Studies on the fine structure of the dorsal vessel of arthropods

Summary The beetle family Altioidae contains some very archaic surviving species, and at the same time, very modern ones. In one lineage from archaic to modern (Fornterita — Alticini — Oedionychini), the number of spermatozoa has been stepwise (according to a geometric series, 2ⁿ) reduced from 256 to 16 per bundle. Comparison with the relatively few data in literature show that this trend is common in insects: more archaic orders have more sperm cells per bundle than the more modern ones, and within an order, the most modern or most specialized groups tend to have least. The reduced number of spermatozoa per bundle may be a sign of reduced sperm production in general, and this probably has adaptive value in limiting genetic variability.A small number of spermatozoa per bundle is sometimes accompanied by an extra large size of spermatocytes. Except for the alticid subtribe Oedionychina, it is hard to see what adaptive value this could have. In the Oedionychines, the large size of the spermatocytes is a prerequisition for the evolution of gigantism of the asynaptic sex chromosomes.Partly supported by a grant (GB-4522) from the National Science Foundation (Washington).I wish to express my gratitude to Prof. Dr. Jan Bechyné, Instituto de Zoología Agrícola, Universidad Central de Venezuela, Maracay, Venezuela, for identification of most alticids mentioned in this paper.