Integument of the velvet-mite, Dinothrombium giganteum, and histopathological changes caused by the fungus Aspergillus flavus

Abstract

Histological and histochemical descriptions of the velvet-mite, Dinothrombium giganteum are presented along with changes noted during infection by the fungus Aspergillus flavus. The integument of healthy mites consists of outer epicuticle and inner procuticle. The epicuticle is composed of outer lipid and inner lipoprotein layers. The outer portion of procuticle is in the mesocuticular (presumptive exocuticle) condition with inner blue-staining endocuticle. Quantitative analysis of elements of cuticle shows that sulfur is present in far larger quantity than in similar analysis of insect cuticle. Nevertheless, sulfur does not form disulfide linkages beween cysteine molecules as it does in keratin. The first sign of fungal disease in the mite is a generalized decoloration caused by loss of the usually scarlet-red plumose cuticular setae. The latter confer a velvety appearance to the mite and are shed when germ tubes of the fungus digest the lipoproteinaceous epicuticular layer that fastens the base to the cuticular sockets. Subsequently, analine blue-positive endocuticle of the healthy mite becomes fuchsinophilic with Mallory's triple stain. Mycosis-induced “pseudomesocuticle” behaves like classical mesocuticle in regard to staining reactions, resistance to mineral acids, and histochemical reactions. Moreover, endocuticle of diseased mites becomes positive to periodic acid-Schiff (PAS) reagent. Subsequently, melanins are deposited in the outer procuticle. When melanization occurs, PAS-positivity of the inner procuticle is masked. Furthermore, epidermal cells also display marked pathological symptoms.