1 begins with the two intertwining, then copulating in a reciprocal nature (so each mate's penile bulb is inserted into the female reproductive part of the partner), ending in a unilateral sperm transfer from one mate to the other.
Not only do the slugs inject sperm into each other, they also stab each other in the head for several minutes after penetrating each other with their penile bulbs; they remain stabbed and pumping prostate fluid into each other's circulatory systems for the majority of their 40-minute mating period, researchers report on Nov.
These include two images that show the subject standing.
One captures a full length view, in which the subject is unclothed except for a pair of stockings and shoes.
Although developed several decades earlier, photography was rarely used for medical documentation until the 1850s.
In the mid-1850s France, at the request of neurologist Duchenne de Boulogne, photographer Adrien Tournachon documented experiments in which facial muscles were electrically stimulated; another example of clinical photography was documented by German physician Hermann Wolff Berend in an 1855 journal article entitled "Ueber die Benutzung der Lichtbilder für heilwissenschaftliche Zwecke" ("On the Use of Photographs for Therapeutic Research Purposes").
However, by the late 1830s he had left medicine to focus on the printed press, becoming a caricaturist, journalist, and novelist.
Its age has not yet been officially determined but the hunters believe the animal was around four years old.Possibly done on commission by Armand Trousseau, the nine photographs have been described as "probably the first medical photo-illustrations of a patient with intersex genitalia".