Posted by: Rob Viens | May 16, 2012

Beetle Mania! – The Aquatic Edition

May 16th and Darwin is back in the cabin examining his “catch of the day”, simply noting:

“Examined the rich produce of yesterdays collecting. — Earl is considerably better.” (May 16)

What was he examining? – most likely some beetles (among other things), so it’s back to:

Beetle Mania logo

In his Zoological Notebook and letter to William Fox (a couple of days ago) Darwin mentions several water beetles.  Comments include:

“This morning I have taken minute Hydropori, Noterus Colymbetes, Hydrophilus, Hydrobius, Gyrinus, Heterocerus Parnus, Helophorus Hygrotius, Hyphidrus, Berosus &c &c, as a specimen of fresh-water beetles.” (Correspondence to William Darwin Fox, May 1832)

“Amongst the Hydrocanthares were several minute species of Hydroporus, Hygrotus & Hyphidrus & Noterus.” (Zoological Notebook)

“Hydrophilidæ very numerous, & many of species very minute.” (Zoological Notebook)

So today I though I would try to sort out these aquatic Coleoptera a little bit.

First off – “water beetle” is a fairly generic term that refers to beetles that spend all or part of their life in the water. Water beetles fall in both of the large beetle suborders – the Polyphaga and the Adephaga.

Water beetles still need to breathe, and many of the species of come up with interesting adaptations for breathing underwater (as well as for swimming).  Some, such as the whirligig beetle, carry a bubble of air under their abdomen.  Others have modified the exoskeleton to behave like a “gill” which allows them to extract oxygen from the water (like a fish does).

It is important to note a couple of things about Darwin’s beetle notes:

  1. Beetle classification has changed a lot in 180 years, so some of the specific names Darwin uses and how he “lumps” these species into larger families are a bit out of date with our modern taxonomy.
  2. Darwin’s spelling what often more phonetic, so it can be a little troublesome tracking down what some of these beetles are (especially since I am not an entomologist). To be fair, these are his field notes, so it’s not like he always had a reference at hand or had time to go back and correct his spelling.

Ultimately the species Darwin listed seem to fall under the following families:

Predaceous Diving Beetles – Dytiscidae Family (Adephaga Suborder)

“Hydroporus, Hygrotus & Hyphidrus ” (the later correctly spelled Hyphydrus) are all examples of predatory diving beetles of the family Dytiscidae.  These beetles live in the water, and as their name suggests, prey on other insects (releasing a digestive enzyme with their bite). They are often eaten, too – by birds, fish, and even humans.

Hydroporus pubescens (From Wikipedia Commons):
Hydroporus pubescens

Diving beetles (from Fauna Germanica: Die Käfer des deutschen Reiches (vol. I, pl. 38). Farbendrucktafeln von Dr. K.G. Lutz. K.G. Lutz’ Verlag, Stuttgart. (1908))
Water beetles
See key here

There is even a species in this family named after Darwin – Darwinhydrus solidus – though where it is from, I cannot tell.

Water Burrowing Beetles – Noteridae Family (Adephaga Suborder)

Darwin refers to “Noterus” in the same family as the predaceous diving beetles – which really was not a mistake.  Until recently they were lumped together with the Dytiscidae. The other genus Darwin mentions that falls in the family are the Hydrocanthus – of which there are many species found in Brazil.

As their name implies these small beetles burrow into the mud in search of prey (they are largely carnivorous).

Hydrocanthus iricolor (from Iowa State University Bug Guide – photo by Tom Murray):
Hydrocanthus iricolor

Hydrobius fuscipes (from Wikipedia Commons):
Hydrobius fuscipes

Water Scavenging Beetles – Hydrophilidae Family (Polyphaga Suborder)

As Darwin mentions, species of this family are quite common in the area around Rio.

Contrary to their name these beetles eat a variety of food types.

Hydrophilus piceus (from Wikipedia Commons):
Hydrophilus piceus

Whirligig Beetles – Gyrinidae Family (Adephaga Suborder)

These little “gyrinis” already received their own post – see more at   Whirligigs in Ditch.

Taxonomy of Darwin’s water beetles:
beetle phylogeny

Boy – I have to say – sorting out beetles is a bit complex. But I suppose it is to be expected when dealing with hundreds of thousands of species. And it doesn’t make it easy that beetle taxonomy has changed a little in the last 180 years. Still – it gives me even more respect for Darwin’s abilities as a naturalist. (RJV)


  1. Another great post about aquatic beetles! I’d like to add that the larvae of some of these species are quite spectacular. They tend, particularly in the Dytiscidae, to be large voracious ambush predators with hollow needle-like mandibles to inject prey with digestive enzymes and then suck up the remaining fluid. They also breathe air through tubes at the tip of their abdomen. This is similar to how mosquito larvae breath.

    • Very cool – thanks Jason! One of these days I see “Larva Mania!” in your future 🙂

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