Wednesday 23rd September 2009 was a bright and sunny day and I decided to spend it exploring the river Deben. Standing on a hump back bridge North West of Wickham Market I looked down at the water, as I always do when arriving at a site, searching for surface dwelling invertebrates before climbing down the bank with my net. However my eye was immediately taken by several large patches of silver sparkling water, which I could see through my binoculars were whirligig beetles. The number of patches varied because as I watched they would join together or drift apart seemingly at random but, I estimated that together the whirligigs covered around 20 square metres. The space between the individual beetles was only a few centimetres and even a conservative estimate of one beetle per 25 square centimeters would mean that the river was covered in several thousand beetles.

This is by no means an unusual event at this time of the year, as these ‘schools’ or ‘flotillas’of whirligigs often occur in late summer. In the Autumn edition of White Admiral, number 65, I reported a similar occurrence at Lound Lakes.

The flotilla seen at Lound was formed of the species Gyrinus marinus and I wondered if the same species was occurring below me on the Deben. I always carry a plankton net with me as well as a frame net with a handle. The plankton net is cone shaped and designed to pull through the water on a rope. I dropped the plankton net from the bridge onto the nearest of the patches of beetles and found when I hauled it up onto the bridge that I had caugNovember 19, 2011 9:17align="center">

These late summer / early autumn ‘schools’ or gatherings of whirligigs are often a result of the second brood of the year having hatched recently, as happens generally in the autumn. The autumn brood is the one to hibernate. The obvious hypothesis here is that grouping in such large aggregations may well increase breeding success for individual beetles. However the hibernating second brood will not breed until the Spring. Experimental work in the USA by Romey and Rossman suggests that larger groups of whirligig beetles detect approaching predators sooner than smaller groups because there is a greater probability that at least one beetle in the group will see the predator ("the many eyes hypothesis"). The rapid whirling of startled whirligig groups is also thought to function in confusing predators and in advertising the defensive chemicals which they secrete when stressed. Higher temperatures and more abundant food supplies are also thought to increase the likelihood of larger ‘schools’ of whirligigs.

Preserved specimen of Gyrinus aeratus.
Total length from front of head to tip of elytra = 5mm.
(Preservation in alcohol has caused the abdomen to swell beyond the ends of the wing cases).

The large ratio of males to females which is greater than 2 to 1 is a puzzling fact if mating is not a reason for the school to form. The fact that two other species had joined the school possibly supports the ‘safety in numbers’ hypothesis, except that no females of these species were taken. The sample taken from the group should be a random one with the net dropped from a height of about 5 metres onto the centre of the school. There is of course a possibility that females were able to swim away from the dropping net more successfully than males.

The record was a very useful one for the county dataset as both Gyrinus aeratus and G. urinator are nationally notable species. G. substriatus is commonly recorded throughout Suffolk and in fact is as common nationally as all other species put together. Although there are regular occurances of G. urinator it is certainly not a common whirligig. Gyrinus aeratus on the other hand has far fewer records from Suffolk. The last record I can find was from Dedham Mill pond in 1957. Earlier records sent to me by Garth Foster of the Balfour Browne Club were; one from Mildenhall in 1947, another from Dunwich in 1926 and before that from Brandon in 1925. Finding such a large population of this beetle in central Suffolk hopefully means that it may be recorded more widely in future. If anyone has other records of this species I would, of course, be delighted to hear of them.

Of the 12 species of Whirligig beetle in the UK the following have been recorded in Suffolk.