Synthesis and function of the fibrous layers covering the eggs of Siphlonurus lacustris (Ephemeroptera, Siphlonuridae)

Ultrastructural analysis (transmission and electron scanning microscopy) of the eggs of the mayfly Siphlonurus lacustris (Eaton) showed that they are wrapped in a thick coat composed of a network of tightly entwined filaments . Groups of twisted filaments form slightly uplifted buttons that are scatt ered on the coat surface. After experimentally induced egg deposition, egg- water interaction promotes marked cohesion of the eggs and their firm adhes ion to the substrate. Egg masses include numerous gametes; the covering of those located close to the substrate greatly extends to anchor the whole ma ss. Eggs removed from the coat reveal a slightly punctuated smooth chorion and tagenoform micropyles (three to five). The coat increases egg size by a bout 20%. The lack of female reproductive accessory glands in Ephemeroptera transfers the synthesis of the adhesive coats to the follicle cells, which are typically competent for insect egg shell deposition (vitelline envelop e and chorionic layers). This covering results from electron-dense granules that give rise to filaments progressively organized to form superimposed l ayers variously orientated around the egg. In addition to egg adhesion to t he substrate, a trophic function and protection from shear stress are postu lated for this covering.