Last year, before I started my fieldwork, I set some traps in a vernal pool to get some experience. Unfortunately, I dropped my first beetle onto the ground. I couldn’t identify the species before it escaped into the litter. The only thing I was sure about is that it was an Acilius sp., as it was broad, flat and with a yellow band on its pronotum, the part between the head and the wing cases (Picture 1). To memorise my lost Acilius, I carved a sauna label.

There are two Acilius species in Finland, A. canaliculatus and A. sulcatus. Both are common species and can be found even in Lapland. A. canaliculatus (body length 14.0 – 15.5 mm) is smaller than A. sulcatus (15.7 – 18.0mm). Besides the body length, the patterns on their heads (Picture 2) can help you easily identify them.

Last year, I occasionally got some Acilius larvae. I was worried how to identify them to the species level, as diving beetle larvae are not well studied yet. Luckily, Dettner (1982) has provided very good identification key to three instars of the two species. If you look at the labium of a larva (Picture 3), you’ll easily recognise who it is. Maybe you don’t find Acilius larvae cute, but they are efficient predators of mosquito larvae, which may help you have a more positive impression on them.

I was surprised when I got Acilius even in fish ponds. It seems that Acilius has some tricks to avoid predators. A. sulcatus detects predators with both visual and chemical cues during the day when the visibility is good. At night when the visibility is poor, it mainly uses chemoreception to detect predators. When it notices the presence of predators, their activity reduces.

Let’s end the blog with a bad photo I took in the Finnish Museum of Natural History last year. The heads are unclear, so I’m unable to identify the species.

References

Chandra et al. (2008). Biocontrol of larval mosquitoes by Acilius sulcatus (Coleoptera: Dytiscidae). BMC infectious diseases, 8(1), 138. Available online

Dettner, K. (1982). Description of the larvae of Acilius duvergeri (Col., Dytiscidae), with keys to larvae of European species of genus Acilius and of the European Genera of subfamily Dytiscinae. Aquatic Insects, 4(2), 81-88. Available online

Nilsson, A. N., & Holmen, M. (1995). The Aquatic Adephaga (Coleoptera) of the Fennoscandia and Denmark. Ii. Dytiscidae: II-Dytiscidea. Brill. page 161

Rassi et al. (2015). Kovakuoriaisten Maakuntaluettelo 2015 [Provincial List of Finnish Coleoptera 2015]. Sahlbergia, 21(Suppl 1), 1-164. Available online

Åbjörnsson et al. (1997). Responses of Acilius sulcatus (Coleoptera: Dytiscidae) to chemical cues from perch (Perca fluviatilis). Oecologia, 111(2), 166-171. Available online