Females with penislike genitals and males with vaginalike organs are cases of a new extreme reversal of sex roles researchers have discovered in little-known cave insects. These are the first examples of animals with genitalia that reverse the traditional sex roles. The researchers were astonished to discover that the females of four Neotrogla species from extremely dry Brazlian caves had penislike genitals dubbed gynosomes, a complex organ composed of muscles, ducts, membranes and spikes. In contrast, the males possess vaginalike phallosomes. Shown here, the female penis structure of the cave insect Neotrogla aurora.
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Cave Insects Copulating
In desolate caves throughout Brazil live insects that copulate for days, the female's penetrating erectile organ sticking fast in a reluctant male's genital chamber until he offers a gift of nutritious semen. Not all animal species have a male penis, but because the evolution of body parts usually works through slow modification of existing structures, there would need to be a good reason for a female to develop a penetrating organ, says entomologist Kazunori Yoshizawa of Hokkaido University in Japan, a co-author of the study. Entomologist Charles Lienhard at the Geneva Museum of Natural History in Switzerland recognized them as a new genus — and also as possessing unusual genitalia. Hold on tight When the flea-sized winged insects mate, the female mounts the male and penetrates deep into a thin genital opening in his back. Membranes in her organ swell to lock her in, and multiple spiky spines act as grappling hooks to anchor her tightly to the male. When researchers tried to pull apart two mating insects, the female was gripping so tightly that the male was accidentally ripped in half, leaving his genitalia still attached to the female. The key to the anatomy and role reversal might be simple hunger. Yoshizawa and his colleagues are now working to establish a healthy population in the lab, but the biggest challenge will be finding a suitable food to replace the cave-bat droppings, Yoshizawa says.
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Traumatic insemination , also known as hypodermic insemination , is the mating practice in some species of invertebrates in which the male pierces the female's abdomen with his aedeagus and injects his sperm through the wound into her abdominal cavity hemocoel. The process is detrimental to the female's health. It creates an open wound which impairs the female until it heals, and is susceptible to infection. The injection of sperm and ejaculatory fluids into the hemocoel can also trigger an immune reaction in the female. Bed bugs , which reproduce solely by traumatic insemination, have evolved a pair of sperm-receptacles, known as the spermalege. It has been suggested that the spermalege reduces the direct damage to the female bed bug during traumatic insemination. However experiments found no conclusive evidence for that hypothesis; as of , the preferred explanation for that organ is hygienic protection against bacteria. The evolutionary origins of traumatic insemination are disputed. Although it evolved independently in many invertebrate species, traumatic insemination is most highly adapted and thoroughly studied in bed bugs, particularly Cimex lectularius.
In contrast, the male genitalia, which is penetrated by the female, has simple form. It is thought that competition for these nutrients among females facilitated the evolution of the female penis. With few exceptions, for living organisms that perform internal fertilization, the male possesses a penis. We discovered that female Brazilian cave-dwelling insects in the genus Neotrogla order Psocoptera have a penis-like intromittent organ, which is inserted into the male during copulation. During copulation, the male Neotrogla transfers capsules containing nutrients to the female together with seminal fluid. Our observations suggest that the female is more competitive than the male regarding copulation for receiving these nutrients, which facilitated the evolution of the female penis. It is expected that detailed research on sex-reversed insects will provide vital hints for exploring the evolutionary background of the origins of differences between sexes. Associate Professor Kazunori Yoshizawa. Research Faculty of Agriculture, Hokkaido University. Neotrogla curvata, one species of Neotrogla, during copulation.