Dry Weight Of Fresh And Preserved Spiders (Araneida: Labidognatha)

9 Pages·1998·2.4 MB·English

by  R L Edwards

66 ENTOMOLOGICALNEWS
DRY WEIGHT OF FRESH AND
PRESERVED SPIDERS
(ARANEIDA: LABIDOGNATHA)1
RobertL.Edwards2 WendyL.Gabriel3
,
ABSTRACT: Dataonthedryweightfor19taxa(suborderLabidognatha)offreshandpreserved
spidersarepresented. Thevariationinweightatlengthforindividualspeciesisalsoprovided.
WiththenotableexceptionofthegenusTetragnatha,FamilyTetragnathidae,theFamilyTheridi-
idae,andmostofthegeneraoftheFamilyThomisidae,thefamiliesexaminedaresimilartoone
another in their weight-length relationships and are not readily separable on that basis. Dry
weightcanbe approximatedusing eitherthe weightoffresh specimensorpreserved material.
The'typical'spider,basedonthematerialexamined,increasesinlengthfasterrelativelythanit
increasesinweight,anddry weightdecreasesrelativetofreshweightaslengthincreases.
With the exception ofthe papers by Clausen, 1983, that included dataon
fresh and dry weight-length relationships fornine species (five families), and
Breymeyer, 1967,forthree speciesofthefamilyLycosidae, there isverylittle
general information availableonthedry weightofspiders. Thisreportserves
to increasethe informationavailableonspiderweightandexploresthedegree
ofdifference betweentaxafrom the weight-length perspective.
MATERIALAND METHODS
Thebulkofpreservedmaterialwascollectedin 1989and 1990intheFrances
CraneWildlifeManagementarea,Hatchville,FalmouthTownship,Barnstable
County(CapeCod),Massachusetts,inconnectionwithanotherstudy(Edwards
1993). Collection details are provided therein. All were preserved in 75%
denatured ethanol and all had their alcohol replaced at least once, typically
within48hoursofcollection. Thetotallengthwasmeasuredfromtheclypeus
to the distal end of the abdomen using an ocular micrometer for specimens
<12mmintotallengthandverniercalipersforthose>12mm. Thetotallength,
as described above, was measured to the nearest 0.1 mm, and the specimens
damp dried on absorbent paper before weighing. Obviously distorted speci-
menswerenotused. Inthosecaseswhere thepedicelhadelongated,thesepa-
ration ofthe thorax from the abdomen was measured and the total measure-
mentcorrected accordingly.
ThefreshmaterialforthisstudywascollectedinthemonthsofJunethrough
September, 1996, from the same area and habitats as the preserved material
with one exception. The collection ofLeucauge regnyi Simon, Family Tetra-
gnathidae, was made in PuertoRico inSeptember, 1996. Allcollectionswere
1Received April9, 1997. AcceptedJuly7, 1997.
2ResearchAssociate,UnitedStatesNational Museum, Box505,WoodsHole,Ma.02543
3WendyL.Gabriel, NortheastFisheriesCenter, 166WaterSt.,WoodsHole,Ma. 02543
ENT.NEWS 109(1): 66-74,January&February, 1998
Vol. 109,No. 1,January&February, 1998 67
made in the afternoon, the spiders were immobilized in an ethyl acetate col-
lecting jar, identified and measured that day, refrigerated overnight at 3 C,
and weighed the following day on a MettlerA200 balance, accurate to 1 mg.
Followingthis,thematerialwasoven-driedatatemperatureof40 Cforseven
days. Tocheckthe efficacy ofthe drying regime, three samplesof50ormore
mixedspeciessamplesweredriedforanadditional sevendays,withthe great-
estadditional lossofweightobservedofless than 3% in all cases. Specimens
ofall species used in this study have been deposited in the United States Na-
tional Museum.
RESULTS AND DISCUSSION
The families, genera, number of individuals weighed, and the range of
total lengths are provided inTable 1. Atotal of2,315 measurements offresh,
dry fresh, and preserved specimens representing 78 genera and 17 families
were made (Table 1). Note in Fig. 1 that the various taxa are identified with
the firstfourletters ofthe taxon as listed inTable 1.
0.25
FRESH
O DRY FRESH
DRY PRESERVED
-f
DC 0.2-
O
O
o.i5-
t
o.H
Q
O
O
0.05-
n a a a
a a + a a a a
THOMTMAR LINY LEUC ARAN LYCO SALT PISA PHOL TETR
THEROXYO PHIL ANYPAMAUCLUB CORI AGEL GNAP
TAXA
Figure 1.ConditionfactorsforthetaxalistedinTable 1,usingtheequation k=(aLh)IL3. The
taxaareidentifiedusingthefirstfourlettersofeachtaxon. Therearevaluesforeachtreatment
(fresh,dry fresh,drypreserved) withtheexceptionofAMAU,TMAR, PHOL,andOXYO,for
whichtherewasnodrypreserveddataavailable.
68 ENTOMOLOGICALNEWS
Thelogtransformedleastsquaresequation,Inweight=lna+b(lnlength),
was used to estimate weight (mg) at length (mm). The statistical parameters
are provided in Table 2. The coefficient of determination (r2) ranged from
0.743 (Oxyopidae, dry fresh) to 0.985 (Corinnidae, fresh). Averages, mini-
mum, and maximum values are listed forall parameters. The average values
for coefficient In a (intercept) varied considerably and were greatest
(-1.976) for fresh and least for dry preserved specimens (-3.580), while the
average values for exponent b (slope) varied relatively little, from 2.739 for
dry preserved to 2.800fordry fresh specimens (Table 2).
Thenumberofindividualsandgeneraobtainedforeachfamilyvariedcon-
siderably (Table 1). It is clear from the statistical parameters presented in
Table 2 that there was little difference in the weight-length relationships be-
tween taxa. To compare weight at length using different taxa, while taking
intoconsiderationthe differing length rangesoverwhich theparameters were
estimated, we calculated thecondition factorforeach taxon,k = (aLb)IL3 (L
= mid-point of lengths (mm) in sample, a = Exponent In a) and the results
showninFig. 1. Ofthe 19taxathegenusTetragnathaLatreille(Tetragnathidae),
the family Theridiidae, and the rotund crab-like members of the family
Thomisidae(generaXysticusC.L.Koch,OzyptilaSimon,MisumenaLatreille,
Misumenops F.O.P.-Cambridge, and Misumenoides F.O.P.-Cambridge) stand
apart from the rest. The genus Tmarus Simon, family Thomisidae, is rela-
tively slender (less crab-like and rotund), differing in this respect from the
other genera ofthe family Thomisidae listed above and is plotted separately
(TMAR). Similarly,thelesselongategeneraLeucaugeWhite,andPachygnatha
Sundevall,familyTetragnathidae,differinbodyformfromtheelongatemem-
bers ofthe genus Tetragnatha and are also plotted separately (LEUC). The
uniqueness ofTetragnatha was notedby Greenstone, et. al. 1985.
Breymeyer (1967) in a study concerning the dry weight ofpreserved spi-
ders reported that alcohol dissolves and extracts some parts ofspiderbodies.
Thisappearstobethecase inthisstudyaswell. Itisworthnotingthatundried
specimenspreservedindenaturedalcoholweighconsiderablymorethanfresh
material (Edwards 1996). Clausen (1983, p. 143-144) noted that"the ratioof
dry over wetweight increases with decreasing size of specimens", and sug-
gested that "With decreasing size, the exocuticle may make up a relatively
greater part of the animal's weight because of the relatively greater surface.
And, there may be a minimum thickness ofthe cuticle, which, in effect, will
givethe sameresult." TotestClausen's (op.cit.) suggestionfurther,theentire
data set available was examined by treatment, i.e. fresh, dry fresh and dry
preserved. The statistical parametersforeach treatmentare provided inTable
3.Thepercentoffreshweightfordryfreshanddrypreservedmaterial isshown
in Fig. 2. The results are consistent, forthe average spider, with the observa-
tions of Clausen (op. cit.), with the caveat that more than the cuticle is un-
Vol. 109,No. 1,January&February, 1998 69
y
0.35
LJJ
X
CO
LJJ
DC
0.25-
oLJJ
0.2-
DC
LJJ
Q.
0.15
10 15 20
LENGTH
(MM)
DRY FRESH DRY PRESERVED
Figure 2. Percent offresh weight represented by dry fresh weight and dry preserved weight,
basedonaveragedataforeachtreatment(Table3).
doubtedly involved since drying does not reduce the specimens to the cu-
ticle only. Other tissues are involved which may also vary in the degree to
which they are present indifferent taxa.
Thevariability seenin individual species intheratioofdryweighttofresh
weightwasexaminedfor 11 speciescollectedinOctoberandNovember, 1996.
The individual speciescollectionswereeachmadewithinanhourinrestricted
localities to reduce environmental variabilities as much as possible. The col-
lectionswere treatedandanalyzedasdescribedearlier(seeTable 4). Onaver-
age the ratio ofdry to fresh weight was 0.314 0.038, varying from 0.250 to
0.390. The average slope (b) ofdry on fresh weight was slightly in excess of
1 with two notable exceptions, that of Pardosa lapidicina (b = 1.224) and
Phidippusclarus(b= 1.537). Theaverage intercept(Ina)valuesforthesetwo
species were also well in excess of the average value, -2.015 and -3.161 re-
spectively. Bothofthese collectionswereofimmature individualsthat would
have matured the following year, although the Tmarus angulatus collection,
alsoofimmature individualsdue to mature the following year, did not have a
similar departure from the average values. The average r2 for the weight-
lengthregressionsofthese specieswas0.892forfresh and0.864fordryfresh.
70 ENTOMOLOGICALNEWS
For studies requiring precision, e.g. those of a single or a set of closely
related species, it would be best to use a sclerotized part ofthe body such as
the head capsule, to reduce the problems associated with measurement error
(cf. Jocque 1981). Whatevermethod is used, it is obvious that the weightof
individual spiders is highly variable.
Inonesurvey(Edwards 1993)over 12,000specimenswerecollected. Many
ofthese were archived against the future. It was encouraging to find out that
preservedmaterialalsoservedthepurposeofrealisticallyestimatingdryweight.
Table 1. Freshandpreservedspidersexamined. Numberofindividuals=n,numberofgenera-
genera,lengths(mm)includedinsample-range. FamilyTetragnathidaeissubdividedintothe
genusTetragnathaandasecondcategoryofrounderbodyforms,includingthegeneraLeucauge
and Pachygnatha, listed as Leucauge. The family Thomisidae includes all genera sampled
exceptthegenusTmaruswhichislistedseparately.
Fresh Fresh,dry Preserved,dry
Taxon n genera range n genera range n genera range
Agelenidae
Vol. 109,No. 1,January&February, 1998 71
Table2. Statistical parametersforspiderweight-lengthequations(Inweightug=Ina+b(In
lengthmm),forfresh,dryfreshanddrypreservedmaterial. No.ofindividuals-n,standarderror
-SE.,coefficientofdetermination-r%exponentofIna=ExpIna.
Freshspecimens
Taxon
72 ENTOMOLOGICALNEWS
Vol. 109,No. 1,January&February, 1998 73
s g -^
74 ENTOMOLOGICALNEWS
ACKNOWLEDGMENTS
TheassistanceofEricandAnnabel Edwardsin the fieldwasgreatlyappreciated.Thecom-
ments and suggestions ofJess Zimmermann, University ofPuerto Rico, Los Piedras, and Stan
Sady, MassachusettsGeneral Hospital, Boston, whokindlycritiquedthe initial manuscriptwere
mosthelpful. Wethankthetwoanonymousreviewersfortheircommentsandsuggestions. Nancy
McHugh,NortheastFisheriesCenter,WoodsHole,kindlymadelaboratoryfacilitiesavailable.
LITERATURECITED
Breymeyer,A.1967.Correlationsbetweendryweightofspidersandtheirlengthandfreshweight.
Bull.acad.Pol. Sci. CI-II,XV(5):263-265.
Clausen, I.H.S. 1983.Weight-length relationsofeight speciesofspiders(Araneae) fromDen-
mark.Ent. Meddr50:139-144.Copenhagen,Denmark 1983.
Edgar,W. D. 1971. Seasonal weightchanges, age structure, natalityandmortality inthe wolf
spiderPardosalugubrisinCentral Scotland. Oikos22:84-92.
Edwards,R.L. 1993.Canthespeciesrichnessofspidersbedetermined?Psyche. 100:(3-4)185-
208.
Edwards,R. L. 1996. Estimating live spiderweightusing preservedspecimens.J.Arachnol.
24:161-166.
Greenstone,M.H.,C.E.MorganandA.Hultsch. 1985. Ballooningmethodology: equations
forestimatingmassesofsticky-trappedspiders.J.Arachnol. 13:225-230.
Jocque,R. 1981. Sizeandweightvariationsinspidersandtheirecologicalsignificance.Biol.
Jb.Dodonaea. 155-165.