Table Of Content2003. The Journal of Arachnology 31:428-432
SHORT COMMUNICATION
WATER SURFACE LOCOMOTION BY SPIDERS:
DISTINCT GAITS IN DIVERSE FAMILIES
Robert B. Suter: Department of Biology, Vassar College, 124 Raymond Avenue,
Poughkeepsie, New York 12604 USA. E-mail: [email protected]
Gail Stratton: Department of Biology, University of Mississippi, University,
Mississippi 38677 USA
Patricia R. Miller: Department of Biology, Northwest Mississippi Community
College, Senatobia, Mississippi 38668 USA
ABSTRACT. Pisaurids such as Dolomedes triton (Walckenaer 1837) are well known as inhabitants of
ponds and streams and are adept at locomotion on the water surface. In a broad survey of water surface
locomotion in spiders, we have found that mosttaxado not use specializedgaits underthesecircumstances.
However, some tetragnathids, araneids, and salticids (three families that are outside of the superfamily
Lycosoidea to which the pisaurids belong) resemble D. triton to the extent that they do use specialized
gaits when on the water surface. Ofthese, the tetragnathids are particularly accomplished at water surface
locomotion, achieving velocities that exceed those of D. triton when it rows, but not when it gallops.
Keywords! Aquatic locomotion, gaits, Pisauridae, Tetragnathidae, Philodromidae, Salticidae,Araneidae,
Lycosidae, Gnaphosidae
Spiders in the family Pisauridae typically fre- in some taxa the walking gait characteristic ofmost
quent the edges of bodies of water and many of terrestrial spiders on land can work very well on
them rely on the water surface for foraging, escape the water surface. In what follows, we describe the
from predators, and the detection and pursuit of specialized gaits of Tetragnatha spp. Latreille 1804
mates. Forthese spiders, the perception ofbeing on (Araneae, Tetragnathidae), Dolomedes triton (Walc-
the surface of water elicits either a rowing gait or kenaer 1837) (Araneae, Pisauridae), two jumping
a galloping gait (Shultz 1987; Barnes & Barth spiders, Habronattns hallani (Richman 1973) and
1991), the former used exclusively on water and Phidippiis sp.C.L. Koch 1846 (Araneae, Salticidae),
under conditions that do not require rapid locomo- two orb-weavers, Neoscona sp. Simon 1864 and
tion, and the latterused when speed is required. The Larinioides sp. Caporiacco 1934 (Araneae, Aranei-
natural contexts in which pisaurids use these gaits dae), and Tibelliis sp. Simon 1875 (Araneae, Phil-
(McAlister 1959; Shultz 1987; Gorb & Barth 1994; odromidae). We follow those descriptions with an
Suter & Gruenwald 2000a, b) and theirbiomechan- analysis of the efficacy of the gaits in contrast to
ical properties (Suter et al. 1997; Suter & Wildman the walking gaits used on water by Geolycosa ro-
1999) suggest that the ancestors of the pisaurids gersi Wallace 1942 (Araneae, Lycosidae) and Her-
succeeded in part because of the efficacy of these pyllus ecclesiasticus Hentz 1832 (Araneae, Gna-
specialized gaits. A number ofother spiderfamilies phosidae). Voucher specimens are deposited in the
also have representatives that frequent the edges of Mississippi Entomological Museum at Mississippi
ponds and streams (e.g., Tetragnathidae) and still State University, Starkville, Mississippi.
others, while strongly terrestrial in habit, must con- We studied the locomotion of the spiders by re-
tend with periodic flooding (e.g., gnaphosids). leasing each into aglass orplastic arenawith afloor
In the context ofa broad study ofthe occurrence area of approximately 0.16 m^ covered to a depth
of pisaurid-like aquatic locomotion in the Araneae, of at least 2 cm with water (tap water whenever
we have noticed (a) that several families outside of distilled water was unavailable; surface tension dif-
the Lycosoidea(which includes the Pisauridae)con- ferences between distilled and tap water are negli-
tain members that exhibit a specialized aquatic gait gible in this context). Video images of the move-
different from that used by pisaurids, and (b) that ments of each spider, captured from directly above
428
SUTER ET AL.—AQUATIC LOCOMOTIVE GAITS IN DIVERSE TAXA 429
—
Table 1. Gaits used in locomotion across the water surface. When a specialized gait is not employed
all of the time, the usual alternative in each case is a gait resembling walking and thus scored as 0 0 0 0
(see text for explanation of scoring). Video clips of the aquatic locomotion of D. triton, Tetragnatha spp., a
salticid, and G. rogersi, can be viewed at the following website: http://faculty.vassar.edu/suter/comparisons/.
*For Tibellus, we are skeptical that this constitutes a gait specialized for aquatic locomotion (see text).
Leg-pair Incidence of
Taxon Specialized aquatic gait score specialized gait
D. triton Pisauridae Legs I and IV held motionless; legs III and II, in 1111 Always
that order, provide rowing propulsion, mem-
bers of each pair sweeping in unison
Tetragnatha spp. Te- Legs I sweep out and to the side, alternating, and 0 0 0 0 Always
tragnathidae provide propulsion; legs II, III, and IV move
in alternation
Tibellus sp. Philod- Legs I, III, and IV move in alternation; legs II 0 10 0 Always*
romidae sweep out and to the side, in unison, but ap-
pear not to provide more propulsive power
than do the other legs
Pooled salticids Saltic- Legs III, in unison, sweep out and to the side, 1111 Occasional
idae providing rowing propulsion; legs I and II also
sweep, but with little power; legs IV trail be-
hind
Pooled araneids Ara- Legs II and I, in that order, provide rowing pro- 1111 Rare
neidae pulsion, members of each pair sweeping in
unison; legs III also sweep in unison, but with
little power; legs IV trail behind
G. rogersi Lycosidae (terrestrial walking) 0 0 0 0 Never
H. ecclesiasticus Gna- (terrestrial walking) 0 0 0 0 Never
phosidae
the arena, were digitized at 250 frames/sec by a terized as involving pair-wise rowing motions by
high-speed motion analysis system (MotionScope S legs III and II, in that order, while legs I and IV are
series, Redlake Imaging Corporation) and then re- held approximately motionless in front of and be-
corded in VHS format at 30 frames/sec (JVC HR- hind the spider, respectively. In contrast, when D.
S5400U). The movements ofthe spiders, slowed to triton walks on land, it employs an alternating tet-
&
about 1/8 the natural speed, could be qualitatively rapod gait (Barnes Barth 1991; Shultz 1987) in
analyzed on a video monitor. We performed more which the members of a leg pair move alternately
detailed measurements (e.g., of absolute velocity) and all eight legs are involved in propulsion. When
via frame-by-frame analysis in NIH Image, an im- D. triton gallops across the water surface, only legs
age analysis shareware product available from the IV remain relatively motionless while the three an-
National Institutes of Health (http://rsb.info.nih. terior pairs of legs stroke approximately in unison.
gov/nih-image/). Detailed descriptions of the mea- Thus in both ofD. triton's aquatic gaits, legs I are
surement method have been published elsewhere non-alternating (they are either still ormove in uni-
(Suter et al. 1997; Suter & Wildman 1999). son), legs II and III move in unison, and legs IV
In our qualitative analyses, we considered the are non-alternating. We found it useful in visualiz-
following: for each pair of legs, did the members ing these gaits and in comparing other gaits to
of the pair move in unison or alternately, or were them, to assign the value 1 to a pair of legs if the
the legs held motionless relative to the body ofthe members ofthe pair moved in unison or were still,
spider? In our analyses of velocity, we used the and the value 0 to a pair of legs that moved alter-
linear measurement tools in NIH Image to deter- nately. Using that set ofrules, both rowing and gal-
mine the distance traveled over several strides dur- loping by D. triton are scored as 1 1 1 1, whereas
ing relatively straight-line locomotion, and divided walking on land by the same animal is scored as 0
that distance by the elapsed time as shown in ms 0 0 0
.
on the recorded high-speed images. Table 1 provides not only brief descriptions of
The rowing gait ofD. triton, with which we were the aquatic gaits employed by the species in this
already familiar from earlier studies (e.g., Suter et study but also leg-pair scores for these gaits. Note
al. 1997; Suter & Wildman 1999), can be charac- that the scores shown in the table ignore variability
430 THE JOURNAL OF ARACHNOLOGY
within and between individuals because the purpose of these legs affect aquatic locomotion (c.f., crite-
ofthe scores in this paper is to provide a shorthand rion b). We studied videotapes of Tibellus'^ loco-
characterization of specialized aquatic gaits rather motion on land and observed precisely the same
than to evaluate the frequency with which they are pattern of leg motion (c.f., criterion a). We have
used. A detailed analysis ofgait variability will ap- tentatively concluded that, forthis spider, legs IIare
pear elsewhere (Stratton et al. in preparation). We used as antennae, monitoring the environment as it
have included in the table a column to provide a passes by to either side. Certainly we see no evi-
qualitative estimate of the incidence of each spe- dence that these legs function in propulsion on the
cialized gait. Video clips ofthe aquatic locomotion water surface.
of representative taxa used in this study can be Absolute and relative velocities give some indi-
viewed at the following website: http://faculty. cation of the efficacy of locomotion, that is, how
vassar.edu/suter/comparisons/. well it works. For the spiders in this study mean
Weconsidered agait to be specialized foraquatic velocities ranged between 0.019 ± 0.010 m/s (mean
locomotion if (a) it differed qualitatively from the ± S.E.) and0.172 ± 0.015 m/s (Fig. 1), andrelative
gait used on land and (b) we could detect in the velocities ranged between 2.74 ± 1.26 body lengths
motions of the spider evidence that the novel leg per second and 29.13 ± 2.58 bl/s (Fig. 2). A para-
motion produced thrust. On land, for example, Te- metric ANOVA on the absolute velocity data re-
tragnatha spp. move rather awkwardly, propelling vealed significant {P = 0.0001) between-taxa vari-
forward in the alternating tetrapod gait that char- ation. Post-hoc tests (Fisher’s PLSD) showed that
acterizes the locomotion of most spiders on solid Tetragnatha spp was significantly faster {P < 0.05)
substrates (Barnes & Barth, 1991; Shultz, 1987). than each ofthe other taxa except Tibellus sp., and
On the water surface, the spider uses its legs in the that D. triton was significantly faster {P < 0.05)
same sequence (the members of each contralateral than the salticids, the araneids, G. rogersi and H.
pairoflegs alternating), but the locomotion is dom- ecclesiasticus. There were no significant absolute
inated by the alternating backward sweeps of the velocity differences among the five taxa excluding
two anterior legs. Their motion is in the plane of Dolomedes and Tetragnatha.
the watersurface (on land, theirmotion is in aplane An ANOVA on the relative velocity data re-
nearly perpendicular to the substrate) and causes a vealed significant {P = 0.0001) between-taxa vari-
pronounced, rhythmic “wag” in the animal’s elon- ation. Post-hoc tests showed that Tetragnatha spp.
gate body as the spider surges forward with the was significantly faster {P < 0.05) than each ofthe
stroke of each leg. Thus the gait is qualitatively other taxa, and that there were no other significant
different from what is used on land and the sweep relative velocity differences.
of each anterior leg causes a noticeable change in Our assumption at the start of our investigation
the motion of the spider (in this case, both a wag was that spiders such as Tibellus sp., G. rogersiand
and a surge forward). H. ecclesiasticus, which have the same walkinggait
Criteria (a) and (b) were met by Dolomedes tri- on land and on the water, would underachievewhen
ton, Tetragnatha spp., by the salticids and by both their aquatic locomotion was compared to that of
species of araneids used in this study (note that spiders with specialized gaits. Why, otherwise,
many of the salticids and araneids we tested in the would specialized gaits have evolved? That ratio-
broad survey did not adopt qualitatively different nale for the presence of specialized gaits is sup-
gaits than those used on land). Among these, Te- ported by our data on Tetragnatha spp. and D. tri-
tragnatha spp. stands out because their aquatic gait ton, both of which perform significantly better in
involves legs in alternation, whereas the aquatic terms ofabsolute velocity than do the two non-spe-
gaits ofthe others all involve propulsion by contra- cialists on which we have adequate data {Tibellus
=
lateral legs stroking in synchrony (Table 1). sp., with n 2, deserves to be ignored). Additional
In contrast, three of the species in this study, Ti- support comes from the Tetragnatha spp. data on
bellus sp., G. rogersi and H. ecclesiasticus, have relative velocity. Given the fact that D. triton can
only one gait on the water surface, and it is the also call upon a second aquatic gait, galloping (Su-
same walking gait that these spiders use on land. ter & Wildman 1999; Fig. 1 inset) when high speed
Thus, criterion (a) was not met by these spiders. In is required, it is reasonable to claim that the data
this respect, Tibellus is particularly interesting be- from both ofthese “specialists” supports the adap-
cause its aquatic gait resembles rowing with legs II. tionist rationale for specialized aquatic gaits.
On the watersurface, this spiderwalks with sixlegs In contrast, the specialized aquatic gaits used
(I, III, IV) in the alternating hexapod gait that char- rarely among araneids and occasionally among sal-
acterizes land locomotion by insects. Legs II are ticids (Table 1) appearto conferno obvious benefit:
longer and are held out to the sides, appearing to the absolute and relative velocities achieved by spi-
stroke backwards in unison. Buttheirstrokes do not ders using these rowing gaits are unremarkable
result in surges in the forward motion ofthe spider, when compared to the walking gaits used by G.
nor is there any other evidence that the movements rogersi and H. ecclesiasticus. To look more closely
SUTER ET AL.—AQUATIC LOCOMOTIVE GAITS IN DIVERSE TAXA 431
Salticid or Araneid
1
Tetragmthaspp. D. triton Tibeliussp. G. fogersi H. eccles. Salticids Araneids
—
Figures 1-2. Velocities attained on water by spiders in seven diverse taxa. 1. Absolute velocities: bars
represent means ± S.E., with n shown at the base ofeach bar. The bar forD. triton represents the rowing
gait only; the galloping gait for that species is shown for comparison as the darker bar in the inset. 2.
Relative velocities expressed in body lengths per second. For a few individual spiders in the salticid and
araneid samples, we also had data on walking (as opposed to a specialized aquatic gait) (inset); here the
dark bars represent walking and the lighter bars represent rowing.
at the efficacy of these specialized gaits, we com- faster than they rowed, and two salticids and the
pared the relative velocities achieved by rowing to one araneid rowed faster than they walked.
those achieved by walking in the five salticids and Our data indicate that specialized aquatic gaits
the one araneid in which the individual spiders used occur in at least three families (commonly in Te-
both gaits during our test runs (Fig. 2 inset). Even tragnathidae, butrarely in SalticidaeandAraneidae)
in these within-individual comparisons, the results that are outside ofthe superfamily Lycosoidea. But
areequivocal: three ofthe salticids walkedon water only in tetragnathids (along with the pisaurids
432 THE JOURNAL OF ARACHNOLOGY
which are part ofthe Lycosoidea) is the propulsion ing and stopping. Journal ofArachnology 22:89-
provided by the gait adequate to suggest that its use 93.
evolved because of its effectiveness. What remains McAlister, WH. 1959. The diving and surface-
is aconundrum: wehave demonstratedthepresence walking behaviour ofDolomedes triton sexpunc-
of behaviors in salticids and araneids that appear tatus (Araneida: Pisauridae). Animal Behaviour
not to be advantageous to theirperformers. Therar- 8:109-111.
ity ofthese behaviors is certainly understandable in Shultz, J.W. 1987. Walking and surface film loco-
that context, but their persistence is not. Further in- motion in terrestrial and semi-aquatic spiders.
vestigations may reveal that physiological cost or Journal of Experimental Biology 128:427-444.
some other as yet unmeasured parameter explains Suter, R.B. & J. Gruenwald. 2000a. Predatoravoid-
the presence of these not very effective aquatic ance on the water surface? Kinematics and effi-
gaits. cacy of vertical jumping by Dolomedes (Ara-
We thank William Miller for his assistance in the neae, Pisauridae). Journal of Arachnology 28:
fliaeblodraatnodry.JeTshsiecastVuidnyowkausrsfuoprphoerrteadssiinsptaarntcebyinVatsh-e Sut2er0,1-R.2B1.0.& J. Gruenwald. 2000b. Spider size and
sar College’s Undergraduate Research Summer In- locomotion on the water surface (Araneae, Pi-
stitute and in part by Vassar’s Class of ’42 Faculty sauridae). Journal of Arachnology 28:300-308.
Research Fund. Suter, R.B., O. Rosenberg, S. Loeb, H. Wildman, &
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