This is the business end of a male seed beetle. The end of his aedeagus (penis) is covered in these spines. The function of these spines is unclear, but there is some evidence that they damage females during copulation. The severity of this damage in terms of fitness costs to females, however, has not been determined (the costs of copulation in these beetles is complicated by behavioural factors like male harassment, which has also been presumed to carry a cost to female fitness. Separating these is difficult to do).

A close-up of the spines.

This is a side-ways view of the aedeagus. The spines, which you can see on the bottom, actually evert and go out the other end during mating.

The wishbone-type structure on the right, which we have deemed an “apodeme” (as it appears to be similar to a structure that goes by that name in flies) lies over the aedeagus (which you can see on the right, this time viewed from above) and is perhaps a support structure of some sort, though it might also have a sensory or stimulatory role as the ends of the apodeme have bristles.

This triangular structure’s role is not known. It might have a sensory or stimulatory role.

This is how the compontents of the male genitalia are arranged. Note again that the spines (on the left) evert during copulation so they come out the end end of the aedeagus. Here is photo of the aedeagus in its erect configuration:

(micrograph image by Andrew Syred, Microscopix; source: Helen S. Crudgington, Mike Siva-Jothy 2000. “Genital damage, kicking and early death”, Nature Brief Communications vol 407 pages 855-856)

This is the female reproductive apparatus. The opening is the right, and the left side shows a bulbous structure that appears to be used for sperm storage or processing. The middle part is where the male’s adeagus fits in.

This is a cross-section of the tract (also known as the bursa copulatrix). I am measuring the area of connective tissue (the light pink region).

A simplified schematic. Note the spines just next to the oval structures. The function of these spines is unknown; an intruiging possibility is that they are used in some sort of antagonistic role. Here they are:

Hope you enjoyed that! On a related note, genitalia are the most diverse and rapidly evolving system in animals. It might seem quiant, but substantial theoretical and empirical research has been devoted to understanding why this should be the case. I may well write some more about this later.