New research is shedding light on why and how females of a Brazilian insect species have penises.
Female Neotrogla insects, which are part of the lice family, have penetrative penises which likely evolved from muscles originally used to stimulate males to release semen during sex, say researchers from Japan's Hokkaido University.
Their study, published in the Royal Society Open Science journal, is titled 'Acquisition of novel muscles enabled protruding and retracting mechanisms of female penis in sex-role reversed cave insects'.
While the females are still fertilised, they do so with a "penis-like intromittent organ called the gynosome", which penetrates the males' "vagina-like" organ during sex to receive the "voluminous and nutritious semen", the study describes.
The occurrence of the gynosome has only been found in the small tribe of cave insects, Sensitibillini, to which the Neotrogla belongs.
Despite this intimate knowledge of the insect's copulation techniques, until now the protruding and retracting mechanisms of the gynosome, including its evolutionary origin, were poorly understood.
The Neotrogla's cave environment is thought to be a key factor facilitating the need for female competition, and as a result, the gynosome.
Female Neotrogla also have twin sperm storage slots, allowing them to receive twice as much semen at the same time, and this renders female-female competition all the more intense.
The reversal of male and female genital function is particularly remarkable, and before now poorly understood, because the evolution of such anomalies must be simultaneous across sex and strongly constrained.
One factor that makes the reversal viable is that males of the Psocodea subgroup of lice usually do not have penetrative penises. The transfer of sperm instead happens via tight contact between the "flat female spermapore plate" and the "non-bulging opening of the male seminal duct".
To understand the origin of the penetrative gynosome, researchers compared the genital muscles of the Neotrogla with similar species with non-penetrative gynosome.
They discovered differences in the Neotroglas abdominal segments, spermapore plates, and subgenital plates.
However, they were unable to determine a possible trigger for the unique penetrative evolution of the female penis, suggesting more analysis will be key "for understanding the evolutionary history".
They were able to conclude that the muscles distinct to the penetrative function of the female penis "may have originally served to allow females to stimulate males so they would release semen during sex, providing the females with a nutritious semen snack".
