larva began to wander. When the small amount of ecdysteroid began to appear in the haemolymph (about 8 hr before wandering), intercellular spaces started to appear in the epidermis which became more prominent throughout the wandering and spinning periods. At the same time, amorphous electron-dense bodies (considered as ecdysial droplets) appeared at the cell border adjacent to the overlaying endocuticle. Such amorphous bodies seemed to move to the inner layer of the endocuticle to form the ecdysial space during the wandering stage. Distinct larval-pupal apolysis which was observed 12 hr after the gut purge become more pronounced throughout the spinning period. Well developed epidermal microvillae observed during the feeding period degenerated after the cessation of feeding. However, the microvillae began to develop again around the critical time of ecdysone secretion from the prothoracic glands. Deposition of pupal cuticle started around the time of completion of cocoon spinning. Based on the relationship between the epidermal changes and ecdysteroid levels in the haemolymph, a developmental time-table was presented for the last larval instar of the wildworm, Bombyx mori.

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