Mating behaviors and multiple mating in the firebrat, Thermobia domestica (Zygentoma: Lepismatidae)

While most terrestrial animals undergo internal fertilization, some insects use external transfer of sperm during mating. Among these are insects belonging to the orders Zygentoma (silverfish and firebrats) and Odonata (dragonflies and damselflies), which employ external sperm transfer. Although the mating processes of the latter group are well understood, those of the former group have remained obscure. Here we present a detailed description of mating in the firebrat, Thermobia domestica (Packard) (Zygentoma: Lepismatidae), involving external sperm transfer. We found that mating behaviors of the firebrat are complex and largely modular, with four distinct recognizable elements: facing, turning around, back and forth, and ejaculation and straddling. Interestingly, males do not guard or show further interest in females after depositing the spermatheca on the ground. To investigate whether firebrats engage in multiple matings, we performed male-informative crossing experiments by using a gene-edited mutant line. Our results revealed that when females were paired with two males of different genotypes, 48% of the recovered F₁ clutches (24 out of 50) were derived from multiple matings, providing strong evidence for frequent multiple mating in the firebrat.