The amazing internal morphology

I've been doing full disarticulations of acidocerines, and so far, it has been fascinating!

For one thing, I wasn't sure about how to do it, but was pretty sure about what I wanted to see. I started trying to find a protocol for this, as I've seen this kind of work presented at national meetings, but maybe I didn't do a good job in my search, because I couldn't find one. And yes, I know the technique (mostly the KOH soft tissue dissolution) vary depending on several factors as the "age of preservation" of the specimen, its size and its hardness, but still I was looking for some kind of standard that I didn't find... at least not like that.

What I did found were several revisionary works on hydrophilids that include detailed morphology for the group in question. My favorite so far, in terms of the description of the methods, is by Minoshima, Komarek & Ôhara (2015) for the genus Megagraphydrus (currently Agraphydrus, which they synonymize in this work). Regarding morphological treatment and nomenclature of the head and its appendages, my favorite is Anton & Beutel for the genus Helophorus. Other useful resources are the revision of Afrotropical Anacaena by Komarek 2004 and the nice pictures provided by Fikáček & Vondráček (2014) on their revision of Pseudorygmodus.

My previous experiences with dissections were on broad-nosed weevils, of minimally 4mm which are harder (thicker cuticle) than the acidocerines, and my procedures involved nearly-boiling water and a quick "cooking" of about 10 minutes. The same treatment with water beetles can result in a disaster of two kinds: one, you overcook them in which case dissection is a very delicate and risky process where you end up tearing sclerites that you were not supposed to, or two, you undercook them, and then dissection is difficult and messy because there is soft tissue everywhere and you end up not being able to clearly see the structures under the microscope anyway. For my first experiments I used non-target specimens (I mean hydrophilids other than acidocerines from which we had a bunch sitting on vials on alcohol).

Then I made a patience commitment accompanied of deep breaths and performed the procedures as described in Minoshima et al (2015), which involve a temperature of around 60°C during about one hour. At the beginning I was afraid of overcooking but when I was able to relax and let it be, the results were quite good! I even tweeted it in excitement!!

I also have had to modify procedures, so now I separate head, prothorax and abdomen before cooking the specimen, also open the abdomen on one side (when possible), so the KOH goes through all the abdominal grease and tissues, and now I try to not cook the wings (something I should have done before... oh well).

So far I have 10 species fully dissected (one specimen each). There are interesting characters everywhere!: mouthparts (mandibles are particularly awesome!), metafurca, abdominal tergites and sternites, wings. And then of course, it helps a lot when you have a good microscope available to take a closer look!

For now, I will leave you with this mosaic of structures of Helochares abbreviatus. [structures not necessarily to scale regarding each other, images not edited -only cropped to fit slide-]

Helochares abbreviatus: 1. Labium; 2. Labrum; 3. Maxilla; 4. Mandible; 5. Antenna; 6. Prosternal process; 7. Abdominal tergites I-III; 8. Abdominal tergites V-VIII; 9. Abdominal sternite VII; 10. Abdominal sternite VIII; 11. Scutellum; 12. Metafurca; 13. Abdominal sternite IX; 14. Abdominal tergite IX.

Disarticulation allows you to see structures that are usually overlooked because you cannot see them without breaking the specimen, such as the prosternal process for example. And then of course, as with some specimens you can't identify the sex, "by accident" you dissect a female and also find a bunch of structures that apparently not many hydrophilid workers take a look at. Again, this is only starting! the tricky next part is to start coding these things.

References

• Anton, E., & Beutel, R. G. (2004). On the head morphology and systematic position of Helophorus (Coleoptera: Hydrophiloidea: Helophoridae). Zoologischer Anzeiger-A Journal of Comparative Zoology, 242(4), 313-346.

• Fikáček, M., & Vondráček, D. (2014). A review of Pseudorygmodus (Coleoptera: Hydrophilidae), with notes on the classification of the Anacaenini and on distribution of genera endemic to southern South America. Acta Entomologica Musei Nationalis Pragae, 54(2), 479-514.

• Komarek, A. Taxonomic revision of Anacaena Thomson, 1859 I. Afrotropical species. Koleopterologische Rundschau, 74, 303-349.

• Minoshima, Y. N., Komarek, A., & Ôhara, M. (2014). A revision of Megagraphydrus Hansen (Coleoptera, Hydrophilidae): synonymization with Agraphydrus Regimbart and description of seven new species. Zootaxa, 3930(1), 1-63.