SOME LIKE IT HOT?

The world of the antlion larva is circumscribed by sand. It needs dry sand to make the pit in which it traps its prey. This depends on the property of dry sand grains which when piled up come to an ‘angle of repose’, an unstable condition where any disturbance by a small animal leads to an avalanche depositing the unfortunate at the bottom of a pit within the reach of the antlion jaws (Kirby 2001). On the other hand, wet sand is much stronger and more stable, a property used by children building sandcastles. In wet sand the antlion, which is at most about 10 mm long, cannot make its trap and is more or less stuck until the sand dries. If the sand is wet for long periods, the antlion is unable to trap its prey which will presumably affect its growth rate and survival.

The success of the antlion and its distribution therefore depends on sand and its properties in various environmental conditions. After rainfall some water may drain from the sand and the surface will dry out. The potential ‘trapping‘ time available to the antlion larva may depend on both the frequency of rainfall and how quickly the sand dries out after rainfall. The main factors which affect the rate of evaporation from the sand are temperature (incoming radiation) and windspeed. In particular, high temperatures speeds drying because evaporation is exponentially proportional to temperature. Sand temperature will also affect antlion activity.

To test this hypothesis further some facts were needed about rainfall frequency, rates at which sand dries after wetting and factors affecting drying, particularly temperature. In this context I have started to investigate the feasibility of measuring some of these factors. The investigation has been done at a site in the Dunwich Forest with the permission of The Forest District Manager, East Anglia Forest District.

Laboratory methods to estimate soil water content involve weighing sand samples before and after heating to complete dryness. Nowadays, microwave ovens are replacing the traditional hot air ovens so that suitable equipment was available in the kitchen (with the permission of the Kitchen Manager). To measure the rate of drying after rainfall, sand water content was measured by carefully scooping up sand to a depth of about 20 mm (antlion pit depth) placing it in a waterproof film canister, weighing it immediately on returning home, drying to constant weight and re-weighing. My electronic letter balance proved too insensitive to get a sufficiently precise weight, but I was fortunate enough to have the loan of a laboratory balance from Otley College for a short period. Over the season only two convenient episodes of rain occurred (Westleton has experienced a very dry summer) but it is clear that the disturbed stand around antlion pits dries very quickly in warm sunny, breezy conditions.

Systematic measurements of temperature in the laboratory usually employ a small probe, a thermocouple or a platinum thermometer, and electronic recording of the data (a temperature datalogger). When I last used these professionally, dataloggers were bulky and expensive, but a search of the Web showed that the cost has tumbled (thanks, I believe, to the food health and safety regulations). It is now possible to get a datalogger, temperature probe and software for under £300 and with the help of a SNS bursary I bought a Tinytag Ultra datalogger (Omni Instruments). Sand can become almost too hot to walk on in bright sunlight and so a probe with a wide temperature range (–30 to +50° C) was selected. The datalogger was not delivered until July, too late to make measurements at the height of summer but those observations made so far have produced some interesting and surprising data. Most of the temperature recordings have been made in dry sand in cloudless conditions and have revealed that the temperature below the sand surface can go above 50 °C. For example 15 July 2002 was a clear, completely cloudless day with a maximum screen temperature at Westleton of 24°C.
The probe was buried among several antlion pits at about 5 mm below the sand surface (a depth which I judged was about that at which the antlion larvae waited for their prey) and the logger was concealed amongst bracken and brambles. There was an initial drop in temperature as the probe was put in place and recording proper started at about 09.10 (Fig.). At 09.22 the shadow of a pine tree cleared the probe and the temperature then rose quickly until at 11.19 the temperature reached 52°C, the maximum, which the datalogger could record. It remained above this value until 14.04 when the probe was again in shadow. At 15.31 the probe emerged from the shadow for a short time after which the temperature fell until the equipment was retrieved at 16.20. An analysis of the trend indicated that the temperature may have approached 60°C. Checking through the possible causes of error I was convinced that this was a true record. The depth of the probe (5 mm) was checked before and after recording.

The probe cable was buried for some distance from the probe to obviate heat conduction from the part exposed to the sun. Factors which were conducive to high temperature were, first, the site was almost directly south facing, secondly the bank where antlions dwelt was about 30° to the horizontal, so that the sun was almost normal to the sand surface during the middle part of the day and finally the sand contained organic matter giving it a dark colour which absorbed heat more readily.

Clearly this needs confirmation with further measurements but it seems that antlion larvae may have a hot time. Maximum temperatures at which insects can survive are round about 40 – 45° C, even for those insects adapted to desert conditions (Wigglesworth). This raises questions about antlion behaviour; often the head and jaws of a larva can be seen at the bottom of the pit ready to grab its prey.

Does the larva orientate itself to the shady side of the pit or retreat deeper beneath the sand surface as the temperature rises?

(This an interim report. Antlion larvae will probably cease activity by the end of September and another season is required to answer even a few of these questions.

The SNS bursary asks for a paper within 12 months but a full account for Suffolk Natural History will not be possible until 2004. Meanwhile progress will be reported in the Newsletter)

References

Kirby, E. J. M. (2001) Antlions in the Suffolk Sandlings. Trans. Suffolk Nat. Soc. 37 - 65

Wigglesworth V. B.(1972) The principles of insect physiology. 7th ed.. London, Chapman and Hall

E. J. M. Kirby, The Studio, Blythburgh Road, Westleton, Saxmundham, IP17 3AS

Don't forget you can download the whole of each White Admiral as a .zip file.
(Typically under 700 kb)   ~ See the top menu.