Ricania speculum (Walker, 1851)

Description.

Mature spermatozoa of R. speculum are similar to those of P. shantungensis in morphology insofar as they also have a number of spermatozoa (totally 128 spermatozoa per spermatodesm) organized into sperm bundles with their heads embedded in a homogenous matrix (Fig. 5A, C–D). The individual sperm is filiform, measuring 196 µm in average length (Fig. 5B), with a linear head and distinct flagellum, approximately 24 µm and 172 µm in length, respectively.

The sperm head of R. speculum is elongated and filiform, formed by a short acrosome and an elongated nucleus (Figs 5B, 6B). The conical acrosome contains a dothideoid acrosomal vesicle and perforatorium with the latter made of electron-dense fiber substructures (Fig. 6B). A transition region is visible between the acrosome and the anterior portion of the nucleus (Fig. 6B). Anteriorly the nucleus is surrounded by the acrosome which is filled with numerous fibrous substructures; posteriorly it increases in diameter and changes from a mushroom-shape to a meniscus shape and finally to an oval-shape (Fig. 7 B–D). The nucleus, approximately 0.99 µm in diameter, is filled with compact chromatin and takes on different shapes (Figs 6C, 7 B–I).

In the nucleus-flagellum transition region, the centriole and centriolar adjunct that lie next to the nucleus are abrupt (Fig. 6A, C). The centriole is formed by dense microtubules that originate from the end of the pyknotic nucleus and end above the front of the axoneme (Figs 6B, 7 G–H). The centriolar adjunct is composed of moderate electron-dense substances, connecting mitochondrial derivatives with the nucleus (Figs 6B, 7 E–I).

The cross-section of the flagellum region consists of an axoneme, two symmetrical accessory bodies and two mitochondrial derivatives (Fig. 8A, B). The axoneme of the flagellum of R. speculum has a typical 9 + 9 + 2 microtubule pattern, comprised of two central microtubules, nine inner doublet microtubules and nine outermost single accessory microtubules (Fig. 8C). The mitochondrial derivatives have evident parallel cristae arranged in the periphery and are formed by three different portions: a serrated electron-dense region, a central clear area and a mitochondrial cristae region (Fig. 8B). The cristae are perpendicular to the axis of the derivatives and are at regular intervals (about 46 nm) between adjacent derivatives (Fig. 6D). Between the axoneme and the mitochondrial derivatives are large, fishhook-shaped accessory bodies (Fig. 8A, B); they are composed of electron-dense material (Fig. 8B). Close to the posterior sperm tip, the mitochondrial derivative is first to end (Fig. 8E), followed by the accessory bodies, while the axoneme gradually becomes disorganized (Fig. 8B, D–F).