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(You can click on the picture
to see a short ▶movie).
On water beetles you often find white, stringy flakes of growth, which sink together like a jelly when coming above water. I used to think that the beetles were moldy, but they are colonies of bell-shaped animalcules, sessile peritich ciliates. They may be of the genus Opercularia, possibly O. articulata, so I saw in the unsurpassed book by Streble (1973). The photo above left is a close-up of a group on a yellow-edged water beetle (Dytiscus marginalis), click on the image to see an enlargement, if you like. For the image on the right, a group has been plucked loose from the beetle and placed under the microscope. When you click on the image you can see a short ▶movie.
Sessile peritrichs are unicellular organisms, see Vorticella. An important difference with that genus: the stems are not contractile. That is why 'swaying bunches' are always visible (when under water). Furthermore, Opercularia articulata can only be found on water insects.
There are more species of bell-animals that live epizoically , on animals: on the web I came across the genus Orbopercularia at the NIES (see literature) , with species that often seem to be confused with Opercularia. The only difference is the nucleus (the Macronucleus), which in the first-mentioned species is elongated and usually curved in the middle, Orbopercularia on the other hand has a round nucleus. But unfortunately the nucleus is not visible with certainty in the photos here. Another possibility is Epistylis, with species that also resemble Opercularia, but live on other animals (fish, copepods) and have a 'lip' at the upper edge, that this is missing can be seen in the photo.
Move the cursor over the picture on the left (or tap it). In the diagram shown, what I think (!) is the Macronucleus, is indicated in yellow (N), the cell mouth in brown (S) and the cilia in green (D).
Click on the picture above for a small ▶movie.
The collage below: at the arrow you see two cells, close together on the stem, like buds on a tree branch. I found many such pairs in the microscopic slide.
A number of spots on the stems are clearly abandoned. Whether this is due to the pulling of the colonies with the tweezers, I do not know. In any case, they were firmly attached to the beetle, which was dragged along by the threads, much to its displeasure. In the picture on the right you can see how big such a forest of growth can become. How much trouble does the beetle have from it? In any case, its progress is slowed down. Cleaning with its legs does not help, that does not make them come loose. If you look at the picture on the top left of this page, it looks as if they attach themselves, via the pores in the pronotum and elytra, under these layers, like a tree with roots. But that is just my own idea.
What moves these animals to attach themselves to a water insect? Perhaps the question is also the answer: the insect moves them through the water. This makes them much more mobile. They may also benefit from the food particles that the beetle or bug stirs up, like birds, which like to sit on cows, because of the insects they stir up. When the beetle starts flying, they are also transported to other bodies of water, if they survive the few hours of drought during the flight.
Literature: (See also: Bibliography Microscopic organisms ).
Streble,H & Krauter, D (1973) Das Leben im Wassertropfen. Kosmos Gesellschaft der Naturfreunde. Franckh'se Verlagshandlung Stuttgart 1978. (75-76 and 246-247).
The World of Protozoa, Rotifera, Nematoda and Oligochaeta
The National Institute for Enviromental Studies (Japan),
read 4 apr 2024 op: https://www.nies.go.jp/chiiki1/protoz/morpho/ciliopho/orboperc.htm)