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figure 1: Water beetle with Ancylus spp. attached (photo: Joan Saul)
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Light traps are widely used to catch moths, but occasionally attract other types of night flying insect. On 19th September 2009 some members of the Norfolk Moth Group operated a mercury vapour light trap near Lyndford Hall, Norfolk (NGR TL8293). In addition to moths, the catch that night included a large beetle, which was examined, photographed and released. From the photograph Garth Foster was able to identify the insect as a male water beetle, either Discus semisulcatus or Dytiscus dimidiatus (more precise identification was not possible as the distinguishing anatomical feature was not visible in the photograph). The light trap had been set up about 1km south of the River Wissey and even closer to a much modified stream that is a tributary of the Wissey, so the capture of a water beetle was not a great surprise. What made the record interesting was the large number of freshwater gastropods found attached to the insect’s elytra (see figure 1). Twenty freshwater limpets can be seen in the photograph. Most are Ancylus fluviatilis, with a single specimen of Acroloxus lacustris near the outer margin of the left elytron. |
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H.W. Kew’s The Dispersal of Shells, published in 1893 (opposite), is a wonderful example of meticulous, very polite, Victorian natural history writing and is still a useful source of information well over 100 years later. Kew documented numerous examples of freshwater molluscs, mostly Sphaerium and Pisidium species, but including a smaller number of records of A. fluviatilis, A. lacustris and Bithynia tentaculata, found attached to aquatic insects including Hemiptera (Bugs), Odonata (Dragonflies) and Coleoptera (Beetles). Although most of the insects were found in fresh water, a few of the Coleoptera were caught in flight. An insect brushing against a bivalve and causing it to quickly close its valves could result in the mollusc gripping the insect’s antenna, leg or other appendage and being carried off when the insect crawls, swims or flies away (records of Sphaerium spp. and Pisidium spp.). A prosobranch quickly closing its operculum when contacted by an insect could produce a similar result (record of B. tentaculata). It is less easy to explain how a pulmonate could end up attached to a very mobile and (in molluscan terms) very fast moving insect. Kew (1893) referred to an egg capsule of A. fluviatilis “attached to one of the wing-cases of an Acilius, a strong flying water beetle.” He went on to write “The fresh- water limpets...sometimes ride upon the backs of large flying water-beetles!” Writing about the dispersal of freshwater molluscs in Britain over four decades after Kew, Boycott (1936) wrote: “The means of transport were very well discussed by Kew in 1893... and I do not think anything has been seen since which adds anything really new to his survey, though various fresh examples of snails in transit have been observed.” About A. fluviatilis he wrote: “It needs a hard clean surface to sit on (which may be supplied by the shell of another snail or a mussel or the elytron of Dytiscus, on which it has been caught taking an aerial journey, occasionally by plants, as well as by stones) to which it fits snugly because the edge of the shell consists of periostracum only...” |
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To return to the Lynford record, how could the beetle have acquired its burden of snails, bearing in mind the large number of individuals representing two species? Kew (1893) provided evidence that gastropod egg capsules could become attached to water beetles (see above). Emergence from such a capsule, or capsules, could have resulted in a number of snails crowded together on the dorsal surface of the beetle, especially if the molluscs were unable to disperse due to a lack of other suitable substrata. From their sizes it is obvious that the snails had not recently emerged, which raises the questions of how long they had been on the beetle and how they had been feeding.
The dorsal surface of even a very large water beetle provides a very limited area on which gastropods can feed. The ventral surface of the insect is probably unavailable as movement of the insect’s limbs would prevent snails attaching, or soon dislodge any that managed to do so. Water beetles use their limbs to dislodge detritus and small organisms that might attach themselves. In addition to this mechanical cleaning, Garth Foster (pers. comm.) has pointed out that Dytiscus secretes a chemical that acts as an anti-fouling agent that further limits the growth of algae, etc. (at least on those parts of the body that the limbs can reach). Although it is obvious that snails can remain attached despite the beetle’s efforts to dislodge them, even a single individual living on a beetle would soon run short of food.
To return to the Lynford record, how could the beetle have acquired its burden of snails, bearing in mind the large number of individuals representing two species? Kew (1893) provided evidence that gastropod egg capsules could become attached to water beetles (see above). Emergence from such a capsule, or capsules, could have resulted in a number of snails crowded together on the dorsal surface of the beetle, especially if the molluscs were unable to disperse due to a lack of other suitable substrata. From their sizes it is obvious that the snails had not recently emerged, which raises the questions of how long they had been on the beetle and how they had been feeding.
The dorsal surface of even a very large water beetle provides a very limited area on which gastropods can feed. The ventral surface of the insect is probably unavailable as movement of the insect’s limbs would prevent snails attaching, or soon dislodge any that managed to do so. Water beetles use their limbs to dislodge detritus and small organisms that might attach themselves. In addition to this mechanical cleaning, Garth Foster (pers. comm.) has pointed out that Dytiscus secretes a chemical that acts as an anti-fouling agent that further limits the growth of algae, etc. (at least on those parts of the body that the limbs can reach). Although it is obvious that snails can remain attached despite the beetle’s efforts to dislodge them, even a single individual living on a beetle would soon run short of food.
Of course, the snails may have emerged, fed and grown before attaching themselves to the beetle, in which case the cleanliness of the elytra may be the reason so many snails were present. Boycott (1936) noted that A. fluviatilis could not tolerate silt and mud, and Kerney (1999) wrote that the species “avoids muddy substrates or stones coated with mud or thick algae”. The smooth dorsal surface of a male Dytiscus could provide a very good substratum, mechanically and chemically cleaned by the insect and further washed by moving water whenever the beetle crawls or swims. The deep grooves on the elytra of female Dytiscus could make them less suitable as a substratum for snails. That the beetle could provide a means of transport to another habitat would be a potential bonus!
Questions remain. Did the snails attach themselves to the beetle “one at a time” over a protracted period, or did several or all of them attach themselves in a much shorter time interval? How did so many limpets (literally moving at a snail’s pace) attach themselves to a highly mobile insect? What was the beetle doing whilst this was happening? Presumably the insect was immobile or virtually immobile when the snails were moving on to it. Why? How long the process take?
The author of this note would welcome suggestions as to how so many snails ended up on the back of a beetle and/or to hear of other records of molluscs attached to insects.
The image of a motley gang of marauding molluscs lying in wait, concealed in submerged vegetation, and then ambushing (or perhaps hijacking is a more appropriate term) a hapless beetle as it crawled or swam past is, no doubt, merely fanciful, but nevertheless quite amusing.
Acknowledgements
David Lester showed the author a copy of Norfolk Moth Survey Newsletter (not an obvious source of molluscs records!) which mentioned the Lynford record. Discussion with David and Jo Lester and Garth Foster added to and improved this note. Ken Saul provided information about the moth trapping at Lynford. Joan Saul took the photograph that made it possible to identify the beetle and snails and kindly allowed her photograph to be reproduced here.
References
Boycott, A.E. 1936. The habitats of freshwater Mollusca in Britain. Journal of Animal Ecology 5(1), 116-186.
Kew, H.W. 1893. The Dispersal of Shells. An inquiry into the means of dispersal possessed by freshwater and land Mollusca. xiv, 291p. London: Kegan Paul, Trench, Trübner.
Kerney, M. 1999. Atlas of the Land and Freshwater Molluscs of Britain and Ireland. 264p. Colchester: Harley Books.