Responses to temperature and relative hunidity in the harvestman Vonones ornatus (Say) (Arachnida, Opiliones, Cosmetidae)

2007·0.07 MB·English

by  Punzo, F

A.D.Brescovit&M.Simó 17
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Bull.Br.arachnol.Soc.(2007)14(1),17–21
Responses to temperature and relative humidity in Introduction
the harvestman Vonones ornatus (Say) (Arachnida,
There are many terrestrial arthropods for which little
Opiliones, Cosmetidae)
informationisavailableontheirnaturalhistory,ecology
and behaviour (Punzo, 2000). This is especially true for
Fred Punzo*, Chris Farmer arachnids known as harvestmen (Arachnida, Opiliones)
Box5F,DepartmentofBiology, (MacKayetal.,1992).Infact,detailedfieldandlabora-
UniversityofTampa,Tampa,FL33606,USA
tory studies exist for relatively few species of opilionids
(Corey & Taylor, 1987; Elpino-Campos et al., 2001;
and
Hara & Gnaspini, 2003).
Many species of opilionids inhabit mesic habitats
Lisa Dunham
in forests, grasslands and caves, where they are com-
P.O.Box1416,
monly found under or within decaying logs, on the
AppleValley,CA91240,USA
surface or within bark crevices on tree trunks, and
under leaf litter (Rodriguez & Guerrero, 1976; Edgar,
Summary
1990).
Experimentswereconductedtodeterminefecundity,and The harvestman Vonones ornatus (Say) (Opiliones:
to assess the effects of relative humidity (RH) on lower Cosmetidae) occurs throughout central and southern
(LLT50)andupper(ULT50)lethaltemperaturesandcritical Florida where it has been collected from several vegeta-
thermalminima(CTM )andmaxima(CTM ),andthe
min max tive zones associated with pond pine, sand scrub, and
effect of temperature on survival time in adults of the
flatwoodcommunities(Corey&Taylor,1987).Asthere
harvestman Vonones ornatus under controlled laboratory
conditions.Thenumberofeggsdepositedperfemaleranged are few detailed studies on the physiological ecology of
from 27–66 (mean 46.2!6.1SE). Females deposited their opilionids (Edgar, 1971; Hadley, 1994; Punzo, 2000),
eggs within a 7-h period, between 2130 and 0430h, in thisstudywasconductedtodeterminefecundity,andto
several small clusters. The number of clusters per female
assesstheeffectsofrelativehumidity(RH)onlowerand
ranged from 1–4 (mean 2.1!0.2SE); number of eggs per
upper lethal temperatures and critical thermal minima
cluster ranged from 7–32 (mean 15.6!5.8). Females
depositedeggsbetween12and28July.LLT andULT and maxima, and the effect of temperature on survival
50 50
values ranged from 1.73–2.25(C, and 37.04–39.45(C, timeinadultsofV.ornatusundercontrolledlaboratory
respectively, with no significant differences between males conditions.
andfemales.RHhadasignificanteffectonlowerandupper
lethaltemperaturesforbothsexes.Undermesicconditions
(70%RH),adultsexhibitedanabilitytowithstandsignifi-
cantlylowerandhigherambienttemperatures.CTM and Material and methods
min
CTM temperatures for males and females ranged from
max Experimental animals and husbandry
2.33–2.69(C, and 35.31–37.23(C, respectively. As with
lethal temperatures, CTMmin and CTMmax values were Allharvestmenusedinthisstudy,exceptthoseusedto
significantly lower and higher, respectively, under mesic
determine fecundity, consisted of third- and fourth-
conditions,andtherewasnosignificantdifferencebetween
generation individuals which were captive-bred from
thesexesateitherconditionofRH.Harvestmenmaintained
at 23!0.2(C, 69–71% RH, and 12L:12D photoperiod, adults originally collected in 2001–2002. These adults
attainedsexualmaturityafter7nymphalinstars.Longevity were collected in Lettuce Lake Park (LLP) in Tampa,
for adults maintained at 30(C and 70( RH ranged Florida (Hillsborough County, USA). Voucher speci-
from 78.8–83.4 days. Longevity was reduced significantly
menshavebeendepositedintheInvertebrateCollection
for adults tested at 10(C and 70% RH (10.3–13.1 days).
attheUniversityofTampa.LLPconsistsofover400ha
Survival times were significantly reduced when animals
were exposed to xeric conditions as compared with mesic and is bordered on the west by the Hillsborough River,
conditionsatbothtesttemperatures. with adjoining southern hardwood forest and moist
*Towhomallcorrespondenceshouldbeaddressed. hammocks.
18 BrazilianAtlanticForestCtenusspecies
Harvestmen were housed in plastic cages (40"40" placed in the opaque chamber. The number of eggs
30cm) and provided with water ad libitum by placing deposited was recorded for each female and used as an
twowater-soakedcottonballsinaglassdishatoneend index of fecundity.
of the cage. Cages were housed in a climate-controlled
room (23!0.2(C; 69–71% RH; 12L:12D photo-
period regime). Each cage contained 10 adult males Determinationoflowerandupperlethaltemperaturesand
and 10 females, and cage floors were covered with a critical thermal minima and maxima
substrateconsistingofamixedlayerofsphagnummoss
Harvestmen used in all experiments on thermal toler-
and vermiculite. Cages were cleaned with soap and
ances were 6–7 month old adults (from the day of
water on a weekly basis and provided with fresh sub-
emergence from eggs). Experiments for the determi-
strate. A 20-cm long piece of semicircular-shaped syn-
nation of lower (LLT ) and upper (ULT ) lethal
thetic bark strip (Bush Herpetological Supply, 50 50
temperatures were conducted following a modification
Neodosha, Kansas, USA) was placed at the end of the
of the protocol described by Punzo & Rosen (1984),
cageoppositethewaterdish,toprovidearefugeforthe
using Percival Model 816 environmental chambers
harvestmen.
(Boone, Iowa, USA). The door of each chamber con-
Although the natural diet of this species is not
tained a one-way window which allowed observation of
known, adults and subadults were fed ad libitum on a
subjectswithinthechambers.Tosummarise,3replicates
diet consisting of vestigial-winged fruitflies (Drosophila
were run at RH values of 12% (xeric conditions) and
melanogaster), minced earthworms (Lumbricus terres-
70%(mesic).Eachreplicateconsistedof10malesand10
tris) and crickets (Gryllus spp.), and mashed sweet
females subjected to each test temperature. Subjects
potato. Younger nymphal stages and hatchlings were
were tested individually in test chambers consisting of
fedadietconsistingoffirst-instarcrickets(Gryllusspp.)
50-ml vials (Central Scientific, Chicago, Illinois, USA)
andmealwormlarvae(Tenebriomolitor),newlyemerged
whoseopeningswerecoveredwithcheeseclothtoallow
spiderlings (Pardosa spp. and Oxyopes salticus), minced
circulation of air. Food (0.5g of minced earthworms
earthworms, and mashed banana. Food was always put
soaked with water) was placed at the bottom of each
in a circular food dish (2.0cm deep; 5cm in diameter)
vial. For LLT experiments, different subjects were
placed in the open area of the cages. Maintenance and 50
exposed to low temperatures ranging from 1–5(C at
breeding of V. ornatus in captivity have been successful
0.5(Cintervals.Subjectsweremonitoredat1-hintervals
under these rearing conditions (Punzo, unpubl. data).
and time of death was recorded. A subject was con-
Measurements of total body length and width
sidereddeadifitwasfoundonitssideordorsalsurface
weremadetothenearest0.1mmusingaUnitronModel
and failed to respond to gentle prodding with a glass
1100 dissecting microscope fitted with an ocular
stirring rod. LLT s were determined using standard
micrometer. 50
probitanalysis(Finney,1971).Identicalprocedureswere
used to determine ULT s, with the exception that
50
Determination of fecundity subjects were exposed to high temperatures ranging
from 30–55(C, at 12 and 70% RH.
All experiments described in this paper were con-
TheprotocoldescribedbyPunzo&Jellies(1983)was
ducted between 7 July 2004 and 17 August 2005.
used to determine critical thermal minima (CTM )
Fecundity was determined from females collected in min
and maxima (CTM ). To summarise, CTM and
their natural habitat and brought into the laboratory. max min
CTM were defined as the low and high temperature,
Between25Juneand5July2004,264adultfemaleswere max
respectively,atwhichlossoftherightingreflexoccurred.
collected in LLP, brought back to the laboratory, and
For CTM determinations, environmental chambers
housed individually in identical plastic cages min
were programmed to start at 10.0(C and decrease to
(17"17"15cm). Cages were provided with the same
#2.0(C at 1(C/min. For CTM determinations,
substrate as described above, and the females were max
chambers were programmed to start at 30(C and
maintainedonthesamedietandunderidenticalclimatic
increase to 60(C at 1(C/min. For both CTM and
conditions as described previously for adults. A cube- min
CTM experiments, different groups of subjects
shaped opaque chamber (7cm per side), containing a max
were tested under conditions of 12 and 70% RH. We
small circular opening (1.5cm in diameter), was placed
conducted four replicates at each RH, each replicate
inonecornerofeachcage.Theopeningofthechamber
consistingof20malesand20females.Ineachreplicate,
faced the centre of the cage. The floor of this chamber
subjects were placed individually in 50-ml condiment
was provided with moistened blotting paper which pro-
cups with a water-soaked cotton ball placed at the
vided a substrate on which females could deposit their
bottom of each cup to provide a saturated atmo-
eggs.PreviouspilotstudieshadshownthatfemalesofV.
sphere.Subjectsweremonitoredcontinuouslyuntilthey
ornatus always deposited their eggs in these darkened,
collapsed (knock-down or torpor point).
moistchambersratherthanontheopenfloorofthecage
(Punzo, pers. obs.). The blotting paper was checked
daily for the presence of eggs. If no eggs were present,
EffectoftemperatureandRHonsurvivaltime(longevity)
thepreviouspaperwasremovedandreplacedwithfresh,
moistenedblottingpaper.Ifeggswerepresent,theywere Adult males and females were obtained from
removed and counted, and fresh blotting paper was laboratory-bred cultures as described previously. In
F.Punzo,C.Farmer&LisaDunham 19
theseexperiments,testingbeganonthedayimmediately Lethal temperatures and critical thermal minima and
following the animal’s moult to the adult stage. In this maxima
species, adults can be readily sexed as males have
ResultsforLT determinationsareshowninTable1.
markedly longer palpi than females, with a distinctive 50
LLT andULT valuesrangedfrom1.73–2.25(C,and
spur on the femur. Based on results obtained for lethal 50 50
37.04–39.45(C, respectively. No significant differences
temperature determinations, the following stressful (but
were found between males and females (p>0.60). How-
non-lethal) ambient test temperatures were chosen:
ever, RH had a significant effect on lower and upper
10(C and 30(C. Subjects were exposed to these tem-
lethal temperatures for both sexes. Under mesic con-
peratures under xeric (12% RH) and mesic (70% RH)
ditions (70% RH), adults exhibited an ability to with-
conditions,inenvironmentalchambersidenticaltothose
stand significantly lower and higher ambient
used in the experiments on upper and lower lethal
temperatures (p<0.05).
temperatures.Animalswereplacedindividuallyin50-ml
CTM and CTM values for males and females
glass vials and were monitored daily to determine mean min max
are also shown in Table 1 and ranged from 2.33–2.69,
survival time in days as a function of temperature and
and 35.31–37.23(C, respectively. As with lethal tem-
RH. Subjects were considered dead when they were
peratures, CTM and CTM values were signifi-
foundlyingmotionlessonthebottomofthevialanddid min max
cantly lower and higher, respectively (p<0.05), under
notrespondwhenproddedwithaglassstirringrodafter
mesicconditions,andtherewasnosignificantdifference
a period of 24h.
between the sexes at either condition of RH (p>0.50).
Statistical analysis
Survival time for adults
All statistical tests followed procedures as discussed
Theseharvestmen,maintainedat23!0.2(C,69–71%
bySokal&Rohlf(1995).ABartlett’stestforhomogen-
RH, and 12L:12D photoperiod, attained sexual
eity of variances was performed on all data and indi-
maturity (adulthood) after 7 nymphal instars. The
cated that parametric tests were appropriate because
effects of ambient test temperature and RH on survival
assumptions of normality were met.
timeforadultsareshowninTable2.Meanlongevityfor
adult harvestmen maintained at 30(C and 70% RH
ranged from 78.8–83.4 days. Longevity was reduced
Results
significantly for adults tested at 10(C and 70% RH
Deposition of eggs and fecundity (10.3–13.1 days). Survival times for both sexes were
significantlyreducedwhenanimalswereexposedtoxeric
Allfemalesthatdepositedeggs(n=60)wereofsimilar
conditions as compared with mesic conditions at both
size (body length: mean 5.5mm!0.05SE, range 5.3–
test temperatures (p<0.01). There was no significant
5.7mm; body width: mean 4.2mm!0.03SE, range
difference in mean survival times between males and
4.0–4.4). At 23!0.2(C, 65–70% RH, and 12L:12D
femalesatambienttemperaturesof10(Cor30(Cunder
photoperiod regime, the number of eggs deposited per
xeric or mesic conditions (p>0.50).
female ranged from 27–66 (mean 46.2!6.1SE). All
females deposited all of their eggs within a 7-h period,
between2130and0430h(EasternStandardTime,EST),
Discussion
in several small clusters on the blotting paper. The
number of clusters per female ranged from 1–4 (mean One important aspect affecting the survival of terres-
2.1!0.2SE); the number of eggs per cluster ranged trial arthropods is the extent of their ability to resist
from 7–32 (mean 15.6!5.8). Females deposited eggs desiccation by limiting evaporative water loss (EWL)
between 12 and 28 July. (Edney, 1977; Hadley, 1994; Punzo, 2000). These ani-
Lethaltemperatures((C)
LLT ULT
50 50
%RH Males Females Males Females
12 2.21!0.05a 2.25!0.03a 37.04!0.42a 37.22!0.39a
70 1.73!0.03b 1.79!0.02b 39.12!0.47b 39.45!0.22b
Criticalthermalminima(CTM ) Criticalthermalmaxima(CTM )
min max
Males Females Males Females
12 2.68!0.03a 2.69!0.02a 35.31!0.19a 35.47!0.29a
70 2.33!0.04b 2.36!0.03b 37.15!0.27b 37.23!0.33b
Table 1: Lower (LLT ) and upper (ULT ) lethal temperatures, and critical thermal minima and
50 50
maxima, for adult males and females of the harvestman Vonones ornatus under xeric (12%
relative humidity, RH) and mesic (70% RH) conditions. Data are expressed as means!SE.
Values in columns followed by different letters are significantly different (p<0.05). Level of
significancecomputedusingaStudentt-test.
20 TemperatureandhumidityeffectsinVononesornatus
Survivaltime(days)
Ambienttesttemperature
10(C 30(C
%RH Males Females Males Females
12 2.6!0.3a 3.1!0.2a 33.7!6.2c 36.5!5.8c
70 10.3!2.4b 13.1!3.3b 78.8!8.6d 83.4!4.8d
Table2: Effectsofambienttemperatureandrelativehumidity(RH)onsurvivaltime(indays)ofadults
ofVononesornatus.Dataexpressedasmeans!SE.Valuesinrowsandcolumnswithdifferent
lettersandsymbolsaresignificantlydifferent(p<0.01)basedonStudentt-tests.
mals lose water through their cuticle and/or respiratory regions in Australia are strongly nocturnal in their diel
surfaces. Because ambient temperature and relative periodicity and frequently aggregate near ephemeral
humidity both affect rates of EWL, terrestrial arthro- pools of water and permanent sink holes. Although
pods must respond effectively to gradients of tempera- some species of neotropical harvestmen have been
tureandmoisturewithintheirmicrohabitatsinorderto reported to form intra- or interspecific aggregations
survive. It is well known that the integument represents consisting of several to many individuals (Machado
amainavenueforEWLinarthropods,andthedegreeof et al., 2000; Elpino-Campos et al., 2001: Pereira et al.,
cuticular permeability determines, to a large extent, the 2004), no instances of aggregation were observed for
rate at which EWL occurs (Punzo, 1991a). Terrestrial V. ornatus in central Florida.
arthropods utilise various behavioural mechanisms of A relatively detailed study of a Nearctic harvestman
thermoregulation to escape potentially stressful con- was conducted by MacKay & Grimsley (1992) on
ditions (Cloudsley-Thompson, 1991; Punzo, 1998). TrachyrhinusmarmoratusBanks,aspeciesfoundinNew
For example, many terrestrial arthropods avoid high Mexico (northern Chihuahan Desert) in a variety of
ambient temperatures by nocturnal activity patterns. habitats including tobosa-black grama grasslands and
Otherssimplymoveawayfromanareawhenunfavour- desert shrub consisting predominantly of creosote-
able conditions manifest themselves. Some respond to lechuguilla plant associations. Rainfall is typically
harsh conditions by burrowing beneath the ground <25cm/year, and occurs primarily in late spring, late
surface or seek refuge under leaf litter, logs, or rocks. summer, and early autumn. Air temperatures range
Because opilionids typically have lightly sclerotised from <0(C in winter to over 40(C in late summer.
cuticles that are not particularly effective as barriers to Population densities peaked by late May, and then
EWL (Edney, 1977; Edgar, 1990), they are especially declinedrapidlyduringthehottestmonths.Mostadults
susceptible to desiccation. mated in early May, before spring rains, and deposited
The results of the present study show that survivor- their eggs most frequently within 36–72h after spring
ship and longevity of V. ornatus adults are significantly rains. With the appearance of higher ambient tempera-
reduced when they are exposed to xeric conditions at tures in late May, surface activity declined sharply.
both lower and higher temperatures. This is in agree- Thus,eventhisdesertspeciesdepositsitseggsincooler,
ment with previous observations that opilionids are moister seasons. Although detailed information on the
typically found in microhabitats where moisture levels phenologyofV.ornatusfromFloridaislacking,prelimi-
arehigh(Phillipson,1959;Juberthie,1965;Edgar,1971, nary observations indicate that, although juveniles are
1990;Rodriguez&Guerrero,1976;MacKayetal.,1992; most abundant in mid-spring and adults in August
Gnaspini, 1995). In a previous study, Corey & Taylor (Punzo, pers. obs.), because of the high rainfall levels in
(1987) showed that several species of harvestmen from the state harvestmen can be found throughout most
central Florida, including V. ornatus, were strongly months of the year.
nocturnal and most frequently found in moist micro- Inviewofthepaucityofinformationavailableonthe
habitats: under moist leaf litter, within moist decaying physiological ecology of harvestmen, future studies
logs, and on the surface of leaves containing numerous shouldfocusonthecombinedeffectsoftemperatureand
water droplets following summer rains. This is also in RH on survivorship of embryos, rate of embryonic
linewithresultsofthepresentstudyshowingthatadults development,postembryonicgrowthrates,ageatsexual
of V. ornatus exhibited a greater resistance to low and maturity, fecundity, temperature preferenda, rate of
hightemperaturestressundermesicconditions,andisin cuticular water loss, metabolic rate, and longevity.
generalagreementwithpreviousstudiesonotherterres- Furthermore, these studies should include individuals
trial arthropods, including spiders (Pulz, 1987; Punzo, from different taxa that are found in a diversity of
1991b), insects (May, 1985; Nation, 2002), centipedes microhabitats.
(Cloudsley-Thompson,1991),andmillipedes(Crawford,
1981, 1991; Punzo, 2000).
Acknowledgements
In species of harvestmen that inhabit desert regions
characterised by high seasonal temperatures and low We thank B. Garman for consultation on statistical
relativehumidity,eggdepositionmostfrequentlyoccurs analyses, L. Ludwig, J. Price, P. Merrett, and anony-
during rainy seasons (Crawford, 1981; Cokendolpher, mous reviewers for commenting on an earlier draft of
1990). Hunt (1991) reported that harvestmen from arid the manuscript, and M. Lopez for assistance in main-
F.Punzo,C.Farmer&LisaDunham 21
taining animals in captivity. Much of this work was MACHADO, G., RAIMUNDO, R. L. G. & OLIVIERA, P. S.
madepossiblethroughaFacultyDevelopmentGrantto 2000: Daily activity schedule, gregariousness, and defensive
behaviour in the harvestman Goniosoma longipes (Opiliones:
FP from the University of Tampa.
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