The Paragenital Organ of Stylopidae (Insecta: Strepsiptera) and the Functional Incorporation of the Secondary Larval Exuvia

Females of the insect order Strepsiptera are known to be traumatically inseminated. Traumatic insemination is the process of insemination by sperm transfer through a wound inflicted by the male in the female's integument, rather than by the male transferring sperm through the female's genital opening. Females fertilised by traumatic insemination are likely to exhibit morphological adaptations that help them to reduce the fitness costs associated with the integument wounding. One such adaptation is the presence of a paragenital organ. It has been described in traumatically inseminated bugs of the superfamily Cimicoidea and in species of the Strepsiptera genus Stylops. Although the paragenital organ appears to play a critical role in the mating biology of Stylops species, its phylogenetic roots are unknown. Here, we show that the paragenital organ in Strepsiptera may be an autapomorphy of the family Stylopidae, where we found it present in all species of the genera we studied (i.e., Eurystylops, Halictoxenos, Hylecthrus, Kinzelbachus). Our data thus refute the notion that the paragenital organ in Strepsiptera is exclusive to the genus Stylops. Integument relative thickness assessment based on µCT data revealed that regardless of the presence of a paragenital organ in Strepsiptera, penetration sites in the female's integument are thickened relative to control sites. In addition, we found evidence for the lateral processes of the secondary larval exuvia stabilising the paragenital organ. Our study contributes to the basic understanding of the evolution and the function of the paragenital organ in Strepsiptera and suggests potentially important morphological characters for a species-level phylogeny of the Stylopidae.