Cheap Transport for Fishing Spiders (Araneae, Pisauridae): The Physics of Sailing on the Water Surface

8 Pages·1999·8.9 MB·English

by  Robert B Suter

1999. The Journal of Arachnology 27:489-496
CHEAP TRANSPORT FOR FISHING SPIDERS
(ARANEAE, PISAURIDAE):
THE PHYSICS
OF SAILING ON THE WATER SURFACE
Robert B. Suter: Department of Biology, Vassar College, Poughkeepsie, New York
12604 USA
ABSTRACT. Many pisaurid spiders inhabit the edges of bodies of fresh water and actively propel
themselves across the water surface using bothrowing and galloping gaits. They also sail across thewater,
taking advantage ofthe wind and their nearly frictionless interaction with the water surface. The physical
interactions of Dolomedes triton (Walckenaer 1837) (Araneae, Pisauridae) with moving air, in a wind
tunnel in which the floor was water, formed the core of the present investigation. Spiders in an elevated
(sailing) posture were subjected to greater drag forces attributable to air motion than were spiders in a
prone (non-sailing) posture and therefore were transported substantially faster than prone spiders. In the
context oftransport velocity, the benefit ofadopting an elevated posture was substantially greater(relative
to mass) for small spiders than for large ones, although even under the relatively steady flow conditions
of the wind tunnel the velocities of the small spiders in the elevated posture were more variable than
either smallprone spiders orlarge spiders. The efficacy ofadopting anelevatedposturewas aconsequence
of the steep air velocity gradient that existed above the surface of the water in the wind tunnel and that
also exists above any pond over which the air is moving. Taken as a whole, the data indicate that sailing
is a remarkably cheap form oftransportation forDolomedes, but that, at least at the edges oflarge bodies
of water, it involves risks because it is directionally uncontrolled.
Locomotion by spiders includes ordinary erature on arachnids (ballooning: Humphrey
terrestrial modes such as walking, running, 1987; Suter 1991, 1992; aquatic locomotion:
andjumping, and quite unusual modes of air- Suter et al. 1997; Suter 1999a).
borne and aquatic locomotion. Although the Fishing spiders, Dolomedes triton (Walck-
eight-legged stepping patterns of spiders on enaer 1837) (Pisauridae), the subjects of this
land obviously differ in detail from those of paper, actively propel themselves across the
insects (Cocatre-Zilgien & Delcomyn 1993), water surface using two distinct gaits: rowing
the biomechanics of terrestrial locomotion by propels the spiders horizontally at velocities
insects andby spiders probably differlittlebe- < 0.27 m/sec (McAlister 1959; Shultz 1987;
cause they share size (Price 1984; Pennycuick Suter et al. 1997) and galloping, usedby these
1992), exoskeletal architecture, and important spiders during some kinds of prey-capture
&
aspects of the nervous system (Osorio et al. (Gorb Barth 1994) and during escape from
1995, 1997). Consequently, the rich literature predators (Suterunpubl. data), propels the spi-
on the biomechanics of terrestrial locomotion ders at horizontal velocities up to 0.75 m/sec
in insects (e.g.. Full & Tu 1990; Full et al. (Suter 1999a). In both rowing and galloping,
1995) contributes substantially to our under- the spideraccelerates forwardbyrapidlymov-
standing of terrestrial locomotion in spiders. ing its propulsive legs backwards, transferring
The same cannot be said of the biomechanics momentum to the water through the genera-
of aerial dispersal via ballooning and of lo- tion ofdrag. These active aquatic gaits aredis-
comotion on the water surface, forms of spi- tinct from the alternating tetrapod locomotion
der locomotion that are shared by only a few used by the spiders on solid substrate (Barnes
insects (ballooning: McManus & Mason & Barth 1991; Shultz 1987).
1983; Cox & Potter 1986; aquatic locomotion: Fishing spiders also move across the water
Andersen 1976). For these unusual modes of surface propelled by air movements. Deshefy
locomotion, most of our knowledge of the (1981) reported on one distinctive form ofthis
physics and biomechanics comes from the lit- “sailing” in which the spider extends and el-
489
490 THE JOURNAL OF ARACHNOLOGY
evates its most anterior pair of legs, taking D converter was supplied by a 12 V lead-acid
advantage ofthe increased wind speed 2-3 cm battery.
above the water surface. I have observed a The analysis of air speed as a function of
second distinctive form of sailing in pisaurid distance to the water surface was complicated
spiders, in which the spider extends and de- by the high variability in wind speed, presum-
presses all ofits legs, thereby raising its body ably due to turbulence, at any single height.
well above the water surface and allowing the Althoughthe mainstreamvelocity (sensuDen-
body and proximal leg segments to interact ny 1993) was unknown for the data collected
with more rapidly moving air currents. What at the pond, the air’s velocity in the boundary
follows are analyses of (1) the wind velocity layer above the water must decrease to zero
gradient in the boundary layer above a pond’s as height approaches zero (Denny 1993). Ac-
surface, (2) the drag forces acting on elevated cordingly, I used logarithmic curve fits to
vs. prone spiders, and (3) the velocity changes characterize the relationship between height
that result from modification ofposture during and velocity both for the pond data and for
sailing. the wind tu—nnel data (below).
METHODS Spiders. The adult Dolomedes triton (Ar-
aneae, Pisauridae) used in these experiments
Pond measurements.—I used a hot-wire were collected from small ponds in Mississip-
anemometer (Thermonetics Corporation mod- pi, and the juvenile was the progeny of one
el HWA-103) to measure wind speed just of the field-caught females. All were held in
above the surface of a pond at The Rockefel- my laboratory under conditions described
ler University Field Research Center, Mill- elsewhere (Suter et al. 1997).
brook, Dutchess County, New York. At the In the experiments described below, I in-
time and location of the data collection, the vestigated the motion of, and the forces acting
water surface was upwind of the pond’s usual upon, killed and dried spiders oftwo sizes. A
shore but was separated from the shore by third-instar juvenile that had been in the lab
about 4 m of mud flat. I arranged the ane- since hatching (wet mass 0.013 g, 0.126 mN)
mometer assembly (below) so that its sensor and two adult males of approximately the
pointed upwind (away from the shore) and same size (0.186 g, 1.82 mN; 0.243 g, 2.38
was over the water 0.5 m from the edge of mN) were anaesthetized with CO and killed
2
the mud flat. As a result of the location and by freezing. After post-mortem thawing, the
orientation of the sensor, it measured the spiders were immobilized in a prone posture
m
speed of air that had traveled at least 120 (the juvenile and one adult) or an elevated
across the pond’s surface unimpeded by struc- posture (the second adult) and allowed to air
tures other than the surface ofthe water itself. dry for several weeks. During the weeks of
The height of the sensor above the water sur- experimentation, the postures ofthe adult spi-
face was controlled by a motorized cam ders (Fig. 1) remained unchanged. During
which, as itrotated, raised and loweredthe0.5 measurements ofhorizontal velocity, however,
m boom to which the sensor was attached. the posture of the juvenile was changed from
The resulting motion of the sensor tip was prone to elevated to make possible directcom-
vertical (in space) and sinusoidal (in time), parisons of the same spider in two postures:
with a period of 4.3 sec, an excursion from to accomplish the posture change, I softened
0.5 to 8.9 cm, and a maximum velocity of the spider’s mostproximal legjoints by moist-
0.06 m/sec (a small fraction of the recorded ening them, and then repositioned the limbs
air velocities). and air dried the spider for several days.
Analog signals from the anemometer were The weights of the dried adult spiders (el-
digitized at 10 Hz by an analog-to-digital (A/ evated, 0.736 mN; prone, 0.959 mN) were
D) converter (Vernier Software Co., model matched more closely by fastening withepoxy
ULI 5.0) under the control of data logging a small, flat coil ofnichrome wire (0.221 mN)
software (Vernier Software Co., Logger 3.04) to the dorsal surface of the cephalothorax of
running on a laptop computer (Apple Corpo- the spider in the elevated posture. The adult
ration, model PowerBook 5300c). Power for spiders then both had weights of 0.96 mN.
the computer was supplied by its inboard bat- Temporary weight modifications were accom-
tery and power for the motorized cam and A/ plished by hanging the same short length of
SUTER—PHYSICS OF SAILING IN DOLOMEDES 491
—
Figure 1. Adult spider postures as digitized from photographs of the large dried spiders used in the
experiments reportedhere. Forcomparison, the small spiderintheprone andelevatedpostureshadheights
of2.1 mm and 4.4 mm respectively (filled bars at left). A spider in the prone posture (left) is exposed to
lower horizontal air velocities than is a spider in the elevated posture (right). The three curves are the
same as those shown in Figure 5, but with the axes reversed.
30-gauge copper wire over the cephalothorax A micromanipulator, mounted on the outside
of each of the adult spiders. All experiments of the chamber, facilitated adjustment of the
with the dried spiders were conducted at lab- position of the anemometer’s sensor relative
oratory temperatures between 20-23 °C, to the water surface.
Wind tunnel measurements.—The hori- At the beginning of a sailing trial, a dried
zontal wind tunnel used in this study had an spider was placed gently on the water surface
experimental chamber measuring 20 X 20 X approximately 42 cm from the upwind end of
87 cm (length). The floor of the chamber had the chamber (6 cm from the upwind edge of
a 0.6 cm deep cavity beginning 36 cm down- the water surface) and at the side-to-side cen-
wind from the air inlet and having horizontal ter of the water surface. The spider was held
dimensions of 18 X 40 cm. When the cavity at that location by a pair of eichrome wires
was filled with water, the surface ofthe water assembled in an inverted “V” and attached to
and the surface of the remainder of the floor a probe that could be raised several cm, re-
of the experimental chamber formed an un- leasing the spider. The spider was released
broken, flat surface. Both ends of the experi- only after the tunnel fan was turned on and
m.ental chamber were fitted with 2 cm thick the chamber had reached a constant nominal
furnace filter fiber to suppress turbulence. velocity of0.65 m/sec. To measure the sailing
With the exception of the filter, the upwind velocity of a dried spider, I recorded its lo-
end of the tunnel was open to room air. Ten cation in the horizontal plane, beginning when
cm beyond the filter at the downwind end of the spider crossed a line 2.5 cm downwind
the tunnel the air entered a 10.1 cm (diameter) from its release site, using an SVHS video
polyvinyl chloride (PVC) pipe on the end of camera (Panasonic model AG-455) and Image
which was mounted a small fan oriented so (National Institutes of Health software, ver-
that it pulled air through the tunnel. Although sion 1.55 f) as a frame grabber and image dig-
valving in the pipe leading to the fan allowed itizer (at a rate of 20 frames per second). I
me to control air speed in the experimental digitized the location of the center of the spi-
chamber, I conducted all tests at a nominal air der’s cephalothorax using tools resident in Im-
speed of 0.65 m/sec (measured at the center age, and then calculated velocity as the dis-
ofthe air stream, 52 cm from the upwind end tance moved divided by the frame interval.
of the experimental chamber). I used a horizontal balance (Suter et al.
I used a hot-wire anemometer (Thermonet- 1997) to measurehorizontal drag forces onthe
ics Corporation, model ITWA-103) to monitor adult dried spiders in the wind tunnel. The
air velocity in the chamber and to measure the balance employed an electronic clinometer
airspeed profile as a function of the distance (Applied Geomechanics Inc. model 900 Bi-
mm
from the water surface. The 5 (diameter) axial Clinometer) with a resolution of 0.01°
anemometer probe was inserted through a 6 (1.75 X 10 rad) to measure small angular
mm
(diameter) hole in the top of the experi- displacements that were directly proportional
mental chamber, 52 cm from its upwind end. to the horizontal force applied to the spider.
—
492 THE JOURNAL OF ARACHNOLOGY
Time(s)
—
Figure 3. During aperiod ofrelatively constant
average windvelocity (23 secto 50 sec intopgraph
of Figure 2), velocity of the air just above the
— pond’s surface (A, Up) varied approximately as the
Figure2. Thehorizontal wind speed(solidline)
within the 10 cm deep layer of air just above the logio of height (B, assuming, as is required by
boundary layer physics, that Up at the surface must
surface of a pond varied significantly with height be zero; Up = 0.16 log,o height + 0.83, r’ = 0.225,
tohfetehfefescetnwsoars(sdmaaslhled(0.l0in4e)<in e<ac0h.1c2a,sePa<lt0h.o0u5g).h n = 267, P = 0.0001).
The residual variability is assumed to be a conse-
quence ofthe turbulence in the air. height ofthe sensor was significant {P < 0.05
in each sample in Fig. 2). For a subsample of
velocity data in which large-scale fluctuations
The end of the vertical arm of the balance in Up were relatively small, the influence of
consisted of a pair of nichrome wires assem- sensor height was more prominent and Up var-
bled in an inverted “V” which immobilized ied approximately as the logio of height (Fig.
the spiders without applying any vertical force 3). The logarithmic relationship between Up
to them. Clinometer output (in volts) was dig- and height can be seen most clearly during
itized by an A/D converter (National Instru- short segments ofthe pond data (Fig. 2) which
ments Corporation, model NB-MIO-16L) include only a halfcycle or a full cycle ofthe
driven by a LabView 3 program (National In- sensor and therefore do not conflate velocities
struments) on an Apple microcomputer (Pow- that are widely separated in time (Fig. 4), and
er Macintosh 7100/80AV). is entirely in accord with boundary layer the-
ory (Schlichting 1979).
RESULTS
In the wind tunnel, constant fan velocity
Pond and wind tunnel air movement, and suppression of some of the turbulence
Velocity measurements of the air within 10 made the influence of height on wind tunnel
cm of the surface of the pond (Vp) indicated velocity (U^) much more detectable: logarith-
that the air was turbulent, with large fluctua- mic curve fits on three data sets indicated that
tions in velocity at several scales (Fig. 2). sensor height explained more than 90% ofthe
Only a small amount of the variability in Vp variation in Up (Fig. 5; 0.91 < r- < 0.97).
could be attributed to the changes in the po- Sailing vel—ocity and drag forces in the
sition of the anemometer’s sensor (0.04 < wind tunnel. Images ofdried spiders sailing
< 0.12), although the small effect of the downwind in the wind tunnel indicated that,
SUTER—PHYSICS OF SAILING IN DOLOMEDES 493
Height (cm)
Figure 4.—The relationship between Vp and height above the water is most clearly visible during the
brief periods of time (from Fig. 2) which include one full cycle of the sensor and therefore do not
conflate velocities that are widely separated in time. In the examples shown here, logjo curve fits worked
well: 0.86 < ri < 0.98.
by the time each had reached the line where “2.646, P = 0.008) and the variability in Vj
video digitizing began, it was moving at rel- forthe elevated small spiderwas much greater
atively constant velocity: thus it had reached than that for any other group (Fig. 6), The
the terminal velocity (V^) at which the forces very high variability in for the elevated
propelling it (air-induced drag) were in bal- small spider is probably attributable to small
ance with the forces resisting its motion (wa- eddies in the air stream close to the water sur-
ter-induced drag). Under wind tunnel condi- face that can influence very small objects but
tions, a large spider (0.42 g, wet weight) dried are averaged out when interacting with the
in an elevated posture always had greater ve- much greater leg span of the larger spiders.
locities (Vt) than a similar-sized spider that To test for effects of horizontal orientation
had been dried in a prone posture. At the relative to the direction of air movement in
weights at which the effectofthe two postures the wind tunnel, I released dried large spiders
could be compared directly, being elevated at different horizontal orientations, with the
conferred about a two-fold velocity advantage expectation that any relationship would be ap-
over being prone, but a much greater (3.7- proximately sinusoidal. The orientation ofthe
foM) advantage accrued to the elevated form large dried spiders did not have a significant
ofthe very small spider (0.013 g, wet weight) influence on Vj for either the elevated or the
A
(Fig. 6). stepwise multiple regression ofve- prone spiders (Fig. 7).
locity on height (a function of posture) and In direct measurements of the drag force
weight for the large spider yielded a highly exerted on spiders by moving air, drag on a
significant relationship (P < 0.01, F ~ spider in the elevated posture was significant-
106.96, adjusted F = 0.876) in which velocity ly higher than drag on the prone spider (0.055
varied directly with height and inversely with ± 0.002 mN vs. 0.0'29 ± 0.002 mN; t = 31.4,
weight. Forthe small spider, the differencebe- P < 0.0001) as expected from the results of
tween V^ for elevated and prone postures was dynamic tests (Fig. 6). The L92-fold differ-
highly significant (Mann-Whitney U test, Z ~ ence between the mean values fits well with
—
494 THE JOURNAL OF ARACHNOLOGY
m
E
>.
q>4—if
_o
0
>
0
•C«—>
o
N
O
X
Height (mm)
—
Figure 5. In the wind tunnel, air velocity (V^) Vertical Force (mN)
varied with the logio ofheight above the surface of —
the water. For the three runs shown here, the equa- Figure 6. During wind tunnel measurements of
tions were = 0.36 logjo height + 0.28, = the horizontal terminal velocities (U) ofsailing spi-
0a.n9d87, = 0=.280.l3o9g,looghieoighhetig+ht0.+39,0.2r28,= 0.>9200..99, daenrds,pUostvuarrei.edFowriththsepildaerrgesiszpeiadserwsel(lweats wwietihghmtas=s
1.82 and2,38 mN),theelevatedposture (•)resulted
in a doubling of U relative to U for the prone pos-
the 2.12“fold differences between measure- ture (o) (dry weight = 0.96 mN - elevated: 0.088
pmreonntes sopfideVris faotrthtehier liidgehntteisctawleieglhetvat(Feidg.a6n)d. t±±io00=..00200.7412mm;//ssdeerccy wvvsse..igpphrrtoonn=ee::100...400242m24N±±-00..el00e00v56atmme//dss:eecc0,,.0rr6aa2--
DISCUSSION tio = 2.51). For the small spider (wet weight =
A hshing spider, lying prone on the surface 0fo.l0d13ingc)r,etahseeeilnevUate(dbapsoesdtuorne c(oAm)praersiusltoendoifn mae3d.i7--
of a pond and not anchored to floating vege- ans, designated with dashes) relative to U for the
tation or debris, will move passively across prone posture (+). See text for statistical analyses.
the water at a rate influenced by the spider’s Exponential curves (because V, must approach zero
mass and the velocity of wind over the pond as vertical force becomes very large) fitted to the
(Fig. 6). The actual air velocity to which the data for the large spiders were 0.187 F70242
spider is exposed, however, is strongly influ- vated, r- - 0.873) and 0.104 (prone, F =
enced by the location of the spider’s body 0.766) where F^ is the vertical force. The velocities
parts relative to the water surface (Figs. 4, 5). shown here, for all but the heaviest prone spider,
overestimate U achievable during sailing by live
This connection between elevation and air ve-
spiders because the dried spiders were lighter and
locity means that the velocity atwhich the spi-
therefore created shallower dimples and less water-
der can travel under the influence ofairmove- generated drag (Suter 1997).
ments is closely tied to the spider’s posture
(Figs. 1, 6).
Qu—asi-passive locomotion, like balloon- motion resembles ballooning in which, once
ing. Although the propulsive forces in- airborne, the spider has no control over its
volved in sailing are environmental rather horizontal direction and therefore little control
than physiological, the importance of posture over its destination, and in which posture is
and the stereotyped performance of the pos- crucial (Suter 1992).
tures employed in sailing (above and Deshefy Two otherparallels between sailing andbal-
1981) indicate that this form of locomotion is looning are worth noting: neitherrequires sub-
not purely passive. On the otherhand, because stantial muscular input, and in both forms of
wind direction is not controlled by the spider locomotion, smaller individuals have substan-
and because the keel-less spider apparently tial advantages. The energetic cost of whole-
has no control of its own direction relative to body sailing (cf. Deshefy 1981) can be esti-
the wind (as do humans in sailboats), the sail- mated as the work needed for the spider to
ing spider cannot influence its destination. In raise its body to an effective sailing height
that regard, this quasi-passive form of loco- for the larger spiders used as models in this
SUTER—PHYSICS OF SAILING IN DOLOMEDES 495
Risk assessment.—Fishing spiders are at-
tacked from below by fish (G. Miller pers,
commun.) and are likely also to suffer pre-
dation from above by aeurans and birds. Be-
cause their predators’ feature detectors un-
doubtedly respond to specific cues (e.g.,
shape, size, motion: Lettvin et al. 1959; Ewert
1974; Ewert et al. 1983) the suppression of
any of these cues can result in a reduction in
the probability of eliciting predation. In this
context, sailing can be viewed as an incon-
spicuous form oflocomotion that offers a rel-
ative reduction in predation risk through the
suppression of visual cues: propulsive mo-
tions of the legs relative to the body are ab-
sent, and surface waves caused by rowing and
Figure 7.-—Sinusoidal curve fits onthe velocities galloping locomotion (Suter et al. 1997; Suter
oflarge spiders in elevated (upper) andprone (low-
1999a) are not generated at the velocities
er) postures, afterrelease atdifferenthorizontalori-
entations relative to the direction of air motion in achieved during sailing (Fig. 6; Denny 1993;
the wind tunnel, revealed that orientation had no Vogel 1994).
significant effect on Vj. Each datum consists of Another kind of risk, that associated with
mean ± SD velocities and angles (relative to 0°, motion whose direction is not controlled by
facing directly upwind) forone spider's motionfol- the spider, should rise with elevated uncer-
lowing a singlerelease: variation in V^reflects mea- tainty about the ecological suitability of the
sured changes in velocity between digitized frames destination. In the ponds usually inhabited by
from videotaped records; variation in angle is a D. triton, this risk is minimal because the ul-
consequence of the slow rotation of the released
spider during its movement downwind. For the el- timate destination of a sailing spider will al-
evated posture, Ft == 0.005 sin (angle) + 0.094 (ri ways be an edge of the pond, a location that
= 0.119, n ~ 19, P > 0.05), and for the prone may be unfamiliar to the spider but that is apt
posture. Ft = 0.013 sin (angle) + 0.054 (P - 0.144, to be as ecologically suitable as the spider’s
« - 17, F > 0.05). original location. In contrast, species of Do-
lomedes that inhabit the shores of islands in
study (live weight — 2.1 mN), the work re- the Great Lakes of North America cannot be
quired to raise the center of mass from 4-12 assured of a benign destination if the wind is
mm above the water is about L68 X 10“^ offshore: sailing away from a shore can take
joules, an amount of work that needs to be the spider into open waters where both food
done only once during a sailing episode. In and cover are unavailable and where the di-
comparison, for the smaller of the spiders rection of the nearest shoreline is undetect-
used as models in this study (live weight = able.
0.126 mN), the cost of elevating the center of Function.—“For fishing spiders living atthe
mass 23 mm is about 2.90 X 10“^joules. The edges of ponds, sailing is both energetically
smaller spider's velocity in the elevated pos- cheap and relatively safe (above). But this
ture is approximately the same as that of the form of locomotion has only rarely been ob-
larger spider (Fig. 6), but the cost of attaining served in nature and its function in the context
that posture for the larger spider is 58 times of more controlled modes of locomotion on
as great. Thus, the efficiency of sailing is far the water surface (Suter et al. 1997; Suter
greater for very small spiders, but for any spi- 1999a) remains unclear. Because sailing is
der the cost is very small: consumption of a most efficient for smaller spiders (Fig. 6) and
single fruit fly (Golley 1961) would provide because heavy D. triton are unable to elevate
sufficient energy to elevate the larger spider their bodies into rapidly moving air without
thousands oftimes! Sailing is, like ballooning exceeding the ability of the water’s surface
(Suter 1999b), a remarkably cheap form of tension to support them (Suter 1999a), the pri-
transport. mary function of sailing may be to facilitate
496 THE JOURNAL OF ARACHNOLOGY
dispersal in young spiders, a hypothesis that ior during prey-capture of a fishing spider, Do-
is yet to be tested. lomedes plantarius (Araneae: Araneidae): Gal-
loping and stopping. J. ArachnoL, 22:89-93.
ACKNOWLEDGMENTS
Humphrey, J.A.C. 1987. Fluid mechanical con-
I thank Fernando Nottebohmforpermission straints on spiderballooning. Oecologia,73:469-
477.
to use ponds at The Rockefeller University Lettvin, J.Y, H.R. Maturana, W.S. McCulloch &
Field Research Center, Gary Lovett at the In- W.H. Pitts. 1959. What the frog’s eye tells the
stitute of Ecosystem Studies for educating me frog’s brain. Proc. Instit. Radio Engrs., 47:1940-
about the dynamics of turbulent atmospheres, 1951.
and Edgar Leighton at the University of Mis- McAlister, W.H. 1959. The diving and surface-
sissippi for providing the spiders. The study walking behaviourofDolomedes triton sexpunc-
was supported in part by funds provided by tatus (Araneida; Pisauridae). Anim. Behav., 8:
Vassar College through the Class of ’42 Fac- 109-111.
ulty Research Fund. McManus, M.L. & C.J. Mason. 1983. Determina-
tion of the settling velocity and its significance
LITERATURE CITED to larval dispersal of the gypsy moth (Lepidop-
Andersen, N.M. 1976. A comparative study oflo- tera: Lymantridae). Environ. EntomoL, 12:270-
272.
comotion on the water surface in semiaquatic Osorio, D., M. Averof& J.P. Bacon. 1995. Arthro-
bugs (Insects, Hemiptera, Gerromorpha). Densk. pod evolution: Great brains, beautiful bodies.
BarMneedsd,rW..JD.aRns&kENGat.urBha.rthF.ore1n9.9,1.13S9e:n3s3o7r-y39c6o.ntrol OsoTrrieon,dsD.E,coJl.P..EBvaolc.o,n1&0:4P4.9M-.45W4h.itington. 1997.
of locomotor mode in semi-aquatic spiders. In The evolution of arthropod nervous systems.
LanodcomVeorttoerbraNteeus.ral(DM.eMc.haAnrimssmtsronign A&rthBr.oMp.oHd.s PenAnmyecruiiccka,nCS.cJi.., 18959:22.44N-e25w3t.on Rules Biology: A
Bush, eds.) Manchester Press, Manchester, 340 Physical Approach to Biological Problems. Ox-
Cocpapt.re-Zilgien, J.H. & F. Delcomyn. 1993. A new Prifcoe,rdRUWniv.19P8r4e.ss,InsOexcftorEdc.olIolglyp(p2.nd ed.). Wiley-
method fordepicting animal steppatterns. Anim. Interscience, New York. 607 pp.
CoxB,ehaDv..L,.4&5:8D2.0A-.82P4o.tter. 1986. Aerial dispersal Sch(l7itchhteidn.g),. MHc.GTraw1-9H7i9l.l,BoNuenwdaYroyrkL.ayer Theory
behavior of larval bagworms, Thyridopteryx Shultz, J.W 1987. Walking and surface film loco-
ephemeraeformis (Lepidoptera: Psychidae). Ca-
motion in terrestrial and semi-aquatic spiders. J.
nadian EntomoL, 118:525-536.
Exp. Biol., 128:427-444.
Denny, M.W. 1993. Air and Water: The Biology
Suter, R.B. 1991. Ballooning in spiders: Results of
and Physics of Life’s Media. Princeton Univ. wind tunnel experiments. Ethol. Ecol. & EvoL,
Press, Princeton, 341 pp. 3:13-25.
Deshefy, G.S. 1981. ‘Sailing’ behaviour in the Suter, R.B. 1992. Ballooning: Data from spiders in
fishing spider, Dolomedes triton (Walckenaer). freefall indicate the importance ofposture. J. A-
Anim. Behav., 29:965-966.
rachnol., 20:107-113.
Ewert, J.-P. 1974. The neural basis of visually Suter, R.B. 1999a. Walking on water. American
guided behavior. Sci. Americ&an, 230:34-42. Sci., 87:154-159.
Ewert, J.-R, R.R. Capranica D.J. Ingle (eds.). Suter, R. B. 1999b. An aerial lottery: The physics
P1l98e3n.umAPdrveassn,ceNsewinYoVrekr,teb1r2a3t8epNp.euroethology. nofoLb,al2l7o(oln)i:n2g81i-n2a93c.haotic atmosphere. J. Arach-
Full, R.J. & M.S. Tu. 1990. Mechanics of a rapid Suter, R.B., O. Rosenberg, S. Loeb, H. Wildman &
running insect: two-, four- and six-legged loco- J.H. Long, Jr. 1997. Locomotion on the water
FullmM,oatxiR.oiJn.m.,uJmA..EsxYipna.gmlBaeiuocll.eh,gi1f&5o6r:cD2e.1L5p.-r2o3Jd1iun.cdtriiocnh:. C1o9c9k5-. dsuerrfaDcoe:loPmreodpuelssitvreitmone.chJ.anEixspm.sBoifolt.he2f0i0sh:e2r5s2p3i--
2538.
roaches righting on photoelastic gelatin. J. Exp. Vogel, S. 1994. Life in Moving Fluids (2nd ed.).
Biol., 198:2441-2452. Princeton: Princeton Univ. Press. 467 pp.
Golley, FB. 1961. Energy values ofecologicalma-
terials. Ecology, 42:581-584. Manuscript received 8 June 1998, revised 11 Sep-
Gorb, S.N. & EG. Barth. 1994. Locomotor behav- tember 1998.