Most Meddians have what is called a 'Latch' which protect their genitalia. Though some do not. Some have external genitals or some have a Sheath instead of a Latch. This is made of flesh rather then Chitin. Males with sheaths or external reproductive parts cannot bear young.
The latch itself is like a cover, it can be pulled away and even slid inside either the specially slit inside the wall of a female's vagina or the wall of a male's the pseudo cavity.
Both males and females with Latches can bear children. Those born without latches can only bear young if they are female or herm.
On Males the cavity is like a cloaca so the penis is hidden inside and then slides out when erect or when pushed out voluntarily. When acting as a 'uterus' the cavity expands upward into the position of a real uterus. In this case the penis is no longer in the cavity and is infact outside the cloaca but inside the latch cover. (most common latch. see Thorn, Humina, Melon)
If a Male has a 'Female style' latch (resembles a pseudo vagina. see Honeydew, Fife, Thrush) the penis is merely shifted up and shares the cavity with the developing offspring. A thick membrane is formed as a protective barrier to prevent injury.