ANTLIONS – HABITAT AND HISTORY by Suffolk Naturalists' Society

ANTLIONS – HABITAT AND HISTORY

ANTLIONS – HABITAT AND HISTORY MICHAEL KIRBY The antlion (Euroleon nostras) in Britain is found almost exclusively in the Suffolk Sandlings but pre-1966 there were doubts as to whether it was a native species, breeding in the region. The habitat factors which determine its distribution and history are discussed in this paper. Sand The first essential for the antlion larva is free-running, fine sand. The shape of the abdomen of the larva, a backwards facing wedge and its way of walking backwards enable it to move easily beneath the surface (Kirby, 2001). Its pit in which it traps small arthropods is also dependent on the characteristics of dry sand. As the antlion larva makes its pit, sand is thrown upwards and outwards by the insect and forms a regular inverted conical pit, its sloping sides at the angle of rest (one of the earliest descriptions of pit building was by the Suffolk Naturalist, the Rev. W. Kirby (Kirby & Spence, 1826), although he did not see antlions in Britain). The sloping sides of the pit are only just stable and the slightest disturbance by an insect such as an ant running across the rim of the pit starts a mini avalanche and it slides quickly to the bottom. The avalanche develops in the surface of the sand slope, only a few grains in depth (Daerr & Douady, 1999), so that the insect falls with only a small amount of sand which does not hinder the antlion as it seizes its prey. Origin of the sand Much of the Sandlings was derived from sand and gravel carried by the proto – Thames flowing from the south west (Rose, 2000). It discharged into a shallow sea where the sand and gravel was sorted and deposited as beds several metres thick, As the sea level fell during the ice ages, glaciers diverted the Thames towards its present course. The emerging land was unaffected by glaciation and no further deposits took place in much of this area; for example the topography of the Westleton Beds has not changed and can be seen at present as the rounded, gently undulating sand banks of Westleton Heath. The newly exposed land was eventually covered by climax vegetation, covering and stabilising the sand, the type depending on the prevailing climate. Subsequently agricultural activities produced the cover of heather and grassland seen today. Where heather or grass do not cover the ground, the surface is stabilised by lichens, moss and other organisms. The far travelled sand particles in the Sandlings deposits are mostly quartz and quartzite, polished and rounded by the process of tumbling along the river. When examined under the microscope the sand from antlion sites is seen as smoothly rounded, polished spherical grains (Fig. 1). Small angular fragments of flint and quartzite may also be present. Antlion sites Antlion pits are found where the sand is exposed, usually on a steeply sloping bank, such as exposed quarry faces. Here the sand is not stabilised by vegetation and is constantly renewed by erosion causing falls of fresh sand and by solitary bees and wasps making their burrows in the near vertical banks.

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Figure 1. Sand grains from an antlion pit. Pits are usually found close to the bottom of the bank or on ledges wide enough to accommodate them. They appear to favour places overhung by bushes or in some cases by a layer of glacial till which resists erosion more strongly than the sand stratum beneath. Antlion pits are also found on where a road cuts deeply into the ground and where vehicles such as large lorries or farm vehicles drive close to the side scouring out the vegetation and exposing sand. Following the 1987 gales when many large trees were blown down, the root plate assumed an almost vertical attitude with large lumps of sandy soil adhering to the broken roots. As this weathered on ledges formed by the roots it produced suitable, but transient sites for pit building (Plant, 1998). Site orientation Pits occur on sunny, south facing sites. Otherwise apparently suitable places which face north or are heavily shaded are not colonised. Sand temperatures The temperature of the sand at the level of the antlion, about 10 mm below the surface of sand, was recorded throughout the day (Kirby, 2002). On a sunny day on a bank facing due south, with the sand darkened by an admixture of organic matter and with a slope such that that it zenith the sun’s rays were almost normal to the surface, the temperature quickly rose above 50°C, possibly getting up to about 65°C. At other sites temperatures around 50°C were also recorded. Comparable measurements of temperature relations for a North American antlion gave similar results (Green, 1955). This temperature exceeds the thermal death temperature even for desert insects and the antlion retreats to deeper levels with cooler temperatures, returning as the day advances to refurbish its pits.(Because the high surface temperature hastens

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the rate at which the sand dries, it presumably more than compensates for discomfiture the antlion suffers causing it to move to avoid lethal temperatures.) The temperature characteristics of south facing banks may also be involved in site selection by the egg laying imago. Eggs may be laid on north facing banks or in dense shade and not survive but it seems more likely that the imago can sense the suitability of a patch of sand. Experiments in northern Germany showed that female imagos of Euroleon nostras preferred artificially heated boxes of sand to lay their eggs (Yasseri & Parzefall, 1996). Wet sand Dry sand has extremely low strength, but when wetted becomes much stronger as a bridge of water forms between the sand particles. Thus ‘small quantities of liquid dramatically change the properties of sand, leading to a large increase in the angle of repose’ (Hornbaker et al., 1997). The cohesive forces are such that antlion larvae cannot move or build its pits in wet sand and are immobilised until the sand dries. Gravimetric measurements were made to try to find out how time of year and exposure affected the rate at which sand dried but small local variations in slope and orientation and made it difficult to compare rates of drying at a site. The measurements, however, showed that on fine sunny days the sand in the antlion sites dried very quickly. Weather The balance between dry sand when the antlion is able to make it pits and trap prey and wet sand when it is unable to move or feed will depend on the weather. As little as 5 mm of rainfall will wet sand to a depth of 100 mm, restraining the antlion just as effectively as a heavy rainfall of e.g. 25 mm. Thus the number of days of rain will be a better measure of the wetting potential than rainfall amount. The rate at which sand dries depends mainly on temperature and wind speed. It increases rapidly with increases in temperature and therefore the high surface temperatures of the sand banks favoured by antlions makes for quick drying after rain allowing them to resume activity with minimum delay. Thus hours of sunshine, rather than air (screen) temperature provides the better measure of drying potential. Data for sunshine duration (hours) were obtained from the Meteorological Office web site (http://www.met-office.gov.uk/climate/uk/averages) which provides, for each month of the year, maps of average values for the period 1971–2000. These show that for every month from April to September (months when antlions are active) the Sandlings region was in the highest category (Table 1) except for April when the highest values were confined to the southwest. For May (Plate 5) and July the highest values for sunshine duration were confined, in Suffolk, to the eastern fringe only. Data for rainfall (days of rain = < 0·2 mm rain) were also obtained from Meteorological Office maps. For all months (April – September) the Sandlings region was among the driest parts of the country. Except April and September there was on average more than 20 days/month with < 0·2 mm of rain. For

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Table 1. Rainfall and Hours of sunshine for Cuxhaven* (N. Germany) and Lowestoft (1971–2000) Month April May June July August September

Rainfall (mm) Cuxhaven 45 55 60 87 90 76

Lowestoft 42 40 47 44 51 53

Hours of sunshine Cuxhaven 170 225 250 220 200 150

Lowestoft 151–203 201–243 191–238 204–250 197–242 144–173

*from Yasseri, A.M. & Parzefall, J. (1996) example, in May a narrow strip along the east side of Suffolk, Essex and Kent and a few areas along the south coast of Kent had the fewest days of rain in the Britain (Plate 6). These data show that, from the point of view of two important weather variables, the Sandlings region provides conditions which may be critical for the antlion larva to thrive. Other regions in the south of the country have equally sunny conditions, but the combination of sunny and rain free days is found most frequently along the eastern fringe of East Anglia from the Thames estuary the Lowestoft. Discussion Weather data show that the Sandlings region resembles northern Germany (Table 1) where antlions (Euroleon nostras) are indigenous and common. Both have long rain-free sunny periods which ensure that exposed sand dries quickly and remains dry ensuring that antlion can trap sufficient prey and complete its life cycle. Both regions also have extensive deposits of sand, which, where the vegetation is stripped, make suitable sand banks for antlion larvae (Fig. 8, Yasseri & Parzefall 1996). Before World War II, however, they were few reports from Suffolk of sightings of antlion imagos and none of pits or larvae (Mendel, 1996; Plant, 1998). There may have been undetected breeding antlions, perhaps even since the first report in 1781 (Plant, 1998) but the absence of further reports after the interest engendered by the 1931 sighting (Doughty, 1931) and the consequent report in a local newspaper indicate that few, if any, colonies existed. Mendel (1996) lists the existence of a long established, indigenous population as the fourth option in his list of possibilities of antlion status during this period. Migration of the imago, either ship-assisted or by ‘freak’ weather conditions or migration establishing temporary breeding populations received a higher rating. Not until 1994 were there the further reports of antlions. Soon after the first report substantial numbers of antlion larvae and imagos were found

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(Cottle, et al 1996). At this time there must have been large populations in Westleton parish, as from 1998 pits were found in all of the quarry sites as well as roadside verges and bulldozer scrapes (personal observations; Kirby, 2001). How may this change in antlion numbers be explained? As well as favourable weather, places with exposed sand are essential and these are only found where the covering vegetation is torn away, either by some natural phenomenon or by man’s activity. Before the war, a time of economic recession, there was probably little activity such as large scale quarrying or other activities which would expose sand banks With the onset of World War II there was much activity which provided suitable habitats for antlions. Defence installations along the coast such as anti-glider trenches were made on the heath, extending for considerable distances. At the same time large pits for sand and gravel extraction to were opened up to provide material for aircraft runways. Yet another factor were the gales of 1987 which blew down many trees whose up-ended root plates also provided, for a time suitable places for antlion pits. Westleton Common is an example of the change in topography brought about by quarrying. Small sand and gravel pits marked on the 1885, 1906, and 1927 Ordnance Survey maps indicated that some sand and gravel was extracted over a long period, but probably only in small amounts for local road making and building. On the 1885 map, some are already described as ‘old gravel pit’ and not described in subsequent editions indicating that they had already may had been overgrown. Major work on gravel extraction started in 1940s and the site was abandoned in 1960s (Westleton parish archives). Extraction to the north of the quarry was restricted by Mill Lane, leaving sand cliffs about 10 m high (Fig. 2). Elsewhere the terrain is more or less level or with gently sloping banks, now vegetated, mainly by gorse and heather or mosses and lichens. The steep northern slopes are now partly vegetated by gorse but there are many patches of raw, exposed sand which provide ideal habitats for antlions as well as solitary wasps and bees (Fig. 2). On occasion, more than 200 pits were found, but much of the cliff face is not accessible and the counts underestimated the actual numbers. Other quarries/sand pits in the district probably have similar histories and each now has a thriving population of antlions. As well as antlion habitats the exposed steep sand banks are much used by several species of solitary wasps and bees to make nesting tunnels. More research will provide details on when they were made and how quickly they have become vegetated. Thus the history of the antlion in Westleton parish and probably the Sandlings generally is linked with the history of man’s activity. In the first period possibly from 1781 or before until the 1940s, there were few places where the antlion could make pits and breed. During this time then there have been small, undiscovered colonies or possibly transient colonies deriving from inseminated female imagos blown across the North Sea. Wartime activities created many more suitable sites; those like anti-glider trenches providing routes across the heath along which the insect could spread. As the quarries fell out of use they have progressively become colonised and are major loci for antlions.

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Figure. 2. Westleton Common quarry. The LH (west) and the foreground of the photograph was excavated to a level base and has been colonised, mainly by heather. Extraction to the north was stopped by a windmill, now demolished, and the mill houses and by Mill Lane. The slope to the left of centre has been almost completely grown over by gorse. To the right of centre the slope has several areas of bare sand amongst the gorse and heather, where a large population of antlions make their pits. The slope to the far right hand (northeast) is much used by people on foot and on bicycles, steadily working sand down the slope, preventing growth of plants. (Camera position, grid reference TM4442668466).

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The root plate sites exposed by the gales of 1987 created new sites, many around the RSPB reserve, Minsmere where the concentration of experienced naturalists soon led to their discovery and identification (Plant, 1998). Thus although the first postwar reports were from Minsmere, this may be a secondary site rather than the primary focus, proposed by Plant who apparently did not inspect the quarries as none are listed in his paper. What of the future? A Biodiversity Action Plan was published for the antlion in June 2003 (http://www.suffolkcc.gov.uk/e-and-t/countryside/ biodiversity/action_plan/appendix1.html). In this the local key partners are charged to monitor the populations and establish population dynamics via the Sandlings group. They are also charged with ensuring that the known sites have management plans for the species. The partners have yet to report (May 2005) and no management recommendations have been published. By their very nature, suitable sites have only limited duration. Any remaining root plates thrown up by the gales are now washed free of sand and no longer provide sites for antlion pits. Road verges, damaged by traffic soon revert as the grass grows back. Quarries which harbour the greatest number of antlions are likely to last the longest, but ultimately the steep sand cliffs will erode, either by natural forces or by people running or cycling on them, to more gentle slopes which then become overgrown. The prospect of future commercial gravel extraction seems remote as much of the potentially suitable areas are now owned by conservation bodies. Forestry operations and land clearance often result in banks of cleared roots which are sometimes colonised and together with damage to roadside verges and similar steep banks seem to be the most likely ways in which new sites may be created. Whether these will provide sufficiently large numbers to maintain the population and how quickly and how far the adult females travel and colonise new sites is not known. Acknowledgements The studies of antlions in Westleton was assisted by a Suffolk Naturalists’ Society Bursary which contributed to the purchase of a temperature datalogger. References Cottle, R., Edwards, M. & Roberts, S. (1996). Euroleon nostras (Fourcroy,1785) (Neur.: Myrmeleontidae) confirmed as breeding in Britain. Entomologists Record 108: 299–300. Daerr, A. & Douady, S (1999). Two types of avalanche in granular media. Nature 399: 241–243. Doughty, C. G. (1931). Ant-lion in Suffolk. Trans. Suffolk Nat. Soc. 1: 228. Green, G. W. (1955). Temperature relations of ant-lion larvae (Neuroptera: Myrmeleontidae). Canadian Entomologist 37: 441–459. Hornbaker, D. J., Albert, R., Albert, I., Barabasi, A.-L. & Shiffer, P. (1997). What keeps sandcastles standing? Nature 187: 765. Kirby, E. J. M. (2001) Antlions in the Suffolk Sandlings. Trans. Suffolk Nat. Soc. 37: 57–65. Kirby, E. J. M. (2002). Some like it hot? White Admiral, 53: 29–31. Kirby, W. & Spence, W. (1826). Introduction to Entomology. Longman, Hurst, Rees, Orme and Brown, London.

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Mendel, H. (1996). Euroleon nostras (Fourcroy, 1785) a British Species and notes on Ant-lions (Neuroptera: Myrmeleontidae) in Britain. Entomologists Record. 108: 1–5. Plant, C. W. (1998). Investigations into the distribution, status and ecology of the Ant-lion Euroleon nostras (Geoffroy in Fourcroy, 1785) (Neuroptera: Myrmeleontidae) in England during 1997. Trans. Suffolk Nat. Soc. 34: 69– 79 Rose. J. (2000). Pre-glacial processes and events in Norfolk: a review sketch, in Dixon R. (Ed.) The geological Society of Norfolk 50th Anniversary Jubilee Volume. Pp. 66–75. Yasseri, A. M. & Parzefall, J. (1996). Life cycle and reproductive behaviour of the antlion Euroleon nostras (Geoffroy in Fourcroy, 1785) in northern Germany (Insecta: Neuroptera: Myrmeleontidae). Pure and Applied Research in Neuropterology. Proceedings of the Fifth International Symposium on Neuropterology. Cairo, Egypt, 1994. Canard, M., Aspöck, H. & Mansell, M. W. (Eds). Toulouse, France. 1996. Pp. 269–288. Dr E. J. M. Kirby The Studio Blythburgh Road Westleton Saxmundham Suffolk IP17 3AS

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