PHYSIOLOGICALANDCHEMICALECOLOGY Effects of Insolation and Body Orientation on Internal Thoracic Tem.perature of Nym.phal packardii (: )

DEREK J. LACTIN ANDDAN L. JOHNSON

Land Rl?sourcl? Sciences Section, Agriculture and Agri-Food Canada Research Centre, P.O. Box .3000, Lethbridge, All TlJ 4131 Canada

Environ. Entomol. 25(2): 423-429 (1996) ABSTRACT The effect of insolation un body temperature of nymphal Packard grasshoppers, Melmwpills packardii Scudder, was measured in the field. Live nymphs were each restrained in a Sl?ril?Sof orientations to the sun, and insolation was adjusted using a shade cloth. Internal thoracic temperature was allowed to stabilize and was compared with that of a reference Downloaded from nymph restrained in a sunshade. Equilibrium body temperatures of insolated nymphs exceed- l'd that of the reference nymph by an amount (aT/,) which increased with energy intercepted (ENERGY) and size (SIZE) by a relationship of the form aTb = a . ENERGY· SIZEh. \Vhen insect size was expressed as mass (grams), the estimates of a and b were L8.76 and -0.312, respectively (r2 = 0.6198); when insect size wa~ expressed as length (millimeters), a and b were 826.66 and -1.133, respectively (r2 = 0.6463) These results allow estimation of

l'quilibrium body temperature elevation of M. packardii nymphs from solar radiation, the http://ee.oxfordjournals.org/ zenith angle of the sun, insect size, and the orientation of the insect to the sun.

KEY WORDS grasshopper, body temperature, thermoregulation, size effects, biophysics

MANYTEHRESTHIALECTOTHEHMScan control their insect size on the relationship between energy in- body temperature (Tt,) by exploiting environmental terception and equilibrium T}, elevation. heterogelll'ity (Huey and Slatkin 1976, May 1979). Usually, T" exceeds air temperature (T ) and tends by guest on August 22, 2016 a Materials and Methods to be Ill'ar the optimum for 1 or more ecologically important temperature-dependent physiological Live M. packardii nymphs were restrained in processes (Heinrich 1977, Huey and Kingsolver known orientations to the sun, and equilihrium T" 1989). elevation was compared with the amonnt of solar In diurnal ectotherms of mass <1 g, solar radi- energy intercepted. Insect orientation was manip- ation (insolation) is the most important source of ulated using an experimental device which con- heat gain (Stevenson 1985). In , maximum sisted of 2 wooden boards (60 by 15 by 1.5 cm), equilibrium Tt, elevation (i.e., TIJ - Til) increases mounted one atop the other with the long axes lillearly with radiation intensity, and with the mass connected by hinges. Sixpivots were mounted per- of the insect (Pepper and Hastings 1952, Digby pendicular to the upper board, and one size 14 1955, Henson 1958, StoweI' and Griffiths 1966, cork stopper ("='3.2cm maximum diameter) was Edney 1971, Cena and Clark 1972). Many insects mounted on each with the pivot inserted into a have taxic or kinetic responses that affect the bored hole in the center of the base of the cork. amonnt of radiation intercepted (May 1979, Heath The device was placed in a mowed grass area, and Wilkin 1970); they may thereby exercise con- leveled "='10cm above the ground, and aligned siderable control over Til (May 1982, Kemp 1986, with its long axis perpendicular to the sun vector Carmthers et al. 1992). Therefore, in cctothermic and the hinged side away from the sun. The upper insects, energy interception is determined by in- board was adjusted to match the elevation angle of solation intensity and the area presented to the in- the sun. The orientations of the long axis and the solation source. vVewere interested in quantifYing upper board were adjusted about every 15 min these effects separately, to allow simulation of the during the experiments. ohs{>fVedresponse from the component effects. Each experimental grasshopper nymph was re- This study was conducted on Reid-collected strained by adhering a piece of cork (2 by 2 by 3 nymphal Packard grasshoppers, Melanoplus pack- mm) to its thoracic sternites using upholstery ad- anlii Scndder. The purposes were to quantify the hesive (EC2218, 3M, London, ON). The hind legs effects of insolatioll intensity on equiHbrium Tt) el- of each insect were immobiHzed by gluing them to evation; orientation to the sun on T" elevation; and the abdomen with the same adhesive. The cork

0046-225X/96/042.'3-0429$02.00/0 (c) 1996 Entomological Society of America 424 ENVIRONMENTAL ENTOMOLOGY Vol. 25, no. 2 piece was then attached by an insect pin to 1 of peri mental site that was sheltered from the wind, the large stoppers on the experimental device. One and conducted the experiment only on calm days. nymph was mounted on each stopper. The insects Insolation was measured using a pyranometer were =5 cm above the upper board. (LI-200SZ, LI-COR, Lincoln, NE). Insolation in- Orientation of the insects to the sun was adjust- tensity was manipulatedusillg 42-cm-2 mesh black ed by rotating the large corks on the pivots, align- nylon screen mounted on a wooden frame (l by 1 ing the insects with calibration marks on the ex- m) placed perpendicular to the solar vector, be- perimental device. Orientation was varied tween the nymphs and the sun, 1 m from the independently around 1 of 3 mutually perpendic- nymphs. The shade treatments were 0, 1, 2, or 4 ular axes, which were defined relative to the grass- layers of cloth. hopper. The x-axispassed from the head to the tip In each trial, each of 6 grasshoppers was pre- of the abdomen, the y-axis from side to side, and sented in from 2 to 7 orientations (mode = 4) un- the z-axis from dorsum to venter. Rotations about der each of the 4 shade treatments. Insolation, Til the x-, y-, and z-axes were termed roll, pitch, and and Tb were measured every 0.5 s; these measure- yaw, respectively. ments were averaged and recorded evelY 12 s us- Insect orientation was varied with respect to the ing a data logger (model 21X, Campbell, Edmon- solar vector (i.e., a line joining the insect and the ton, AB). Measurements were taken for at least 3 sun), in increments of 30°. Yaw angles were from min, or until h - 1:' appeared to have stabilized,

0° (x-axisparallel to the solar vector, with the frons as judged by comparing readouts on the data log- Downloaded from toward the sun) to 90° (x-axisperpendicular to the ger. The stabilized value was used as the estimate solar vector, with the left side toward the sun). Roll of equilibrium h elevation, and was compared angles were from 0° (x-axis perpendicular to the with the contemporaneous insolation measure. Be- solar vector, with the dorsum toward the sun) to fore analysis, time-temperature trends were plot- 90° (x-axis perpendicular to the solar vector, left ted, and Tb elevation had failed to stabilize in only side toward the sun). pitch angles were from 0° (x- 1 nm in 1 trial; this case was deleted. http://ee.oxfordjournals.org/ axis parallel to the solar vector, with the frons to- On completion of each trial, 4 physical mea- ward the sun), through 90° (x-axisperpendicular to surements were taken from each insect: length the solar vector, with the dorsum toward the sun) from the frons to the tip of the abdomen; depth to 180°. from the dorsum to the venter; dorsal breadth Grasshopper Tb was measured using 0.127-mm- across the mesonotum; and ventral breadth across diameter copper-constantan, teflon-insulated ther- the widest part of the stenmm, including the width mocouples (Omega, Stanford, CT) inserted to a of the metafemora. Dimensions were measured to depth of =2 mm through an incision between the 0.5 mm using hand-held calipers. The insects were prothora.x and mesothorax on the right side. This then classified by instar, weighed (grams of fresh by guest on August 22, 2016 side of the insect was not exposed to direct sun- body mass), and stored individually at -40°C. light during the experiment. For analysis, solar energy intercepted by each The Tb of insolated grasshoppers was compared grasshopper (in watts, W) was estimated as the with that of a Similarlyrestrained reference grass- product of the pyranometer reading (W . 1l1-2)and hopper in a sunshade. The temperature of this the area of the insect's shadow on a horizontal sur- grasshopper is considered to be Ttl. A grasshopper face (As), in square meters. Shadow area was used was used to provide the reference temperature be- because the pyranometer measures insolation in- cause the temperature of a bare thermocouple cident on a horizontal surface. This estimate was changed too quickly to provide a reliable basis for made by measuring the shadow area (A,,) of the comparison. insect on a plane perpendicular to the solar vector, The sunshade was constructed of 4 parallel alu- then converting this to As using the relationship As minum plates (1 mm thick), which were painted = A,,1cos(Z),where Z is the zenith angle of the flat black on one side and flat white on the other. sun, as calculated from Julian date, solar time and The inner pair of plates measured 10 by 10 cm site latitude (Robertson and Russelo ]968). and were separated by 1.5 Clll. The outer pair of The value 1), was measured using image analy- plates measured 12 by 12 cm; the separation be- sis. The nymphs were mounted under aDage 68 tween inner and outer plates was 1 cm. The top 2 black and white video camera (DAGE-MTI, Mich- plates were mounted with the black side down and igan City, IN), and were presented in the same the others with black side up. The sunshade was angles and orientations relative to the line from mounted within 0.3 m of the experimental device, lens to object, as had they been presented relative with the plates horizontal; it was set at the same to the solar vector in the field trials. Images were height and shaded in the same manner as the re- acquired at standard magnification, input to an im- strained insects. The reference grasshopper, which age analyser (Tracor Northern 8502; Noran, Mid- was restrained in the same manner as the experi- dleton, WI), and converted to digital images (512 mental nymphs, was pinned to a foam cylinder (4 by 512 pixel) with a greyscale range of from 0 (per- cm long by 2 cm diameter) inserted between the fect reflectance) to 256. Two greyscale thresholds inner plates. The insect did not touch the plates. were adjusted to isolate the image of the grasshop- To minimize forced convection we chose an ex- per from the background, and the number of pixels April 1996 LACTIN AND JOHNSON: M. packardii THORACIC TEMPEHATUHE 42,5 comprising til(' image was convelted to AI' using simulations to provide more complete understand- an empi rically derived calibration constant. ing of the effects of insect size, insolation, the ze- Preliminary analyses indicated that Tb elevation nith angle of the sun, and the orientation of the (61/,) was described by a function of the form insect to the sun on Tb elevation, and to allow com- parison to published reports. The procedure was 6T,> = a . ENERGY· SIZEb (1) as follows. wher<' ENERGY is solar energy intercepted, SIZE To allow simulation of a range of insect sizes, is either the mass or the length of the insect, and regression analysis was used to quantify the rela- h < O. The following conclusions were unaffected tionships of insect depth, dorsal breadth, ventral hy 1I1eanreflectance of the insects, as calculated breadth, and mass to insect length. The size of the from the i1l1ageanalyser greyscale reading, proba- simulated insect was specified using either length hly because the value was similar (=40%) in all or mass, and these regression relationships were insects. used to obtain the other measurements. Parameters a and h in equation 1 were estimat- A 2nd estimate of shadow area on a horizontal ed lISing iterative nonlinear regression (PROC surface (A.,,) was obtained geometrically, represent- NUN, Marquardt algorithm, SAS Institute 1989), ing each grasshopper as the trapezoidal prism de- representing size by either insect mass (in grams) fined by the length, deptll, dorsal breadth, and or insect length (in millimeters). To reduce varia- ventral breadth measurements. Contributions of tion suffiently to allow the regressions to converge, sides and faces to the area of a shadow on a surface Downloaded from valnes of ENERGY were rounded to classes of perpendicular to the solar vector, were determined 0.00.5W, insect mass to classes of 0.01 g, and insect by applying trigonometric functions to the angle of length to classes of 0.5 mm, and parameters were the insect relative to the solar vector, and A." was estimated using the class means, weighted by the obtained by dividing this by the cosine of the sim- nll1l1berof observations in each class. ulated solar zenith angle. In PROC NLIN, all observations are treated as Temperature elevation was simulated under a http://ee.oxfordjournals.org/ indqwndent, and the standard errors of the esti- range of insolations (0-1,200 'vV. m-2) for insects mates are calclilated using the number of class with masses from 0.05 to 0.50 g or lengths from means (No,>." varying by analysis). However, the ob- n.5 to 24.5 mm; the extremes of length and mass servations actnally are repeated measures on in- approximately correspond. Simulated Tb elevation sects, and so the standard errors should be based was compared with insolation level (W . m2) and 011the number of tnle replicates, i.e., the number to the amount of energy intercepted by the insect of insects (Ni ••,,). Therefore, all standard errors out- (watts). pllt by PROC NUN were adjusted to reflect the IlI11llberof insects by multiplying by (Nob./Ni ••,) 1/2. by guest on August 22, 2016 Results and Discussion Similarly, model F values were calculated IIsing Ni ••, as the degrees of freedom for the denomina- The experiment was conducted from 12 July to tor. 6 August 1993 inclusive, in 10 trials involving 20 Analysis of covariance (ANCOVA,PROC GLM, fourth-instar and 33 fifth-instar grasshoppers. A to- SAS Institnte 1989) was then used to qllantify the tal of 612 data points was collected. effects of other experimental factors on the rela- For each insect, the relationship between tem- tionship between T" elevationand ENERGY· SIZEb. perature elevation and insect size was linear, but Additional factors tested were trial, inscct nested as insect size increased, the slope and intercept of within trial (insect [trial]), and axisof rotation; all 2- this regression decreased and increased, respec- way intl'ractions were also tested. Imbalance in the tively (Fig. 1). These results indicate that the in- distlibution of instars among trials, which resulted dividuallinear regressions are merely local approx- from natural population phenology of the insects, dis- imations of a general nonlinear trend. allowed direct comparison of the effects of instar on Equation 1 was fitted successfully to the data, the response. Inspection of plots suggested that whether insect size was expressed as mass or any instar effects were related to size differences. length. Parameter estimates are given in Table 1. Equation 1 reveals tllat at a given amount of Although there is clearly considerable variation in energy intercepted, a small insect will attain a the observed trends, the fitted curves have r2 > greater T" elevation than a large insect. However, 0.6. Where insect size was expressed as lcngth, the this interpretation is incomplete, because at a giv- standard errors of tile estimates, as a proportion of en level of insolation (W . m-2), a small insect in- the estimates, were much greater than where in- tercC'pts less energy than a large one. This effect sect size was expressed as mass. This comparison complicates interpretation of the results. FurtllCr- suggests that mass is the most appropriate physical mort" TI> C'levationhas been ~ically expressed as factor to be used, because it is a direct measure of a function of insolation [W . m- ] (e.g., Pepper and the amount of tissuc to be heated. Length is simply Hastings 1952, Digby 1955, Henson "1958,Stower a proxy for mass, and the correlation between and Griffiths 1966, Edney 1971, Cena and Clark length and mass is imperfect. Despite this inflation 1972), and equation 1 is not easily comparable with of standard error estimates, comparison of F and these rqJOrts. Therefore, we undertook a series of r2 statistics indicates that the model in which size 426 ENVIRONMENTAL ENTOMOLOGY Vol. 2.'}, no. 2

with ENERGY· SIZEb). We attribute these results to our measurement of Tb elevation relative to a baseline established using another insect. Appar- ently, differences in thermal equilibrium among in- dividuals resulted in a distribution of baselines for comparison, which translated to differences in in- tercept among trials but did not affect intercepts or slopes within trials. The trial X ENERGY . SIZE/' interaction was significant for both measures of insect size. This result indicates that the slope of the regression of ilh on ENERGY . SIZEb differed among trials. 0.1 0.2 0.3 Several factors could explain this observation. Energy Intercepted [W] First, we used the final T/, measure as tile estimate Fig. 1. Equilibrium body temperature elevation on of equilibrium Tb; the correlation between these energy intercepted for each insect. Each fine line seg- quantities was probably imperfect. Second, we ment represents a linear regression over the range of val- used the contemporaneous insolation reading as ues of the energy measure to which the insect was ex- the measure of the value to which 1'1, elevation had posed. The heavy broken lines represent simulation responded; this is also an imperfect approx.imation Downloaded from predictions for insects of mass 0.05 (left) and 0.50 g. because insolation often varied over time, and there was a time lag in the resulting change in T/,. Third, the temperature of the irradiated and shad- was represented by length described the data ed sides of tile thorax of M. sangllinipes, which is slightly better than that in which size was repre- similar in size and shape to M. packanlii, can differ http://ee.oxfordjournals.org/ sented by mass. Thus length is an adequate mea- by as much as 2.5°C (Pepper and Hastings 1952); sure of size, and may be easier to use than mass. despite our attempts to standardize thermocouple The ANCOVA models testing the effects exper- insertion depth, this internal temperature gradient imental factors on the relationship between il1'b probably contributed to tile range of slope esti- and ENERGY . SIZEb were highly significant, mates. Finally, conditions varied slightly among tri- whether insect size was expressed as mass or als, and slight differences in unmeasured meteo- length (Table 2). In both cases, F = 14.9; df = rological factors, e.g., diffuse radiation, may have 127, 484; P :s; 0.0001; r2 = 0.7962. These overall affected the relationship of 1'1) elevation to EN- model statistics are the same because the ANCO- ERGY . SIZEb. Despite these sources of error, it by guest on August 22, 2016 VAwas performed on transformations of the same was possible to quantify the relationship above the data; differences occur only in the partitioning of statistical noise. variance among model components. Axis of rota- Strong relationships were detected among tile tion had no effect on the relationship between ilTb measurements taken of each insect. The insect and ENERGY· SIZEb, either alone or as an inter- dorsal breadth, ventral breadth, and depth were action (P > 0.7). This result indicates that no ori- each proportional to length; proportions (estimate entation is better than any other at absorbing en- ± SE) were 0.204 ± 0.003, 0.279 ± 0.003, and ergy or converting it to heat, and that the 0.305 ± 0.004, respectively. In all cases, F > 5,000; relationship between Tb elevation and energy in- df = 1, 81; P :s; 0.0001; and r2 > 0.98. Mass (in tercepted is caused entirely by the change in the grams) was related to length (in millimeters) by amount of energy intercepted. equation 2: Regression intercepts differed among trials (sig- nificant trial effect), but the regressions did not In(mass) = c + d . In(length), (2) differ among insects within trials (no significant ef- fect of insect[trial] either alone or as an interaction where c = -10.87 ± 0.489 (±SE) and d = 3.174

Table 1. Parameter estimates for tJ.Tb = a . ENERGY· SIZEb (equation 1 in text) relating body telllI,erature elevation to energy intercepted, using 2 size measures

Parameter estimates

Size meaSure a b F ,.2 Est SE(I)" SE(2)h Est SE(l) SE(2) Mass 18.76 1.57 4.05 -0.312 0.048 0.124 91.3 0.6198 Length 826.66 362.83 935.44 -1.133 0.153 0.394 101.2 0.6463

For both equations, P < 0.0001; df = 2, 50. Est, estimate; SE, standard error. o Value output by PROC NUN (i.e., calculated using number of observations). b Value corrected to represent number of tme replicates (i.e., number of insects). April 1996 LACTIN AND JOHNSON: M. packardii THORACIC TEMPERATURE 427

Tab!., 2. Results of ANCOVAtestinll:relationshil' of body teml.erature elevation to experimental factors

Mean Variance souree dF F P square Size measured by mass ENERGY· MASS 0.312 246.093 1 27.64" :sO.()(Kll Trial 11.583 10 ]4.41" :sO.()(Kll Trial X ENERGY. MASS 0.312 8.\l03 ]0 7.59" :sO.(KKll Inst·('t(trial) 0.804 52 0.84,1 0.7752 Inst·('t(trial) X ENERGY. MASS -0.312 1.173 52 1.l3d 0.13fl8 Axi!'t.t' 0.110 I 0.12,1 0.7344 Axis X I~NEHGY . MASS 0.312 0.013 ] 0.00,1 0.9076 Size measured by len~h ENERGY. LENGTH 1.13.1 2.34.:380 ] 27.5fl" :sO.OOOI Trial 11.716 10 14.5&' :sO.OOO] Trial X ENERGY. LENGTH 1.133 8.494 10 8.76" :sO.OOO] Inst't't{trh,l) 0.804 52 0.84,1 0.7741 Inst'('t{trial) X ENERGY. LENCTH-I.I33 0.970 52 J.02d 0.4467 Axis 0.001 1 O.OO" 0.9705 Axis X ENEHGY· LENGTH 1.133 0.071 1 0.07,1 0.7850 Downloaded from For hoth analyst's, F = 14.fl; df = 127, 484; P :s 0.0001; and ~ = 0.7962. /I Em>r «'nn: trial X ENEHGY . SIZE". " Error tt'nn: inst'ct (trial). (' Error tPrIll: inst'l't(trial) X ENERCY . SIZE"- d Error tt'rm: Illodpl residual. ,. Axis in which inst'('t was rotated. http://ee.oxfordjournals.org/

:':: 0,174. This regression had F = 332.7; df = 1, the intercept and slope were 0.12 and 0.009,5, re- 81; P :S 0.0001; and r2 = 0.8062. spectively. The intercept is significantly >0, prob- By regression analysis, the geometrically ob- ably because the geometric representation of the tained estimate of the shadow ca~t by the grass- insect neglects the contribution of appendages to hopper on a horizontal surface (A..) was strongly the shadow cast. The slope is significantly >0 and correlated with the value obtained by image anal- not significantly different from 1. These results in- ysis (AJ. Where areas were expressed in square dicate that the measured shadow area of the insect millimeters, can be estimated with >91% precision using this by guest on August 22, 2016 geometric representation. As = 15.0 + 0.994 . A,. (3) Simulated body temperature elevations, ob- This regression has F = 7,687.5; df = 1, .565; P :5 tained using equations 1-3, are given in Figs. 2 and 0.0001; and r2 = 0.9128. The standard errors of 3. In these figures, insect orientation is fixed at 90° (perpendicular to the sun vector), and the solar zenith angle is fixed at 45°. When energy was expressed as the amonnt in- tercepted (watts), simulations indicate that at a giv- en amount of energy intercepted, small insects at- tained a greater Tb elevation than did large insects (Fig. 2; to facilitate comparison with observations, simulation results representing the smallest and largest insects are included in Fig. 1). This prob- Insect Mass = 0.50g ably occurs because the surface area to mass ratio increases with decreasing insect size, so that a giv- en amount of intercepted energy heats a lesser mass of tissue in a small insect than in a large one, and thus, a small insect is heated more than a large o o 0.1 0.2 0.3 one. In Fig. 2, the ends of each line segment rep- Energy Intercepted [W] resent Tb elevations at the same insolation level, for insects of a range of masses. Fig. 2 illustrates Fill:. 2. Simulated equilibrium body temperature ele- clearly that at a given level of insolation the vation of M. packardii nymphs as a function of insect size amount of energy intercepted increases with body and energy intercepted. Insects were assumed to be ori· size, as does Tb elevation. Therefore, small body ented perpendicular to the solar vector; solar zenith angle size limits the ability of the insects to elevate TI} = 45°; insolation '= 300-1,200 W . m-2. Each line seg· ment represents the limits to body temperature elevation by limiting the amount of energy intercepted and for one insect. From left to right, insect mass increases this counteracts the advantage held by a small in- from 0.05 to 0.50 g in increments of 0.05 g. sect at a given amount of intercepted energy. 428 ENVIRONMENTAL ENTOMOLOGY Vol. 25, no. 2

a) ergy intercepted increases with body size, and that this trend overcomes the disadvantage of large size that results from the slllface area:vohunc ratio. Body temperature elevation in insects has been 8 generally expressed as function of insolation and body size. Although this approach is simple, it ob- ~ 0' 6 :::l~ scures the underlying physical determinants of T" Ci.i C elevation. We believe that the approach adopted •....0 Q) .- 4 here improves significantly on this historically 0..- E ~ adopted method. Q) Q) 1--[jJ 2 Insol. Acknowledgments [W/m2] We thank n. McLean, E. Kokko, B. Lee, H. C. An- l!) '<1:0 drews, L. Paterson, K. Floate, D. Yu, II. MeMenamin o (all Lethblidge Research Centre), and 2 anonymous re- Mass [g] viewers for their contrihutions. L. KE'nnedy (Kennedy Upholstery) provided advice on adhesivC's. This study was b) fimded in part by grants from the Nat Christie Founda- Downloaded from tion, administered thI'Ough the University of LC'thbridge; the Alberta Agricultural Hesearch Institute; and the Na- tional Sciences and EngineC'ring HC'seareh Couneil. This 8 is contribution No. 38794105 of the Lethbridge Research Centre.

~ 0' 6 http://ee.oxfordjournals.org/ :::l~ co c -•....0 References Cited Q).- 4 0..- E ~ Carruthers, R. I., T. S. Lm'kin, H. Firsten(~('I, and Z. Q) Q) 2 Fen~. 1992. Influence of thermal ecology on the 1--[jJ mycosis of a rangeland grasshoppC'r. Eeology 73: 190- Insol. 204. Cena, K., ami j. A. Clark. 1972. Effect of solar ra- [W/m2] .•... diation on temperatures of working honeybC'es. Nat . .•... o

e\i0 New BioI. 236: 222-223 . by guest on August 22, 2016 C\J Di~by, P.S.B. 1955. Factors affecting the temperature Length [mm] excess of insects in sunshine. J. Exp. Bio\. .32: 279- Fi~. 3. Idealized response surface illustrating simulat- 298. ed equilibrium body temperature elevation of M. pack- Edney, E. B. 1971. The body temperature of teneb- ardii nymphs as a function of insect size and insolation rionid beetles in the Namib desert of Southem Africa. (W . m 2). Insects were orientcd perpendicular to the J. Exp. BioI. 55: 25:3-272. solar vector; zenith angle was 45°. (a) Insect size repre- Heath, j. E., and P. j. Wilkin. 1970. Templ'raturl' sented by mass. (b) Insect size represented by length. responses of the desert cicada Diceroprocta apache The range of masses in panel "a" corresponds approxi- (Homoptera: Cicadidae). l'hysiol. Zool. 43: 145-154. mately to the range of lengths in panel "b". Heinrich, B. 1977. Why have some evolved to regulate a high body temperature? Am. Nat. Ill: 623-640. Henson, W. R. 1958. The effE'cts of radiation on the Where energy was expressed as insolation (W . habitat temperatures of somC' poplar-inhabiting in- 2 m- ), simulated T" elevation increased with both sects. Can. J. Zoo\. 36: 463-478. insect size and insolation (Fig. 3). At a given insect Hney, R. B., and J. G. Kin~solver. 1989. Evolution size, simulated T" elevation increased linearly with of thermal sensitivity of ectotherm performance. insolation, but the rate of change varied with insect Trends Eco\. Evo\. 4: 131--135. size. At a given insolation level, the relationship Hney, R. B., and M. Slatkin. 1976. Costs and bC'nefits between T" elevation and body size was nonlinear of lizard thermoregulation. Q. Hev. BioI. 51: 363-384. when size was expressed as mass, and approxi- Kemp, W. P. 1986. Thennorl'gulation in three rangc- mately linear when size was expressed as length. land grasshopper species. Can. Entomol. 118: 335- 343. These results are concordant with published ob- May, M. L. 1979. Ins('ct thermoregulation. Annu. Rev. selvations that at a given level of insolation, max- Entomol. 24: 313--349. imum T" elevation increases with body size (e.g., 1982. Body temperature aud thermoregulation of the Pepper and Hastings 1952, Digby 19.55, Henson Colorado potato beetle, Leptinotarsa decellliineata. 1958, Stower and Griffiths 1966, Edney 1971, Entomol. Exp. App\. 31: 413-420. Cena and Clark 1972). Comparison of these 2 sets Pepper, J. H., allll E. Hastings. 1952. The effects of of simulations reveals that T" elevation increases solar radiation on grasshopper temperature and activ- with body size simply because the alllount of en- ities. Ecology 33: 96--103. April 1996 LACTIN AND JOHNSON: M. packardii THORACIC TEMPEHATURE 429 nober\>;on, G. W., Ulul D. A. nu..~sdu. 1968. Astro- body temperature of terrestrial ectotherms. Am. Nat. nwt('orolo~cal estimator. AW;c. Can. Tech. Bull. 14: 126: 362-386. 1-22. Stower, W. J., and J. F. Griffiths. 1966. The body SAS Illstitut". 1989. 5A5/5TAT user's guide, version 6, temperature of the desert locust (Schistocerca gregor- 4th ed., vol. 2. SA5 Institute, Cary, NC. ia). Entomol. Exp. Appl. 9: 127-178. Shwellsun, n. D. 1985. The relative importance of be- Received for publicatio1l 26 jallurlY'lj J 996; accepted J:3 havioral and physiological adjustments controlling November J 996. Downloaded from http://ee.oxfordjournals.org/ by guest on August 22, 2016