Why examine genitalia? Genitalia often provide very clear diagnostic features in species identification. This particularly applies to the male genitalia, though there are some groups where female genitalia are of greater value in identification than the male bits. In extreme cases (e.g. the carabids Pterostichus nigrita and P. rhaeticus, and in many non-carabid beetles) identification is only feasible using genitalia characters. In other cases, rather than struggling to evaluate subtle and comparative external characters, it is often quicker and easier to carry out a dissection, and your identifications will be more accurate.
Beginners often shy away from dissection, believing it to be too difficult. It is in fact extremely easy to smash a beetle open and remove the male genitalia. The difficult bit is not wrecking the specimen in the process!
These notes outline my technique for extracting the male genitalia from carabids, with minimal damage. They will be broadly applicable to other larger beetles. For tips on dissecting the tiniest of beetles, see here.
First check your specimen is a male – for most carabid species, males can be distinguished by their broader front feet, though the differences are slight in some species.
Second, brush out all the appendages, ready for carding. For larger beetles (c. 6 mm upwards), place the beetle on its back and hold it under the first and second fingers of your hand, so that the tip of its abdomen just protrudes from between the fingers (Fig. 1). It is usually easier to do this under a microscope. This technique can be used for even the smallest carabids, but it gets increasingly fiddly.
The mounted, hooked pin can then be inserted between the end plates of the abdomen (i.e., between the last sternite and the last tergite), which should part easily. The genitalia lie off-centre on the beetle’s right side, so inserting the pin down the midline, and then hooking round to that side and withdrawing the pin, will often bring the genital capsule out (Fig. 2).
Extracting the genitalia is sometimes easy, but sometimes involves a bit of fishing around! Often the abdomen becomes detached, which makes for a less attractive specimen but is not too disastrous – just card the abdomen alongside the rest of the beetle.
Once the genitalia are out, it is worth teasing away the soft tissues with either two pairs of forceps or a pair of forceps and a mounted pin. You can then arrange the median lobe and its left and right parameres as necessary for identification. An oval ring often emerges with the genital capsule, and should probably be carded with the beetle. However, the ring is easily broken, and very rarely used in species identification.
Genitalia can just be carded alongside the rest of the specimen, using the same glue. There is no standard orientation. Depending on the group, it may be important to look at the median lobe in dorsal view, to look at the median lobe in side view, or to look at the parameres. It is thus best to identify the specimen before gluing the genitalia down, although it is a simple matter to re-wet the glue at a later date if the genitalia need to be looked at from a different angle.
Internal structures of the genitalia
As if that wasn’t enough, there are some cases where it will be desirable to look at the internal structures of the aedeagus. In such cases, it’s important to prevent the genitalia from drying out at all – they very quickly develop air-bubbles inside which conceal the internal structures. So, quickly glue the genitalia beneath the rest of the beetle and while the glue is still drying, blob some DMHF over it using a mounted pin, enough to completely cover the genitalia. After a short while the DMHF diffuses into the internal spaces and makes the structures easier to see. It sets hard meaning that you have a permanent genitalia preparation and is a good way to prepare genitalia routinely even if internal structures are not of interest.
James McGill’s advice is to “use isopropyl alcohol as a solute for DMHF (not water). This mixes much better with the genitalia. It needs to be quite dilute (solute in slightly greater proportion than 1:1); you should be able to pick it up without strings forming from the liquid. Over time the alcohol evaporates so you will occasionally need to add a few more drops of isopropyl alcohol. You need a bit of a blob of DMHF over the genitalia as it tends to flatten as it dries. Try to apply the DMHF around the genitalia and upwards to prevent too many air bubbles forming. These are a nuisance but can be worked upwards with a fine pin or brush and then lifted out on a brush with some patience (if anyone has a better method please Comment)”.
DMHF has been getting more and more difficult to get hold of. I am not currently aware of any UK supplier. DMHF is still available from this Spanish website in a range of quantities up to 1 litre. It’s worth getting a lifetime’s supply before another world DMHF shortage hits, or production ceases altogether.
If and when DMHF becomes completely unavailable, there’s an alternative preparation in use in Germany called ‘Lompe’ (after Arved Lompe): “it consists of PVP, glycerol and water. The big advantages are its high transparency, water solubility and very easy usage (no special treatment of the genitalia is needed before embedding)”. Recipe and instructions in German here, or translated into English here. There is a 2005 paper by Gianfranco Liberti “Improved solutions of two water-soluble media for mounting beetle genitalia” in The Coleopterist 14: 29 – 35. This compares the properties of DMHF and PVP and also describes how to mix the solutions.