Plastron Respiration in the Eggs of Drosophila and other flies

Abstract

IT has long been known that the conspicuous projections near the anterior end of the eggs of Scopeuma, Drosophila, and other flies are concerned in respiration, and the projections have been called respiratory horns. The site of entry of oxygen into the horn has not previously been determined, but where not explicitly stated it has been implied that oxygen enters through holes in the distal end of the horn. Reaumur¹ regarded the respiratory horns of Scopeuma as floats that served to prevent the submergence of the eggs and so their asphyxiation. A similar function has been postulated for the respiratory horns of Drosophila by Wigglesworth and Beament². However, the eggs of Scopeuma stercorarium L. and Drosophila mealnogestar Meig., as well as those of many other species with similar respiratory horns, are heavier than water even when the chorion and plastron are air-filled and besides are normally stuck to the substrate: they do not float if submerged, when under natural conditions they might be washed away from the larval food supply. Portions of cow pats containing eggs of Scopeuma, and Hebecnema umbratica Meig. were repeatedly submerged in water, but the eggs were never detached. Of course these eggs and those of Drosophila and other species can be suspended from the surface film if they are freed from their attachment to the substrate and a line of contact with the water and air is established. Under these conditions their centres and buoyancy and gravity are such that the tips of the respiratory horns often project above the surface film.