Discovery of a complex sclerotized sperm pump in Monotomidae (Coleoptera): Functional morphology and phylogenetic value

Abstract

In insects, a long ductus ejaculatorius surrounded by a muscular sheath transfers seminal fluids and sperms from the testes into the male copulatory organ, the aedeagus. In several unrelated coleopteran families, a section of the ductus has evolved to form a specialized pumping organ. The most complex forms of this "sperm pump" was known in Lepiceridae and two subfamilies of evolutionary distant Staphylinidae; in these groups two sclerotized funnels situated on ends of the sperm pump provide attachment sites for a remarkably robust compressor muscle. A sperm pump with two funnels and a previously unknown spiral ridge, representing the most complex sperm pumping apparatus among Coleoptera, is for the first time reported to occur in the superfamily Cucujoidea, family Monotomidae. The compressor muscle is particularly large, and the pump shows features more advanced than those in any other beetle. Not the entire section between funnels is compressed, but only a pair of specialized thin-walled, unsclerotized and resilinrich regions within the pump. The spiral ridge stores mechanical energy to decompress the pump when the muscle relaxes, and provides a sclerotized scaffold for the entire structure.