Comparative Morphology of Epithemata (Polar Chorionic Structures) in the Eggs of Ephemerella ignita (Ephemeroptera: Ephemerellidae)

Abstract

The eggs of the mayfly Ephemerella ignita (Ephemeroptera: Ephemerellidae) are characterized by a polar adhesive structure, the epithema. The epithema prevents egg drift following deposition in water by extending a network of threads. Mated females lay an aggregate of eggs that is attached to the last abdominal sternites. Within the aggregate, eggs with differently shaped epithemata are arranged in a well-defined pattern. Three types of eggs were defined by three types of epithemata, namely short, bulging, and long. In addition, histological observations confirmed the thread-like composition of the epithemata and their peculiar arrangement in each type. Short and long epithema egg types differed also by their total lengths. Structural changes in the architecture of the epithemata are discussed in relation to a different function of such devices. The fine structure of the egg chorionic pattern in Ephemeroptera has been used, as with other insect orders, to elucidate differences and relationships among species (Alba-Tercedor & Sowa, 1987; Gaino et al., 1987, 1989; Gaino & Mazzini, 1984; Mazzini & Gaino, 1990). In addition, ultrastructural studies on mayfly eggs have indicated the adhesive function of their peculiar projections, which may be involved in preventing egg drift following deposition in water (Gaino & Mazzini, 1987, 1988). As a consequence, shell surface structures may give some insight into the devices developed by different species to survive in their respective habitats (Gaino & Mazzini, 1988). Ootaxonomy is based chiefly on the species-specificity of chorionic architecture, which is constant within the representatives of a species. For instance, the organization of the mayfly egg has been employed to classify the order and to recognize nymph and adult stages in the same species (Koss, 1968). The polar devices, referred to as the "epithema" (this term having been introduced first by Bengtsson, 1913) or "polar cap" (terminology after Koss & Edmunds, 1974), is a unique chorionic structure in eggs of Ephemerella ignita Poda at the posterior polar region and is involved in egg adhesion to the substratum. The occurrence of eggs with adhesive polar devices of different lengths in E. ignita were noted earlier and were considered to be related to the position of the eggs within the ducts (Bengtsson, 1913; Degrange, 1960). This observation might have relevant implications in the field of ootaxonomy and may imply additional functions. The primary objective of this paper is to describe and document the ultraWe thank Prof. C. Falugi and Dr. C. Pesce for their helpful criticisms and F. Di Lauro for his technical assistance. This research was supported by the Italian Ministry for Public Education (M.U.R.S.T.40% and 60% funds). TRANS. AM. MICROSC. SOC., 11 l(3): 255-265. 1992. O Copyright, 1992, by the American Microscopical Society, Inc. 256 TRANS. AM. MICROSC. SOC. structural features of the eggs of E. ignita with emphasis on variations among epithemata. MATERIALS AND METHODS Adult females of Ephemerella ignita involved in oviposition were collected in June 1990 along the Berlino stream near Rossiglione, Genova, at approximately 300 m elevation. Ovipositing females were identified by the green aggregate of eggs adhering to the abdomen. Eggs were fixed in Karnovsky's (1965) fixative and rinsed in sodium cacodylate trihydrate buffer. For histological examination, eggs were dehydrated in a graded series of ethanol and embedded in metacrylate resin (JB4). Sections were stained with toluidine blue. For scanning electron microscopic (SEM) examination, selected material was dehydrated following the above-described procedure for histological study, critical-point dried using liquid CO, in a Bomar apparatus, attached to specimen holders by silver-conducting paint, and coated with gold-palladium in a Balzer Union evaporator. Specimens were observed with a Philips EM 505 scanning electron microscope. The dimensions, including epithemata, were measured in a sample of 120 eggs, subdivided into three groups of 40 eggs each, according to the morphology of their epithemata. Data given are means from each group of 40 eggs.