Oecologia (Berl) (1981) 50:250-255 Oecologia �9 Springer-Verlag 1981

Field Analyses of Insect Heat Budgets: Reflectance, Size and Heating Rates

P.G. Willmer and D.M. Unwin

Department of Zoology, Downing St., Cambridge

Summary. This paper outlines simple techniques for determining rates of heat gain and loss in relat ion to the weight and reflec- tance of insects caught in their natural habitats. In part icular the construct ion of a new ' ref lec tometer ' is described. The results thus obtained permit estimates of the relative importance of size and colour in determining rates of heat exchange and temper- ature excesses, so allowing better predictions of heat budgets for a given species.

Introduction

With a few exceptions, insects are classically regarded as ecto- thermic animals, whose body temperature (and hence activity) is determined largely by the energy received. The thermal econ- omy of an individual should thus be related to its behaviour (the time spent in insolated zones, and or ientat ion to incoming radiat ion and to air currents), to its body form, and to ambient conditions. However, l i terature analysing the actual temperatures of insects in their na tura l habi ta ts is sparse, so that discussions are inevitably speculative and often involve extrapolat ions f rom experiments with model insects (Parry 1951) or with a few insects f rom laboratory cultures under simulated condit ions (Digby 1955). Al though the present documenta t ion of insect heat bud- gets is so poor, there has been much recent work on their thermal ecology and energy balance (cf. Edney 1971 ; Casey 1976; Dreisig 1980; and review by May 1979), and though in some cases this has included spot measurements of body temperatures many other studies have relied on assumptions about rates of heat exchange, and the effects of size, shape or colour thereon, which are poorly substantiated. There is thus a recognized need for a realistic considerat ion of the effects of insect size (weight, and /or linear dimensions) and of colour (determined by emissi- vity or reflectance of the surfaces) on the rates of heat ing and cooling of insects, and on the temperature excesses which they may at ta in in sunny conditions. This paper analyses such factors for a range of c o m m o n insects. It describes a device, suitable for field use, for rapid and accurate measurements of the reflec- tance properties of insect surfaces, which in combina t ion with an equally adaptable microbalance can provide informat ion about the relative importance of reflectance and of size in deter- mining the thermal properties of different species, thus giving data upon which assumptions about temperature budgets may more readily be based.

Methods a) Field Techniques

Initial recordings of insect weights and refleetances were made from fresh specimens, caught in their natural habitats while feeding on

Offprint requests to: P.G. Willmer

flowers or resting in vegetation, in an area of sandy pine forest in the Breckland area of Norfolk. Specimens which were damaged in the net, or which were too small for accurate weighing with the avail- able equipment ( < 2 mg) were discarded. All others were killed rapidly with ethyl acetate, and weighed using a specially constructed field microbalance (Unwin 1980). Their thoracic breadth was measured us- ing a calibrated 7 x magnifying eyepiece, and their thoracic reflectance determined (see below). (The dorsal surface of the thorax was used for both linear measurements and for reflectance, as it overlies the flight muscles and is thus the most crucial zone for thermal balance and activity.) All the measurements required could be completed within 5 rain of the death of the insect. Many of the specimens were then stored and allowed to dry, for subsequent more elaborate laboratory tests of reflective properties.

Rates of heating and cooling, and temperature excesses, were re- corded in the field in natural sunlight with a large number of the freshly killed insects. These tests were all conducted during a sequence of hot sunny days, at times when incoming radiation was high and constant, and wind-speed very low. Extremely fine copper-constantan thermocouples (0.081 mm wires) were used to minimise conduction to and from the specimen, and these were connected to a portable multi-channel indicator (Unwin 1980), each run involving a set of three matched thermocouples. These were glued with a fine coating of gum tragacanth to the insects' thoracic terga, or were inserted through the cuticle to lie just within the underlying flight muscles; the two techniques gave indistinguishable results. Insects were always tested in pairs, with the third sensor recording air temperature. A standard run involved successive 2-5 min periods in full sun and in shade (provided by white reflective sheeting), the transitions being repeated several times. Radiation was recorded with a thermopile solar- imeter (Unwin 1980), and air temperature and humidity were periodi- cally checked with a wet and dry bulb psychrometer (Unwin 1980).

b) Laboratory Tests and Equipment

The surface reflectance of insects was determined using a newly-de- signed device (" reflectometer"), incorporating a source which attempts to mimic the effects of sunlight and a sensor with matched responses sited to take account of both direct reflection and the diffuse reflections from a microsculptured surface.

The construction of the reflectometer is shown in Fig. 1. The insect is held against a small (2.8 mm) hole in a brass plate which is painted matt black. The light source is a 2.5 volt 'lens-end' torch bulb, supplied with 3 volts from a pair of torch batteries. "Over-running" the light source in this way gives a 'whiter ' output than that of a normal tungsten lamp, and the light is thus a better approximation to sunlight. Light is focussed onto the centre of the hole by a lens of about 20 mm focal length. Radiation reflected from the insect then illumi- nates the inner surface of a dome, made from half of a table-tennis ball and painted matt white inside. The surface brightness of the inside of the dome is measured by a pair of ORP12 cadmium sulphide photo- conductive cells, connected to a meter circuit as in Fig. 1 B. To reduce any effects of stray light from the source reaching the photo-conductive cells directly, the lens system is mounted in a thin-walled metal tube, matt black internally, fixed through the apex of the dome; and the outside of the dome is also blackened to absorb and eliminate direct

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input from the light source. The whole array may then be mounted in a light tight box, with a small access-port for the specimens to be tested.

The device may be calibrated with standard reflective surfaces (Kodak), adjusting the 1 K variable resistance (a 10-turn helical poten- tiometer) to give an appropriate meter reading: the response is effec- tively linear. For general use, the spectral response of the instrument with components as described is found to give a reasonable match to that of the sun's radiation. But the overall response is skewed somewhat towards the red end of the spectrum, and for applications where this is important it could be corrected by the use of a filter such as the Wratten 38. When used with small animals such as insects, the exact spectral composition of the source is not likely to be crucial (Digby 1955), and this precaution was omitted. However, some labora- tory measurements of reflectance to different wavelengths were made with selected insects, and for such purposes a range of filters can be inserted as required between the bulb and the dome assembly (Fig. 1 A). In some cases, this necessitated the use of a more powerful fibre-optic light source and the substitution of a 50 gA meter for the 1 mA meter shown in Fig. 1 B to improve sensitivity at low light levels.

Once calibrated, the instrument was generally used to give a single broad-band reflectance reading, by holding the thoracic tergum of an insect against the hole in the sensor hemisphere and rotating the animal gently to allow for any directional effects on reflectance. A mean figure for percent reflectance could then be taken, though in no case was the change in value of this parameter greater than 1.5% during small movements of the thorax, except when the surface was obviously striped; for such animals, the reflectance of the dominant bands was determined.

For all specimens where a broad band reflectance reading was obtained in the field immediately after death, the value subsequently found in laboratory tests with the dried out animal was within 1% of the original figure. Consideration of colour effects could therefore be reasonably continued using some of these dried specimens and the spectral properties of their thoracic surfaces were analysed using a series of filters (Ilford) of known properties, covering the ultra-violet to infra-red range. In every case the reflectometer calibration was first checked with the standard surfaces.

Results

A. Reflectance and Colour

As an initial test of the efficacy of the reflectometer, a wide range of thoracic reflectances of freshly-killed insects were re- corded and compared with their subjectively determined ' co lour ' . This information is given in Table 1, arranged as a series of increasing reflectance. Darker insects had low reflec- tances (absorbing 90-98 % of incoming radiation), while the paler

Table 1. Reflectance and colour characteristics of a range of insects

Insect species Reflectance Dominant thoracic % colour

Dorcus parallelopipedus 2 Black matt Chelonus sp. 2 Black matt Eristalis pertinax 2-3 Dark brown Chloromyia formosa 2 3 Blue-green metallic Delia sp. 2-3 Black matt Chrysogaster hirtella 2 3 Black Alophora hemiptera 3 Dark Brown-black Poecilobothrus nobilitatus ~ 3 Blue-green metallic Hilara sp. 3 Black Arge gracilicornis 3 Dark blue metallic Dolichopus wahlbergi 3-4 Bronze metallic Morellia aenescens 3-4 Dark grey-green Delia sp. 3-4 Dark grey Volucella bombylans 4 Dark brown, paler hairs Poecilobothrus nobilitatus ~ 4 Red-purple metallic Melanostoma scalare 4 Black shiny Cerceris arenaria 4 Black Dolichopus festivus 4 Green metallic Calliphora vicina 4 Blue-green metallic Conops flavipes 4 Black shiny Lonchaea sp. 4 Black shiny Tenthredo acerrima 4 5 Black Lucilia caesar 4-5 Green metallic Paravespula vulgaris 4-5 Black Dasyphora cyanella 4-5 Green metallic Melangyna cincta 4 5 Dark bronze Necrophorus sp. 4 8 Black & red Psen equestris 5 Black shiny Chrysotoxum cautum 5-6 Brown Bombus terrestris 5-6 Brown, pale hairs Helophilus pendulus 5-7 Grey & brown Bombus pascuorum 5-8 Brown, pale hairs Hylemya strenua 6 Grey Syrphus ribesii 6 7 Bronze Crabro cribrarius 7-8 Black shiny Tachina fera 7-8 Grey-brown Psallis varians 7-9 Brown Episyrphus balteatus 7-10 Bronze Lygocoris lucorum 8 Green-brown Meliscaeva cinctella 8-10 Yellow-bronze Apis mellifera 8 10 Orange-brown Oxycera trilineata 9-10 Pale green Rhagio lineola 9-10 Pale brown Macrotylus pakyulli 9-11 Green Cantharis livida 10 Yellow-brown Xyphosia sp. 11 Pale orange-brown Ophion sp. 14 Orange Scathophaga stercoraria 14 Yellow-brown Chrysopa sp. 15 Turquoise Opomyza sp. 16 Yellow-orange Tricholauxania praeusta 17-19 Pale orange Rhagonycha fulva 20-23 Orange

forms, usually in the light brown/orange/yellow colour range, reflected a higher percentage of the light and absorbed only 70-90%. The apparent anomalies in this scheme were the metallic insects, which though appearing paler and shiny nevertheless had very low reflectances. These results match those of Digby (1955), obtained using different and more time-consuming tech- niques, and indicate that the reflectometer device does give mean- ingful results. They also suggest that since even very pale insects still absorb at least 75% of received broad-band radiation, with the majority absorbing or transmitting more than 90%, colour

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Fig. 3A-D. Thoracic temperatures versus time for insects in sunlight (radiation strength 840 900 Watts/m2).) and in shade (2 min intervals; hatched bars show shaded periods)�9 The solid lines show air tempera- tures, and in each case solid symbols represent the less reflective insect of a pair, A (o) Xyphosia sp., r = l l % , (y) Hilara sp., r=3%. B (o) Rhagonychafulva, r=22%, (y) Cantharis livida, r=10%. C (o) Crabro cribrarius, r=8%, (y) Cerceris arenaria, r=4.5%

in itself is unlikely to be a crucial factor in thermal balance for most temperate insects. However, some desert beetles are white in colour, or at least have white elytra, and a reflectance of 74% has been recorded for such forms (Rficker 1933); the calculations of Edney (1971) also indicate that very high values may occur for white insects.

To further test the characteristics of the reflectometer, a range of insects were subjected to lights of varying spectral properties. The results of these analyses are shown in Fig. 2. For each of the ' no rma l ' insects, whose colour arose from pigmentation of the cuticle or its pubescence, the reflectometer gave entirely predictable results, with peak reflectance corresponding to the observed colour of the species (and usually with a high reflec- tance in the ultra-violet range also). But this clearly did not apply for the metallic forms, where the reflectance at different wavelengths showed no consistent relation with apparent colour ; indeed there was often a trough of reflectance at the wavelength corresponding to the colour seen by the human eye. This effect presumably arises from the different methods of colour produc- tion in metallic insects (by scattering, diffraction or interference effects: see Fox and Vevers 1960; Chapman 1969).

B. Temperature Changes of Insolated Insects

As examples of the temperature changes recorded in test insects, three sets of paired runs are shown in Fig. 3, for small (A),

medium (B) and large (C) insects. In each case, insects of similar size and shape but different reflectauces were paired. Several effects are apparent. The temperature excess (T.ex) clearly de- pends principally upon size, but is also progressively more depen- dent on reflectance as the size increases. Similarly the half-times (T1/a) of temperature change increase for larger insects, (there were no significant differences between half-times for heating and cooling), and values again are modified by reflectance in the upper size ranges.

Data of this kind from all the tests performed is summarised in Table 2, where values of T.ex and T1/2 are shown together with reflectance and size, for insects now arranged systematically. Correlations of temperature parameters with weights and reflec- tances are shown in Fig. 4. Both temperature excess and T1/2 were very highly correlated with insect weight (4A and 4B), and correlations with the recorded linear dimension (thoracic breadth) were almost equally good (cf Table 2). On the other hand the correlations of temperature changes with reflectance were poor, and Fig. 4C and D give little indication of any pre- dictable effect of colour on thermal changes. These findings are again comparable with those of Digby (1955), who suggested that colour probably contributed ' no t more than some 25%' to the temperature control of insects. At the most, Fig. 4 might indicate that the more reflective insects do not show high values of T.ex or T1/2, but even this apparent effect arises partly be- cause all the very reflective insects tested were fairly small. In

253

Table 2. Size, reflectance and thermal characteristics of freshly-killed insects. The temperature excess in full sunlight, and the half-time to r . ex/2

achieve this, are given together with the rate of heating K calculated as ~=c/z/z

* = pubescent insects ; ** =metal l ic insects

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Diptera Chloromyia formosa ** 9.7 2.4 2 3 1.4 6 0.117 Rhagio lineola 9.0 1.8 9-10 1.8 14 0.113 Poecilobothrus nobilitatus ** 3.6 1.1 3 0.5 - - E pisyrphus balteatus 21.0 2.4 7-10 2.6 18 0.130 Syrphus ribesii 34,0 2.7 6-7 4.0 19 0.105 Melanos toma scalare 9.5 1.7 4 1.6 7 0.114 Chrysogaster hirtella 42.0 3.0 2-3 4.0 14 0.143 Eristalis pert inax* 105.0 4.6 2 3 10.0 64 0.078 Helophilus pendulus* 82.0 3.2 5-7 7.2 44 0.081 Chrysotoxum cautum 110.0 4.8 5-6 9.0 52 0.086 Volucella bombylans * 118.0 5.0 4 12.1 60 0.100 Dasyphora cyanella** 39.0 3.2 4-5 3.5 20 0.087 Lucilia caesar** 18.0 2.5 4 5 2.2 15 0.073 Calliphora vicina** 75.5 4.2 4 6.2 53 0.058 Tachina fera 48.0 3.2 7-8 5.4 26 0.104 Atophora hemiptera 47.0 3.0 3 6.0 20 0.150 Morellia aenescens 20.0 1.9 3M, 3,7 9 0.206 Hylemya strenua 6.0 1.4 6 1.4 - - Delia sp. 16.5 2.0 2-3 3.2 10 0.160 Delia sp. 14.5 1.8 3 4 3.0 10 0.150 Scathophaga stercoraria 15.0 1.9 14 2.7 18 0.075 Lonchaea sp. 3.5 1.2 4 0.8 - - Opomyza sp. 8.5 1.7 16 1.0 12 0.042 Tricholauxania praeusta 7.0 1.5 17-19 0.7 - -

Hymenoptera Tenthredo acerrima 52.0 2.5 4-5 8.2 18 0.228 Chelonus sp. 5.0 1.2 2 1.8 - - Ophion sp. 17.5 2.2 14 1.4 14 0.050 Crabro cribrarius 90.0 3.4 7-8 7.9 40 0.098 Cerceris arenaria 102.0 3.6 4 10.3 32 0.161 Paravespula vulgaris 117.0 3.9 4-5 11.5 44 0.131 Apis mellifera 93.0 4.0 8-10 7.5 46 0.104 Bombus pascuorum* 155.0 5.9 5-8 12.0 85 0.071 Bombus terrestris* 190.0 6.8 5 6 14.0 115 0.061

Coleoptera Rhagonycha fulva 27.0 2.7 20 23 3.2 27 0.065 Cantharis livida 26.0 2.5 10 4.4 17 0.126 Necrophorus sp. 130.0 5.9 4 8 11.6 58 0.100 Dorcus parallelopipedus 220.0 10.0 2 16.5 115 0.072

Herniptera

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Neuroptera Chrysopa sp. 9.0 2.0 15 0.7 11 0.032

fact, the p a r a m e t e r s o f size a n d ref lec tance are n o t t hemse lves

i n d e p e n d e n t var iab les ; the co r re la t ion coeff ic ient for the s a m p l e

u sed was 0.29 ( n = 4 1 ; p < 0 . 1 0 ) . T o sepa ra t e the effects o f size

and co lou r the re fo re p re sen t s s o m e diff icult ies, as m u l t i v a r i a t e r eg res s ion ana lys i s is n o t s t r ic t ly appl icable . Neve r the l e s s , a

r o u g h guide to the re la t ive c o n t r i b u t i o n s o f the two p a r a m e t e r s cou ld be ach ieved by s u c h an ana lys i s ; th is ind ica ted t ha t co lou r c o n t r i b u t e d on ly a b o u t 10% to the d e t e r m i n a t i o n o f T . e x in

the s amp le o f insects used , t h o u g h the f igure rose to 22% if

on ly the larger ( > 80 mg) insects were cons idered . A fu r t he r m e t h o d by w h i c h the c o n t r i b u t i o n o f ref lec tance

to t h e r m a l ba l ance can be e x a m i n e d is to use the va lues o f K ca lcu la ted in Tab le 2 as the init ial ra te o f hea t ga in or loss

(T.ex/2/T1/2); th is p a r a m e t e r ha s a relat ively res t r ic ted r ange o f values , and is i n d e p e n d e n t o f insect we igh t (r = 0.18). Va lues o f K are p lo t ted aga ins t ref lec tance in Fig. 5, and the co r re la t ion

254

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is reasonably good; more reflective forms clearly gain and lose heat less rapidly. Figure 5 also shows up the anomalies more readily, in that both pubescent and metallic forms appeared to change their temperatures more slowly than predicted. For pubescent insects this can be attributed to the insulative effects of the hairs and of trapped still air (Church 1960). But the metallic forms appear to be genuine anomalies, and the use of simple reflectance recordings for such insects may be inappro- priate; their thermal characteristics are in every case consistent with higher reflectances than those recorded.

Discussion

This paper describes apparatus and techniques suitable for deter- mining size, reflectance, and the heating effects of sunshine for insects in natural conditions. Both the reflectometer and the simple microbalance used have proved to be of practical value, giving results when used in the field comparable to those of earlier laboratory studies, and also showing up some of the anomalies which occur. The present study is limited, though, in that it ignores some of the factors which may be important determinants of thermal ecology (see Willmer 1981). Of the envi- ronmental parameters, windspeed is probably the most crucial variable which has been excluded from consideration here, and this factor should be particularly important in altering the tem- perature excess for the smaller insects whose size lies in the range where natural convection is supplanted by forced convec-

255

tion currents (Digby 1955). However, such small insects will spend much of the time contained within the boundary layer of still air around the surfaces on which they rest, so that macroc- limatic windspeed may have little practical effect on the cooling of many inactive insects. A second important effect which has not been considered, since these tests involved recently dead insects, is that of activity itself: 'metabolic heat ' . Any active insect produces heat internally in relation to the cube of its linear dimensions, whereas radiative heat gain varies with L2; hence metabolic heat is relatively less important for a small insect. Larger insects therefore generally heat up more during flight, whereas smaller forms show little change in body tempera- ture when they take off or may even cool down slightly if evapo- rative and convective losses exceed the heat generated from the flight muscles (Digby 1955).

Thus, although the range of factors controlling insect body temperatures is large, and their interrelations are complex, a knowledge of the two parameters of size and reflectance, in conjunction with good microclimatic data, can be of considerable predictive value in considering insect temperatures. The good correlations of weight and linear dimension with thermal gains (Fig. 4) accord with earlier studies, while the lesser but significant effects of colour are more clearcut and predictable from the present data, (controversies in the earlier literature on this issue are reviewed by Edney 1971). Based on such data, simple predic- tions may be made about the thermal balance of insects. Large insects will both attain, and maintain, high temperature excesses fairly easily. If they are also dark in colour (low reflectance), they will reach a high temperature more quickly, so that a large dark insect will be at an advantage at dawn and dusk, when it may more readily reach a temperature conducive to activity. At greater radiation levels in the middle of the day smaller insects will also be capable of activity, though they may be restricted to sunlit patches while the larger forms extend their range into the shade. At very high temperatures smaller insects may be at a clear advantage, since the larger ones, with tempera- ture excesses of up to 15-30 ~ C (present study, and see Heinrich 1975), may be in danger of overheating during prolonged expo- sure to direct insolation; whereas small insects, having lower temperature excesses anyway, can also take advantage of very local patches of shade more readily and will cool down more quickly therein. When overheating is a real danger, as may be the case in tropical or desert situations, insects may do better if highly reflective, and this should be especially valuable for insects of moderate or large size which are active during the day. Hence large pale or shiny insects are relatively common in the tropics, but rare in the temperate fauna, whereas large desert insects are commonly either black (if crepuscular) or shiny white (Hamilton 1975).

Clearly these predictions represent gross generalizations, and there are obvious exceptions; in some cases these can be traced to known physiological adaptations such as antifreezes (Salt 1969) or endothermie regulatory mechanisms (Heath et al. 1971 ; Heinrich 1974; May 1979; Willmer 1981). But there are some available studies, particularly where these consider related insects of similar shape, which accord well with the predictions outlined here; those of Heinrich (1976), Schlising (1970) and Willmer and Corbet (1981), all on bees or wasps, are suitable examples. Studies of all insect types active at particular sites through a day (Willmer in prep) also support these suggestions. Further- more, the few available studies of different eolour forms of a single species (eg Watt 1968; Cena and Clark 1972) provide good evidence for the thermal effects of colour in isolation from other factors, as do the known cases of colour change within

an individual in response to temperature (Key and Day 1954; O'Farrell 1963). The use of the described field techniques to measure these crucial parameters should therefore prove useful in further studies of insect activity cycles in the field, and of the hygrothermal considerations which underlie then.

Acknowledgements. We would like to thank F.S. Gilbert for helpful discussions about this manuscript, and C.P. Ellington and Dr. W.B. Amos for loans of equipment. PGW is also grateful to the Childerhouse family for permission to work on their land and use their facilities, and to New Hall, Cambridge for continued financial support.

References

Casey TM (1976) Activity patterns, body temperature and thermal ecology in two desert caterpillars. Ecology 57:485-497

Cena K, Clark JA (1972) Effect of solar radiation on temperatures of working honey bees. Nature New Biol 236:222-223

Chapman RF (1969) The Insects - Structure and Function. Ch. 7, Colour. English Universities Press, London, p 107 124

Church NS (1960) Heat loss and the body temperatures of flying insects. II. Heat conduction within the body and its loss by radia- tion and convection. J Exp Biol 37:186 212

Digby PSB (1955) Factors affecting the temperature excess of insects in sunshine. J Exp Biol 32:279-298

Dreisig H (1980) Daily activity, thermoregulation and water loss in the tiger beetle, Cicindella hybrida. Oecologia (Berl.) 44:376 389

Edney EB (1971) The body temperature of tenebrionid beetles in the Namib desert of Southern Africa. J exp Biol 55:253-272

Fox HM, Vevers G (1960) The Nature of Animal Colours. Sidgwick and Jackson, London

Hamilton WJ (1975) Coloration and its thermal consequences for diur- nal desert insects. In: NF Hadley (ed), Environmental Physiology of Desert Animals. Stroudsberg Pa. p 67-89

Heath JE, Hanegan JL, Wilkin PJ, Heath MS (1971) Thermoregulation by heat production and behaviour in insects. J Physiol 63 : 267-270

Heinrich B (1974) Thermoregulation in insects. Science 185:747-756 Heinrich B (1975) Thermoregulation in bumblebees. II. Energetics

of warm-up and free flight. J Comp Physiol 96:155 166 Heinrich B (i976) Resource partitioning among some eusocial insects:

bumblebees. Ecology 57:874 889 Key KHL, Day MF (1954) A temperature controlled physiological

colour response in the grasshopper Kosciuscola tristis Sj6st. Aust J Zool 2:309-339

May ML (1979) Insect thermoregulation. Ann Rev Entomol 24 : 313 349

O'Farrell AF (1963) Temperature-controlled physiological colour change in some Australian damselflies. Aust J Sci 25:437438

Parry DA (1951) Factors determining the temperature of terrestrial arthropods in sunlight. J exp Biol 28:445-462

Rficker F (1933) Die Farben der Insekten und ihre Bedeutung ftir den W/irmehaushalt. Pfl/ig Arch ges Physiol 231:729-741

Salt RW (1969) The survival of insects at low temperatures. Symp Soc exp Biol 23:331-350

Schlising RA (1970) Sequence and timing of bee foraging in flowers of Ipomoea and Aniseia (Convolvulaceae). Ecology 51:1061-1067

Unwin DM (1980) Microclimate measurement for ecologists. Academ- ic Press, London

Watt WB (1968) Adaptive significance of pigment polymorphisms in Colias butterflies. I. Variation of melanin pigment in relation to thermoregulation. Evolution 22:437~458

Willmer PG (1981) Microclimate and the environmental physilogy of insects. Advances in Insect Physiology 16 (In Press)

Willmer PG, Corbet SA. Temporal and microclimatic partitioning of the floral resources of Justicia aurea amongst a concourse of pollen vectors and nectar robbers. Oecologia, In Press

Received February 6, 1981