Spermatozoa of the 'primitive type' in Scutigerella (Myriapoda, Symphyla)

The myriapod class Symphyla is of interest in that insects generally are assumed to be derived from symphylan-like ancestors. Male Symphyla form spermatophores that are picked up by the female. Both euspematozoa and paraspermatozoa are formed. In spite of the mode of fertilization their euspermatozoa were found to be of a kind that is typical of aquatic animals, so called 'primitive spermatozoa;' these are characterized by a short sperm head with a bilayered acrosome, a midpiece containing a few unmodified mitochondria, and a 9 + 2 flagellum. Scutigerella are unique among terrestrial arthropods in having 'primitive spermatozoa'; and together with horseshoe crabs they are only the second case in Arthropoda. Two further sperm plesiomorphies not found in other myriapods or insects are (1) the presence of a cytoplasmic canal housing the proximal flagellum and (2) the existence of microtubular triplets in centrioles, one of which acts as a basal body. Symphyla and Diplopoda both have a striated structure in the center of the subacrosomal material. The paraspermatozoa lack acrosome and nucleus but have a prominent crystal, a single mitochondrion, and two membrane systems. The structure of Scutigerella euspermatozoa is consistent with Symphyla being close to the stem group of Myriapoda plus Insecta.