In terrestrial isopods the 1st 2 pairs of abdominal limbs (pleopods) of males have evolved to perform the function of a penis. In many animal groups, natural selection has created species-specific morphologies in organs that are involved in semen transfer or reception. Such adaptations prevent or decrease the incidences of breeding between similar species, thus contributing to the genetic isolation of the species from each other, especially during the early phases of speciation. Expectedly, such organs with species-specific morphologies are of great help when it comes to distinguishing species from each other. The modified limbs of male isopods are no exception.
Last nite's accomplishment was the removal of the 1st pair of pleopods of a male Trachelipus rathkii, an isopod widespread in North America although it is believed to be an introduction from Europe. This happened to be my first attempt at this procedure, which is not really that difficult if you have a stereo microscope and a pair of fine tweezers.
Here they are.
This is a composite image of the 1st pair of pleopods and the outer branch (exopodite) of the right (animal's left) pair that came apart during the removal operation (each pleopod has an outer and an inner branch). The length of the organ from top to bottom is about 1.3 mm.
Now, compare my photo with this figure for T. rathkii from Van Name*.
The general agreement with Van Name's drawing confirms the identification, initially based on other characteristics, as T. rathkii.
*Van Name, W. 1936. The American Land and Fresh-water Isopod Crustacea. Bull. Am. Mus. Nat. Hist., 71:1-535. pdf