Thermal Tolerances and Preferences of the Crab Spiders Misumenops asperatus and Misumenoides formosipes (Araneae, Thomisidae)

11 Pages·1999·11.9 MB·English

by  Victoria R Schmalhofer

1999. The Journal of Arachnology 27:470-480
THERMAL TOLERANCES AND PREFERENCES OF THE CRAB
SPIDERS MISUMENOPS ASPERATUS AND
MISUMENOIDES FORMOSIPES (ARANEAE, THOMISIDAE)
Victoria R. Schmalhofer: Graduate Program in Ecology and Evolution; Department
of Ecology, Evolution, and Natural Resources; Rutgers University, Cook College; 14
College Farm Road, New Brunswick, New Jersey 08901-8551 USA
ABSTRACT. Misumenops asperatus (Hentz 1847) and Misumenoidesformosipes (Walckenaer 1837)
are diumally-active, flower-dwelling crab spiders (Thomisidae) commonly inhabiting open fields. In lab-
oratory experiments, both species remained active over a temperature range of approximately 46 °C. The
spring-maturing M. asperatus tolerated significantly lower temperatures than the summer-maturing M.
formosipes (CT^jn = ~1.4 °C and 2.2 °C, respectively), whileM.formosipes tolerated significantly higher
temperatures than M. asperatus (CT^a^ = 48.2 °C and 45.1 °C, respectively). Misumenops asperatus
displayed thermal discomfort over a broader range of temperatures (36-44 °C) than did M. formosipes
(41-46 °C). In a laboratory thermal gradient apparatus M. asperatus tended to prefer cooler temperatures
thanM.formosipes (14.4°C and 18.4 °C, respectively). Regression analysisofliteraturedatafor21 species
of spiders showed a significant positive relationship between the thermal preference of a species and its
CTn^ax- For their CTn^a^’s, which were high compared to most other spider species, M. asperatus and M.
formosipes preferred low temperatures. The coupling of low thermal preferences and high thermal toler-
ances displayed by M. asperatus and M. formosipes is unusual for ectothermic organisms.
Spiders are strict ectotherms (Humphreys ences, upper tolerance limits, or thermoregu-
1987; Pulz 1987) and are important predators latory behaviors of spiders inhabiting temper-
in many terrestrial systems (Riechert 1974; ate regions (forexceptions see Almquist 1970;
Wise 1993), yetthe thermal ecology ofspiders Sevacherian & Lowrie 1972; Tolbert 1979;
is generally less well understood than is that Suter 1981; references from Table 2 in Pulz
of their insect prey (Humphreys 1987). Ther- 1987).
mal relations (i.e., thermoregulatory behav- Information regarding an animal’s thermal
iors, thermal tolerances and preferences) have tolerances and preferences is necessary to de-
been examined in less than 0.1% of spider scribe the thermal ecology of the animal and
species (Humphreys 1987), and most research to evaluate the thermal suitability ofits habitat
has concentrated on large tropical and sub- (Hertz et al. 1993). In the experiments report-
tropical orb-weavers (e.g., Krakauer 1972; ed here, I examined the thermaltolerances and
&
Carrel 1978; Robinson Robinson 1978), de- preferences of two flower-dwelling crab spi-
sert-dwelling spiders (e.g., Mouer & Eriksen ders (Thomisidae),Misumenopsasperatus and
1972; Seymour & Vinegar 1973; Humphreys Misumenoidesformosipes. These experiments
1974, 1987; Riechert & Tracy 1975; Lubin & were part of a larger study investigating tem-
Henschel 1990; Henschel et al. 1992; Turner perature effects on crab spider microhabitat
et al. 1993), and winter-active species (e.g., selection and hunting performance. Because
Aitchison 1978). Studies of temperate-zone M. asperatus and M. formosipes prey primar-
spiders inhabiting moderate environments ily on insect pollinators, most foraging op-
have generally focused on cold resistance and portunities and predation events occur diur-
super-cooling capabilities (e.g., Almquist nally (Schmalhofer 1996). Compared to
1970; Kirchner 1973, 1987; Schaefer 1977; nocturnal spiders, which are active when heat
Duman 1979; Bayram & Luff 1993) or tem- stress is minimal, M. asperatus and M. for-
perature effects on growth and metabolic rate mosipes, as well as other diumally-active in-
(e.g., Anderson 1970; Hagstrum 1970; Moul- florescence spiders, likely experience greater
der & Reichle 1972; Li & Jackson 1996). temperature variability and extremes of tem-
Fewer studies have examined thermal prefer- perature. Consequently, diurnal spiders might
470
SCHMALHOFER—CRAB SPIDER THERMAL ECOLOGY 471
tolerate greater thermal stress. This study spe- August 1993. A given spider was used in only
cifically addressed the following questions: 1) one experimental manipulation.
What are the thermal tolerances and prefer- Because hunger has been shown to influ-
ences ofM. asperatus and M. formosipesl 2) ence thermal sensitivity in spiders (Pulz
When comparing field-active adultfemale spi- 1987), I maintained the experimental spiders
ders, does the summer-maturing species, M. on a diet of one housefly per week for three
formosipes, display higher tolerances and weeks prior to the initiation of the experi-
preferences than the spring-maturing species, ments. This regimen equalized hunger-states
M. asperatusl 3) How do the thermal toler- among individuals of a given species and
ances and preferences of these flower-dwell- maintained spiderbody mass atrelativelycon-
ing thomisids compare with the thermal tol- stant levels (Anderson 1970; Schmalhofer un-
erances and preferences of other spiders? 4) publ. data).
Does data drawn from the literature indicate Misumenops asperatus and M. formosipes
that diumally-active spiders, in general, have were not acclimated to similar temperatures in
higherthermal tolerances andpreferencesthan the lab prior to testing. Although this con-
noctumally-active spiders? founds species identity with seasonal temper-
Information concerning the thermal rela- ature differences when comparing the two
tions ofM. asperatus and M.formosipes is of species, I was not attempting to separate these
additional interest because thomisids rank influences, but, rather, was interested in com-
among the most diverse families of spiders paring the responses of field-active adults.
(Coddington & Levy 1991) and are among the Also, other researchers (e.g., Sevacherian &
most common cursorial spiders found in the Lowrie 1972; Seymour & Vinegar 1973) have
herb stratumofopen fields. However, the ther- found that temperature acclimation does not
mal ecology of this family is virtually un- affect thermal preferences or upper tolerance
known, consisting only ofa few anecdotal ob- limits of spiders. However, I maintained spi-
servations (Morse 1979; Lockley et al. 1989). ders at ambient field temperature, rather than
METHODS a constant temperature, to reduce any possi-
bility ofspiderresponses to experimentallyin-
—
Study animals.^ Misumenops asperatus duced temperature changes being influenced
and M. formosipes are sit-and-wait ambush by acclimation to an artificial temperature re-
predators that use their raptorial forelimbs, gime. Under natural conditions, the two spe-
rather than a web, to restrain prey. In central cies experienced different temperature re-
New Jersey, M. asperatus matures in spring gimes; average daily temperature, based on
(mid-April to early May), females lay a single data collected from the weather station at the
egg sac in late May or June, and spiderlings Hutcheson Memorial Forest Research Center
emerge in June and July and overwinter as (HMF) in Somerset County, New Jersey (the
late instar juveniles (pers. obs.). Misumeno- site of later field experiments), for 30 days
idesformosipes matures in mid-summer (Au- prior to the initiation ofexperiments was 16.5
gust), females lay a single egg sac in Septem- ±8.7 °C for M. asperatus and 22.8 ±7.9 °C
ber or early October, and spiderlings generally for M. formosipes. —
overwinter in the egg sac (pers. obs.). Thermal tolerances. Critical thermal
Only adult female spiders were used in maximum (CT^^ax) critical thermal mini-
these experiments. Like many other spiders, mum (CTn^in) describe the highest and lowest
male M. asperatus and M.formosipes seldom temperatures, respectively, at which an animal
capture prey as adults, instead spending their is capable of displaying coordinated locomo-
time searching for and guarding prospective tory behavior (i.e., the animal can still escape
mates (Foelix 1996; Dodson & Beck 1993; unfavorable temperatures). An animal’s ther-
Pollard et al. 1995). Spiders were collected in mal tolerance range is delimited by these crit-
Middlesex and Somerset Counties, New Jer- ical temperatures. Outside its tolerance range
sey, and voucher specimens have been depos- an ectotherm enters a state of heat stupor or
ited at the American Museum ofNatural His- cold torpor, which may result in death if ex-
tory. Experiments involving M. asperatus posure to extreme temperatures is prolonged.
occurred in May and June 1993, and experi- Critical temperatures of M. asperatus and M.
ments involving M. formosipes occurred in formosipes were determined by placing spi-
472 THE JOURNAL OF ARACHNOLOGY
ders confined in petri dishes in a controlled was recorded with a Bailey BAT-12 thermo-
temperature box (VWR Scientific model 2015 couple thermometer.
low-temperature incubator) initially set to 25 Spiders were released singly into the center
°C. Box temperature was then raised to 50 °C of the arena, and the number of each square
(to determine or lowered to —5 °C (to in which a spider stopped and the amount of
determine Temperature changed at a time it stayed within the square was recorded
A
rate ofapproximately 1 °C every five minutes. during a 10 minute period. spider’s pre-
This rate of change was similar to that used ferred temperature (Tp) was determined ac-
in other studies examining tolerance limits cording to the formula
m(eo.gu.r, &AlVmiqnueigsatr 11997730);. FKorrakeavueerry t1e9m7p2e;raSteuyr-e Tp = 2[(r,)(f,/f,)]
change of 1 °C, the spiders were prodded to where T, is the temperature (°C) of square i,
ascertain their ability to initiate an escape re- is the time (s) spent in square i, and is the
sponse. The last temperature at which a spider total time (s) a spider was quiescent {n = 9
responded to prodding (by moving away from spiders per species). The preferred tempera-
the probe, raising its raptorial forelimbs, or ture range of a species was calculated as one
grabbing hold of the probe) was recorded as standard deviation aro—und its average T^.
its critical temperature. Spiders were removed Field temperature. TemperaturHeMdFata ob-
from the temperature box once they ceased to tained from the weather station at were
respond to prodding. Spiders are known to averaged over three years (1993-1995) to de-
termine average monthly high and low tem-
spontaneously initiate vigorous activity at
peratures. These data were compared with the
high temperatures preceding this activ-
preferred temperature ranges of M. asperatus
ity indicates thermal discomfort as maximal
& and M. formosipes. By graphing the data and
tolerance is approached (Lubin Henschel
calculating the appropriate area encompassed
1990). The temperature ranges over which M.
within the high-low temperature curves, a
asperatus and M. formosipes displayed ther-
rough estimate of the frequency with which
mal discomfort were noted. Five spiders of
mature female spiders experienced ambient
each species were used to determine CTn^,,^,
temperatures (shaded air temperature) within
and five different spiders ofeach species were
= their preferred ranges was determined. Aver-
used to determine CT^^j^ (total n spiders
= age daily temperatures calculated over a ten
per species). Another set of spiders {n 9)
day period at the beginning and end of a spe-
was prodded at five minute intervals under cies’ adult stage were compared to determine
constant temperature conditions (25 °C) to as-
the seasonal temperature shifts experiencedby
certain whether a lack ofresponse to prodding adult M. asperatus and M. formosipes.
was due to habituation rather than tempera- Analyses.—Due to small sample sizes,
turTe.hermal preferences.—Atemperature are- nthoenpraersualmtest.rAicMsatnatnis-tWichsitwneerye uUsetedsttowaasnaulsyezde
na was established by placing a box (50 cm to compare CT^^in data ofM. asperatus andM.
X 50 cm), the bottom of which was demar- formosipes. Because CT^^^x and thermal pref-
cated into numbered squares (2 cm X 2 cm), erence data of M. asperatus and M. formosi-
in a controlled temperature room at 2 °C. By- pes were also compared to and thermal
tac Teflon® paper lined the sides of the boWx preference data of other spider species taken
to prevent spiders from escaping. A 650 fromthe literature, Kruskal-Wallace tests were
photoflood lamp (equivalentcolortemperature used, and all possible pairwise posthoc com-
3400 K) mounted on a ringstand and suspend- parisons were made using Mann-Whitney U
m
ed 1 above a comer of the arena served as tests, with aBonferroni correction formultiple
a radiant heat source. A temperature map of comparisons (adjusted a = 0.009). Spiderspe-
the arena was created by determining the tem- cies for which data were available in the lit-
perature of each square within the arena; a erature were separated into two groups: diur-
spider model (freeze-dried female thomisid) nally active and nocturnally active. For
with a fine thermocouple attached to the ven- comparative purposes, species that were ac-
tral side of its abdomen was placed in each tive both nocturnally and diurnally were
square and, after two minutes, its temperature counted as diurnally active. Information con-
SCHMALHOFER—CRAB SPIDER THERMAL ECOLOGY 473
—
Table 1. Critical thermal tolerances (CT^^^’s and CT^^in’s) and preferred temperatures (Tp’s) of Misu-
menops asperatus and Misumenoidesformosipes, and the range of temperature over which spiders dis-
played thermal discomfort. Values are in °C. Critical thermal values and preferred temperatures are given
as means (± 1 SD). For CT^^n data, a significant difference occurs at a = 0.05; for CT^j^^ and data,
adjusted a = 0.009.
Mann-Whitney
Measurement M. asperatus M. formosipes U test P-value
CT^min -1.4 (0.6) 2.2 (1.8) -2.660 0.0098
CT„„ 45.1 (1.3) 48.2 (0.2) -2.627 0.0086
T, 14.4 (3.4) 18.4 (5.4) -1.810 0.0703
thermal discomfort 36-44 41-46
ceming the activity times of the various spe- played thermal discomfort over a wider tem-
cies came from the original studies and from perature range than did M. formosipes (Table
Roberts (1995). In cases where information 1). No evidence of low-temperature thermal
concerning a species’ activity time could not discomfort was observed during the CT^^jn ex-
be found, the species was presumed to be noc- periments (i.e., there was no period of spon-
tumally active. Literature values forCT^,^ and taneous activity).
Tp were subjected to simple linear regression Values for the CT^^ax’s of other spider spe-
to determine whether CT^^ax increased with in- cies are presented in Table 2. Comparing
creasing Tp. values of M. asperatus and M. formo-
RESULTS sipes with literature data for diurnally-active
— and noctumally-active spiders revealed signif-
Thermal tolerances. Cessation of spider icant differences (Kruskal-Wallace test: H =
responses to prodding was due to temperature, 16.271, df = 3, P = 0.001). Diurnal spiders
not habituation to prodding. Under constant and nocturnal spiders had similar CT^^ax’s, and
temperature conditions, spiders did not be- diurnal spiders and M. asperatus had similar
come habituated in 45 consecutive prodding CTjnax’s (Fig. 1). However, CT^^ax’s of M. as-
trials. In the and experiments, a peratus and nocturnal spiders differed signif-
given spider was prodded less than 30 times. icantly, and M. formosipes had a significantly
Also, spider posture changed noticeably when
higher CT^^ax thanM. asperatus, nocturnal spi-
critical temperature was reached. Throughout
most of the experimental temperature range, ders, or diurnal spiders (—Fig. 1).
Thermal preferences. Temperature inthe
spiders maintained the classic crab spider
experimental arena ranged from 9-57 °C. Spi-
hunting posture (raptorial forelimbs held out-
ders encountering hotter areas moved rapidly
stretched, slightly upraised, and approximate-
to cooler areas, holding their bodies well-el-
ly perpendicular to the long axis ofthe body).
Within approximately 1 °C of upper or lower evated above the substrate (stilting) until they
critical temperature, spiders abandoned the settled in a cooler section of the arena. Al-
classic hunting posture and huddled with their though spiders moved more slowly (relative
forelimbs held close to the body. to the speed at which they vacated hot areas)
Misumenops asperatus and M. formosipes at the cooler end of the arena, no evidence of
proved to be broadly temperature tolerant. cold-trapping (i.e., spider activity was so re-
Misumenops asperatus had a significantly ducedby the cold thatthey could not emigrate
lower CT^^in than did M.formosipes (Table 1). from cooler areas) was observed. Spiders did
Conversely, M.formosipes had a significantly not stilt in areas cooler than 30 °C. Misumen-
higher CT^^^x than did M. asperatus (Table 1). ops asperatus preferred cooler temperatures
Spontaneous initiation of escape behavior by thanM.formosipes, but this trend was not sig-
spiders (i.e., a spider moved vigorously nificant (Table 1).
around the petri dish without prompting from Preferred temperatures of other spider spe-
the investigator) was observed during the cies are presented in Table 2. Comparing Tp’s
CTn^a,^ experiments, and M. asperatus dis- of M. asperatus and M. formosipes with lit-
474 THE JOURNAL OF ARACHNOLOGY
not included in this analysis. Addition of M.
y asperatus and M. formosipes data to the re-
§ gression resulted in a loss of the significant
relationship (F = 1.615, = 1, 21, F =
0.2177, F = 7.1%). For similar reasons, data
for winter-active spiders were also excluded.
Field temperature -—Misumenoides for-
mosipes experienced preferred temperatures
under field conditions more frequently than
did M, asperatus. Ambient temperature fell
within the preferred temperature range (PTR)
of adult M. asperatus 43% of the time, ex-
40 ceeded PTR 47% of the time, and fell below
PTR 10% of the time (Fig. 4). In contrast,
,e> ambient temperature fell within PTR of adult
M.formosipes 65% ofthe time, exceededPTR
10% of the time, and fell below PTR 25% of
the time (Fig. 4). During their adult phase, M.
— asperatus experienced an increase in average
Figure L Critical thermal maxima (CTn^a^’s) of daily temperature of 5.3 °C, and M. formosi-
Misumenops asperatus, Misumenoidesformosipes,
pes experienced a decrease in average daily
noctumally-active spiders and diumally-active spi-
ders. Data for nocturnal and diurnal spiders were temperature of 6.5 °C.
taken from Table 2; winter-active species were not DISCUSSION
included in the analysis. Values with different let-
ters are significantlydifferentusingpost-hocMann- Comparing the thermal tolerances and pref-
Whitney Utests withadjusteda = 0.009. Errorbars erences of the spring-maturing M. asperatus
represent one SE for nocturnal and diurnal spiders with those of the summer-maturing M. for-
and one SD for M. asperatus and M. formosipes. mosipes yielded the expectedresults. Adultfe-
Standard errors could not be used forM. asperatus male M. asperatus, which experienced lower
andM.formosipes since species averages were cal- ambienttemperatures than did adultfemaleM.
culated from individual values. In contrast, noctur-
nal and diurnal averages were calculated from spe- formosipes, had a lower CT^^j^, while M. for-
cies averages, making use of the standard error mosipes had a higher CT^a^- Thermal prefer-
appropriate. ences ofthe two crab spider species were sim-
ilar. Of greater interest is a comparison ofthe
thermal tolerances and preferences of these
erature data for diurnally-active and noctur- flower-dwelling spiders with those of other
nally-active spiders revealed significant dif- spider species.
ferences (Kmskal-Wallace test: H = 23.878, There is little information available regard-
df = 3, P < 0.0001). Preferred temperatures ing CTn,in in spiders. In the single study of
were similar between M. formosipes and M. which I am aware, Hagstrum (1970) reported
asperatus, and between M. formosipes and a CT^jn of 6 °C for a southern California wolf
nocturnal spiders (Fig. 2). However, Tp’s ofM. spider, Alopecosa kochi (Keyserling 1877) (as
asperatus and nocturnal spiders differed sig- Tarentula kochi in Hagstrum 1970). Much
nificantly, and Tp of diurnal spiders was sig- more data is available concerning lower lethal
nificantly higher than that of M. formosipes, temperatures and temperature effects on de-
M, asperatus, or nocturnal spiders (Fig. 2), velopmental rates (e.g., Almquist 1970; Li &
Relationship between and thermal Jackson 1996). Data from other studies indi-
preference.— Using the literature data shown cates that temperate-climate spiders are gen-
in Table 2, a significant positive relationship erally capable of activity at relatively low
was found between a spider species’ and temperatures. Ford (1978) showed that a Eu-
its (F = 6.707, df = 1, 19, P - 0.018, ropean wolf spider, Pardosa amentata Clerck
= 26.1%) (Fig. 3). Because M. asperatus 1757, remained active at 5 °C, and Moulder
and M.formosipes had unusually high CTj^^^’s & Reichle (1972) obtained similar results for
for their Tp’s, data for these two species were the litter-spider fauna of a Tennessee Lirio-
SCHMALHOFER—CRAB SPIDER THERMAL ECOLOGY 475
dendron forest. Aitchison (1984) found that One would expect that because diumally-
both winter-active and winter-inactive Cana- active spiders experience higher temperatures
dian spiders fed at 2 °C, with winter-active than noctumally-active species, diumally-ac-
species continuing to feed at temperatures as tive spiders would prefer higher temperatures.
low as “5 °C. These studies, coupled with the Evaluation of data from the literature showed
CTn,in values calculated for M. asperatus and that this was indeed the case (Fig. 2). How-
M. formosipes, suggest that temperate-zone ever, Tp’s of M. asperatus and M. formosipes
spiders from moderate climates may generally were lower than those of other diumally-ac-
be expected to have CTj^jn’s near 0 °C. tive species, and Tp of M. asperatus was also
Misumenops asperatus, and particularly M. lower than that of noctumally-active species!
formosipes, had high thermal tolerances. In Pulz (1987) suggested that, barring winter-ac-
general, CTn,ax’s of these flower-dwelling tive spiders, lower thermal preference corre-
thomisids were more similar to the upper le- lates with lower thermal tolerance. Regression
thal temperatures than to the CTj^^x’s of other analysis of the available literature data sup-
spider species (see Table 2). Ofthe 27 species ported Pulz’s hypothesis ofapositive relation-
for which data was available, only six species ship between and CTj^^. Interestingly, the
had comparable PhuroUthusfestivus CTjnax's of M. asperatus and M. formosipes
(C.L. Koch 1835), Euophrys frontalis (Wal- predicted from the regression equation (40.2
ckenaer 1802), Cyrtophora citricola (ForskM °C and 41.3 °C, respectively) were much low-
1775), Zelotes longipes (L. Koch 1866) (as Z er than the measured values; alternatively,
serotinus in Almquist 1970), Hogna caroli- predicted Tfs (32.1 °C and 43.4 °C, respec-
nensis (Walckenaer 1805) (as Lycosa caroli- tively) were much higher than the measured
nensis in Moeur & Eriksen 1972), and Seo- values. Thus, depending on how one looks at
thyra henscheli (Dippenaar 1991). Misu~ it, M. asperatus and M. formosipes have ex-
menoides formosipes had the second highest ceptionally high thermal tolerances or excep-
CTn^a,, recorded for a spider; only S. henscheli, tionally low thermal preferences. This com-
an eresid from the Namib desert (Lubin & bination of high thermal tolerance and low
Henschel 1990), had a higher thermal toler- thermal preference is unusual for an ecto-
ance. The natural histories of M. asperatus therm; preferred temperature is usually nearer
and M. formosipes may provide an explana- the upper than the lower tolerance limit (May
tion for their unusually high thermal toleranc- 1985).
es. These thomisids do not stalk prey, but, Broad temperature tolerances and relatively
rather, position themselves close to a flower’s low thermal preferences displayed by M. as-
nectaries and/or anthers (pollen-bearing struc- peratus and M. formosipes may be viewed as
tures) in order to ambush flower-visiting in- adaptations that facilitate their diurnal preda-
sects (pers. obs.). On the plants used by M. tory lifestyles in potentially thermally stress-
asperatus and M. formosipes, the floral sur- ful habitats (sun-exposed flowers), Hymenop-
face from which nectaries and anthers are ac- terans anddipterans comprise mostoftheprey
cessed by insects is typically exposed to the captured by these thomisids (Schmalhofer
sun (pers. obs,). Under conditions of high ra- 1996). Dipterans are well known for their
diant intensity and low wind speed, body tem- ability to fly at low temperatures (reviewed in
peratures ofspiders on sun-exposed floral sur- Heinrich 1993); and large hymenopterans,
faces can exceed ambient temperature by 15 such as honeybees and bumblebees, require
°C or more (Schmalhofer 1996). A high ther- thoracic temperatures of 30-35 °C in order to
mal tolerance would allow M. asperatus and fly (reviewed in Heinrich 1993). The capacity
M. formosipes to continue hunting at ambient to endothermically generate heat by shivering
temperatures near 30 °C, when floral surface wing muscles allows these bees to fly at low
temperatures couldbe in excess of40 °C, Am- ambient temperatures; honeybees can fly
bient temperatures approaching 30 °C are not when ambient temperature is as low as 15 °C,
an uncommon occurrence in late spring and and some bumblebees can fly when ambient
summer in central New Jersey: from April temperature is less than 10 °C (reviewed in
through September in 1993-1995 there were, Heinrich 1993). Both M. asperatus and M.
on average, 52 days per year having a daily formosipes prey on honeybees, and M. for-
high temperature of at least 30 °C. mosipes is also capable of capturing bumble-
1
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476 THE JOURNAL OF ARACHNOLOGY
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478 THE JOUR2^AL OF ARACHNOLOGY
301
CT
max
25
cd
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ab
e
20
o
15 CT
0) min
10
/ /
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—
Figure 2, Preferred temperatures of Misumen-
ops asperatus, Misumenoides formosipes, noctur-
nally-active spiders and diumally=active spiders.
Data for nocturnal and diurnal spiders were taken
from Table 2; winter-active species were not in-
cluded in the analysis. Values with different letters
are significantly different using post-hoc Mann-
Whitney Utests withadjusteda = 0.009. Errorbars
represent one SE.
—
M.formosipes Figure4. Therelationshipbetweenaveragedai-
ly high andlowtemperatures andthepreferredtem-
perature ranges (PTR’s) of Misumenops asperatus
and Misumenoidesformosipes. Periods of adult ac-
tivity are demarcated with vertical bars. (up-
per dashed line), CT^un (lower dashed line), and
zone of thermal discomfort (TD, dotted lines) are
indicated for each spider species. The bounds of a
species’ PTR (dot-dashed lines) were calculated as
one standard deviation around mean T^.
bees (Schmalhofer 1996). Presumably many
of the other bees used by these spiders, such
as anthophorids and megacMlids, display tem-
perature-flight relationships similar to those
— shown by honeybees and bumblebees. The
Figure 3. Linear regression of preferred tem- broad temperature tolerances shown byM. as-
perature against CTj^a^- The regression was based peratus andM.formosipes allow these spiders
oeqnualtiitoenra:tuyre=dat0a.2p7r5exsen+ted36i.n26T.abAlleth2o.uRgehgrneostsiionn- to increase their foraging time, both daily and
seasonally. Coupled with their ability to hunt
cluded in the regression, for comparative purposes
data points for Misumenops asperatus and Misu- equally well over a wide range of tempera-
menoides formosipes were included in the graph. tures (Schmalhofer 1996), and their low ther-
Squares () represent other spiders species, circles mal preferences, broad thermal tolerances
(•) represent M. asperatus and M. formosipes. benefitthese spiders by affording themthe op-
SCHMALHOFER—CRAB SPIDER THERMAL ECOLOGY 479
portunity to hunt prey that is itselfactive over (pers. obs.), spiders must remain near the an-
a wide range of environmental temperatures. thers and/or nectaries in order to have access
Assuming that an ectotherm benefits by to prey. In the open held habitats where M.
maintaining body temperature within some asperatus and M. formosipes are typically
preferred range of temperature (Hertz et al. found, the upper surfaces of flowers, where
1993), comparing preferred temperature rang- the anthers and nectaries are located, are gen-
es (PTR’s) ofM. asperatus andM.formosipes erally sun-exposed. Thus flower-dwelling
with the average range of temperatures nor- thomisids may be faced with a trade-off be-
mally experienced by these spiders allows one tween maintaining access to prey and avoid-
to make predictions concerning the likelihood ing high thermal stress.
that the spiders will experience thermal stress ACKNOWLEDGMENTS
(i.e., unfavorable temperatures that might lim-
it activity or impair performance). Data indi- T. Casey, R Morin, M. May, D. Morse, P.
cate that M. asperatus experiences high ther- Harris, C. Kaunzinger, J. McGrady-Steed, J.
mal stress (ambient temperature > PTR) more Fox, M. Cole, W. Schmalhofer, Y. Lubin, and
frequently than low thermal stress (ambient an anonymous reviewer gave helpful critiques
temperature < PTR), while M.formosipes ex- ofearlier drafts ofthe manuscript. N. Platnick
periences low thermal stress more frequently provided information concerning currently ac-
than high thermal stress. In neither case did cepted species names and authorities. I thank
average daily low temperature fall below spi- W., L., and V. Schmalhofer for their encour-
der Thus, since ambient temperature agement and moral support. Partial support
did not fall to levels that would inhibit spider was provided by a National Science Founda-
movement, M. asperatus and M. formosipes tion Dissertation Improvement Grant (DEB
could alleviate low thermal stress by engaging 93-11203), and grants from the Anne B. and
in behaviors designed to elevate body tem- James H. Leathern Scholarship Fund and the
perature (e.g., basking in the sun). Normal William L. Hutcheson Memorial Forest Re-
hunting behavior (i.e., sitting near a flower’s search Center.
nectaries and/or anthers, provided the position LITERATURE CITED
was exposed to the sun) would serve to
Aitchison, C.W, 1978. Spiders active under snow
achieve this result. in southern Canada. Symp. Zool. Soc., London,
High thermal stress appears to be more of 42:139-148.
a concern for these thomisids since ambient Aitchison, C.W. 1984. Low temperaturefeedingby
temperature typically falls within or above a winter-active spiders. J. ArachnoL, 12:297-305.
spider’s preferred range, and even when am- Almquist, S. 1970. Thermal tolerances and pref-
bient temperature falls within the preferred erences of some dune-living spiders. Oikos, 21:
range, floral-surface temperatures, and thus 230-236.
spider body temperatures, may be much high- Anderson, J.F. 1970. Metabolic rates of spiders.
er, potentially approaching Most diur- BayCroammp,.AB.i&ochMe.mL..PLhuyfsfi.ol.1,99333.:5C1o-l7d2-.hardiness of
nally-active spiders avoid high, stressful tem-
wolf-spiders (Lycosidae, Araneae) with particu-
peratures by some behavioral mechanism
lar reference to Pardosa pullata (Clerck). J.
(Pulz 1987). Ground- or vegetation-dwelling Therm. Biol., 18:263-268.
cursorial (non-web-building) spiders can Carrel, J.E. 1978. Behavioral thermoregulation
move to shaded areas under twigs, leaves, during winter in an orb-weaving spider. Symp.
stones, etc., while web-building spiders may Zool. Soc., London, 42:41-50.
have a shaded retreat associated with the web. Dodson, G.N. & M.W. Beck. 1993. Pre-copulatory
In both cases, these spiders still have access guarding of penultimate females by males crab
to prey when in shade and can continue to spiders, Misumenoides formosipes. Anim. Be-
hunt. This option of behavioral thermoregu- hav., 46:951-959.
lation (shuttling between sun and shade) may Duman, J.G. 1979. Subzero temperature tolerance
in spiders: Role of thermal hysteresis factors. J.
not be available to flower-dwelling thonusids Comp. Physiol. (B), 131:347-352.
if the spiders are to maintain access to prey. Foelix, R. 1996. BiologyofSpiders. HarvardUniv.
Because M. asperatus and M, formosipes do Press, Cambridge. 330 pp.
not strike at prey unless it approaches within Ford, M.J. 1978. Locomotory activity and the pre-
a few millimeters of the spider’s chelicerae dation strategy ofthewolf-spiderPardosaamen-