If anyone is looking for a new hobby, I can thoroughly recommend gerrid-watching! It might not be as popular as bird or butterfly watching, but observing the movements and behaviour of Gerridae – commonly known as pond skaters or water striders – is a fascinating and rewarding pastime! These amazing bugs – yes they are true bugs! – occur all around the world, usually on fresh water, although some ever skate on the surface of the open ocean. They do all sorts of exciting things! The males ride around on the backs of females (who are not too happy about this, but have to put up with it for a while). In fact, the males and females are in a constant state of conflict, often with violent struggles as the females try to dislodge the harassing males. There is always something exciting going on in the gerrid world; there are often little squabbles and fights as one individual impinges on the personal space of another (below); or squabble over who owns the fly that just dropped down into their domain! Interactions such as these are challenging to capture, which makes it fun to try!
The water surface can be a messy place – especially when it is glassy smooth and unperturbed, as in the following two photographs (below) – but it also a habitat full of potential finds and opportunities; it is where gerrids find their food and live their lives. Lots of insects fall into the water, get trapped by the surface tension, and end up as food for surface dwelling species such as pond skaters. When gerrids get hold of a prey item, they appear to plunge their large proboscis into it and suck the insect dry!
Water striders can have long or short wings (or no wings at all). This polymorphism varies according to site and region. Short, or reduced-winged forms tend to predominate in permanent habitats, as there is no advantage in being able to migrate. Where habitats and unpredictable and tend to dry up and disappear, wings become a distinct advantage, allowing individuals to move on to new pools and rivers. In practice, there may be individuals without wings (apterus), or with very tiny wings (micropterous), with or short wings of variable lengths, or with long wings! See Vepsäläinen, K. (1974). In these photographs, most individuals appear to be apterous or micropterous.
When I was taking these photographs (in Spain, see below) I was surprised to see many of the gerrids covered in small, orangy-red balls, rather like tiny red balloons of different sizes stuck to their bodies. These are water mites, almost certainty the species Limnochares aquatica (Hydrachnidia, Acari), which inhabit slow-flowing rivers and lakes throughout Europe. They have an extraordinary life cycle and are both predatory and parasitic, at different stages of their lives. In early summer, the female mites deposit their eggs on submerged plant roots; the larvae emerge and float to the surface of the water where they attach themselves to a host (not just gerrids, other aquatic bugs as well). Once attached, they feed on the body fluids of the water striders and can have a negative effect, even causing the death of the early instar gerrids (Smith, 1989). As they develop, the mites lose they appendages and truly become little red blobs – called the nymphophan stage – before, sometime in late summer, they develop into adults and then leave their gerrid hosts.
The pond skaters must must be mightily revealed to finally be free of these red, blood-sucking parasites! Particularly, when the mites had been clustered around the head and eyes of the gerrid, as shown below.
These pond skaters are, I think Gerris lacustris, but there are a number of different species in Galicia (NW Spain) including Gerris lateralis, G. argentatus, G. gibbifer and G. thoracicus as well (Pérez-Bilbao et al., 2012).
As well as watching gerrids, say with binoculars, it is also good fun to try and get good photographs of them. I am still trying to get better photographs; but there is always next year, hopefully!
I took these photographs on 6th Sept 2019 on the Rio Sor, Galicia, Spain. Interestingly, the pond skaters I photographed in another place I visited on the river, did not show any signs of mite infestation.
Henry, S. M. (Ed.). (2013). Symbiosis: associations of invertebrates, birds, ruminants, and other biota. Elsevier.
Pérez-Bilbao, AMAIA, Benetti, CJ, & Garrido, JOSEFINA (2012). New contributions to the knowledge of aquatic heteroptera (Heteroptera: Gerromorpha and Nepomorpha) in wetlands of Galicia (NOT Spain). Bulletin of the Spanish Entomology Association , 36 (1-2), 87-107.
Smith, B. P. (1989). Impact of parasitism by larval Limnochares aquatica (Acari: Hydrachnidia; Limnocharidae) on juvenile Gerris comatus, Gerris alacris, and Gerris buenoi (Insecta: Hemiptera; Gerridae). Canadian journal of zoology, 67(9), 2238-2243.
Vepsäläinen, K. (1974). The life cycles and wing lengths of Finnish Gerris Fabr. species (Heteroptera, Gerridae). Acta zoologica Fennica 141.