The giant sperm of a minute beetle

The mature sperm of Ptinella aptera is a helically coiled, flagellate gamete ca. 1.4 mm long-twice the length of the beetle itself. The rod-like acrosome, comprising the anterior part of the sperm, is 0.4 micrometer thick but is expanded as a flange around the nucleus and the base of the tail, increasing the diameter of the sperm to 2 micrometer. The bulk of the tail consists of a pair of bodies with a characteristic ultrastructure of longitudinal tubules beneath a lamellar cortical layer. These bodies are probably homologous with the mitochondria derivatives of other insect sperm. The axoneme is helically coiled and is flanked by a single accessory body. One of the 'structured bodies' is connected to the acrosome and the other to the accessory body. The sperm move actively in the female reproductive tract. The functional significance of this behaviour and of the evolution of the large gamete itself is discussed in relation to existing hypotheses.