r Predation in Porcellio
Spide
BSTRAC
A population of the theridiid spider Steatoda
grossa in Pacific Grove, California was studied to
observe its predatory behavior towards the terrestrial
isopod Porcellio scaber and subsidiary prey items.
The isopod and spider are both found to be nocturnally
active. The carcasses of consumed prey are cut from
the spider's web to collect in a debris pile which
can be examined for an indication of long term
dietary consumption. The isopod, though rejected
as prey by most spiders because of repugnant tegumental
glands, composes 84% of the diet of this spider as
determined by web analysis. The spider from laboratory
experiments was found to go about five days between
meals but able to tolerate periods of three weeks or
more without feeding.
Spider Fredation in Porcell:
INTRODUCTION
Cloudsley-Thompson (1958), reported birds, reptiles
amphibia, and many other insectivorous animals including
spiders, harvestmen, mites and centipedes as predators
of terrestrial isopods. For central Californian
woodlice, Miller (1938) listed as vertebrate predators
"at least two species of salamanders, several species
of reptiles, birds, and insectivores; among the
invertebrates, the black widow spider (Latrodectus
mactans) and various species of centipedes". Gorvett
(1956) has stressed the importance of the tegumental
glands as defense mechanisms in isopods; these organs
only occur in terrestrial species and the fact that
their secretions are distasteful to many spiders
suggests that spider predation has provided the
strongest selective pressure for their evolution.
Spiders thus appear to be at least potentially the
major predators on woodlice, yet detailed studies
of spider predation on isopod populations are almost
non-existent.
Porcellio scaber Latreille, 1804 is the
terrestrial isopod which shows the greatest development
of the tegumental glands (Gorvett, 1951). This species
is a member of the cryptozoan community, a term coined
by Dendry (1895) to describe the assemblage of small
terrestrial animals found dwelling in darkness beneath
stones, rotten logs, bark of trees and other similar
situations. Though cosmopolitan in distribution.
and one of the commonest isopods in the United
States, little study has been devoted to its interspecific
relationships, particularly those of predation.
Several groups of arachnids were initially
Spide
edat
in Torcellio
observed in their contacts with Pscaber under laboratory
conditions; these included solifugids, harvestmen,
wolf spiders, orb weavers, pholcids and theridiids.
The theridiid house spider Stoutoda grossa (C.L. Koch)
was found to be the most active predator on P. scaber
of all arachnids studied. It is the purpose of this
paper to discuss the natural history of this spider,
particularly its predatory behavior towards P. scaber
-

All studies were carried out at the Hopkins
Marine Station and around private homes in Pacific
Grove, California during the period April 27 to
June 9, 1973.
P. scaber populations were collected from both
the grounds of the marine station and from a private
estate near Asilomar Beach. The populations were
maintained in the laboratory in 30 x 20x 10cm plastic
tubs. A thin layer of dirt was spread over damp paper
towels on the bottom; this was covered with a layer
of grass and pine needles; and this, in turn, topped
with small pieces of decaying wood and bark. Tubs
were covered with aluminum foil and humidity was
maintained at a high level. As long as the paper
towels were kept damp an ideal environment was
maintained.
Most work on S. grossa was conducted in garages
and other human habitats. The main study area consisted
of one wall of a garage of a private home near Asilomar
Beach (fig. 1), which supported 115 individual webs
with 76 observed spiders.
Other methods used in specific investigations
are indicated in the appropriate sections which follow.
spi rodation in orie
IErIVTTY PTTEENS OE TONCTIOE
Initially activity pattorns of the isopod were
monitored in a natural onvironment on the grounds of the
Hopkins Marine Station. Twenty-two traps consisting
of plastic drinking cups were buried, with the lip
of each cup at ground level, in dirt and sand surrounding
a large patch of iceplant (Fosembryanthemum) bordering
the east beach on Mussel Foint. These traps effectively
captured and retained all isopods which wandered over
the lip. In addition, four 0.25m2 arcas of nearby
ranite rock surface were also monitored. Counts
were made every hour for 24 hours on numbers of
. scaber trapped in cups or detected on the rock
surface. Since activity proved largely nocturnal.
these populations were subsequently monitored on three
successive nights, and data gathered on physical
parameters. Temperature readings were taken with a
thermocouple heat probe and relative humidity with a
Honeywell portable relative humidity indicator from
Scm above the ground surface. Temperature was also
recorded from the iceplant habitat proper.
The results of these studies (figs. 2, 3) show
that activity in the P. scaber population increases
at sunset and ceases with approaching sunrise. Relative
humidity consistently shows a peak near 0300, with a
corresponding temperature minimum. Activity is maximum
earlier and it declining at this time. Activity from
sunrise to sunset is negligible.
P. scaber is sensitive to desiccation (Heeley, 1941);
in this connection it maintains a nocturnal activity
pattern, avoids dry places, and remains in a humid
environment. Judging from the data in fig. 3, activity
is most closely related to the absence of light
ther than to particular conditions of temperature
Spider Prodation in poi
or humidity. The light regime provides the most
constant parameter of the environment and one easiest
for them to dotect. Numidity may also be of importance,
however; on the second night, and especially on the
third night of study (when bocause of heavy fog, the
humidity remained close to 100%) nocturnal activity
continued until a later hour at night.
ATI
TY PAT
ETERN OT GTEATODA CROSCA
Acr
The activity pattorns of 76 3. grossa were
monitored in the garage study area near Asilomar
Beach (fig. 1). An atomiser which sprayed a very
fine mist of water wha used to delingate the extent
of each web. The dreplets adhered to the web without
damaging it and without harm to the spider, and refracted
the light brilliantly. Within 20 minutes evaporation
restored the web to its original state.
Each spider was observed every two hours over
à 24 hour period. Spiders were scored as inactive
if hiding in their daytime retreats, and as active
if they were in a typical predatory stance in the web
or were actively out and moving about. Temperature,
relative humidity and light exposure were recorded
every hour from one typical web. During the night
spiders were observed with a flashlight covered with
a red filter, a procedure which appeared not to disturb
them.
The results (fig. 4) showed there is much
variance among the individual spiders with respect
to their periods of activity. Some individuals were
active in the predatory stance during the whole
observational period; others were not active at all
Spider Predation in Porcollio
although they had been seen previously. However, on
the whole the population showed much greater activity
between sunset and sunrise than during the day (fig.5).
Relative humidity and temperature showed a correlated
maximum and minimum respectively, between 0600-0800.
Thus it was found that S. grossa, like P. scaber,
is principly nocturnal. Spiders are potentially
diurnal having an integument containing a waxy epicuticle
which acts to prevent desiccation. The more primitive
groups of spiders are secondarily adapted to nocturnal
habits, probably as a result of competition with more
efficient species (Cloudsley-Thompson, 1958). The
fact that S. grossa feeds upon P. scaber, a species
rejected by more advanced spiders, may further indicate
its inability to compete with these forms. In S. gross
as in P. scaber, the period of activity corresponds
best with the period of darkness rather than the
temperature or humidity directly, emphasizing that
behavior is cued mainly to light.
Predatory behavior is dealt with in a separate
section to follow, but a few observations of other
aspects of S. grossa activity and behavior are included
here.
Several of the spiders maintained in the lab
were observed to spin egg cocoons, one spinning a
second two and a half weeks after the first (fig. 6). One
typical egg sac contained 129 eggs. The eggs are
enclosed within a hollow,silk sac with a cavity of
larger volume than the eggs. When rotated the sac
shows the eggs to sometimes be tightly packed together
in a mass or else loose and able to roll around.
Four weeks after formation of the cocoon, the eggs
within had hatched and gone through at least one molt
Spider Fredation in Porcellio
to produce unpigmented pre-spiderlings with definite
body form and capable of limited movement.
Since S. grossa is sedentary, the question of
where and how it obtains water is of interest. During
experiments with the fine mist atomizer, the spiders
were observed to drink water droplets from the threads
of the web. The legs and pedipalps were also used
to collect droplets from the web and bring them to
the mouth. This suggests that collection of dew
condensing on the webs provides an important source
of water for the spider.
PREDATORY B
HAVIOR OF ST
ATODA GROSSA
Members of the family Theridiidae characteristically
build irregular webs from the threads of which they
suspend themselves in an inverted position while they
await their prey (Levi, 1968). This is what I term
the predatory stance in S. grossa (fig. 7). Both
males and females were observed to spin webs, sex
determined in males by the presence of pedipalps
swollen at the distal end with the reproductive apparatus.
All of the larger web spinners were females as evidenced
by egg laying activity. The web of S. grossa is a
more or less closely woven sheet extending in a single
plane and consisting of threads running in all directions
with no apparent regularity (fig.8). The sheet
extends outwards from a hidden retreat in a crack or
crevice, in which the spider is generally found
when not in the predatory stance. These are the two
basic states of activity. Occasionally a spider
prowls its web laying new threads and actively repairing
damaged portions of the web. Litter or dead carcasses
are cut out of the web and dropped, where they form
Spider Predation inT
an accumulated garbage heap.
As is typical of primitive web builders (Kullman.
1972), the web is used only as a means of getting
information about the position of approaching prey,
and does not use special viscid threads to snare its
victims. Much silk is required and the webs are rather
complicated and irregular. Vertical tangles of
interconnecting scaffolding lines above, and drop
lines below and to the sides anchor the main sheet
of the web in place and provide additional means
for detecting and entangling passing prey. Sometimes
the webs are so compacted in a corner that they lose
their characteristic shape.
Once alerted to the presence of prey by vibrations
of the web, the spider advances rapidly and touches
the isopod or other prey with the pedipalps and first
pair of walking legs. At this point the prey may
be either accepted or rejected. If accepted, S. grossa
proceeds to swath the ventral surface of P. scaber with
viscid threads pulled from the spinnerets by the
fourth pair of walking legs. The prey is not rotated
as seen in species of Araneus, and not even held, but
rather is crisscrossed ventrally with silk in the place
of entanglement. The spider places occasional quy
lines to the dorsal surface of the prey and attaches
them to its web, acting to hitch the prey up into the
web. It may pause occasionally and if the struggling
is strong it continues to swath the prey in silk.
A series of short bites with the fangs is then
made through the ventral surface. One assumes venom is
being injected but it is not always sufficient to kill
P. scaber. The spider may then move a short distance
away from the prey for a period of several minutes or
less. If struggles continue it adds additional silk.
Spider Predation in Porcellio
It then sinks its chelicerne through the ventral surface
of the proy and remains in this position for 1½ - 3
hours. Details of the feeding process were not followed;
presumably enzymes are injected, and suction applied to
ingest the organic soup created. The dead carcass is
eventually cut from the main sheet of the web.
If the packaged prey is small it may be carried
from the web periphery to the center of the main sheet
for consumption. While walking the spider carries the
package with one of its third pair of walking legs, and
it is held in a position between the spider and the
web. If the prey is larger (and isopods up to five
times the size of the spider were effectively preyed
upon) then it is eaten in the place of capture.
Additional details of predatory behavior were
gained from a study of 25 specimens of S. grossa,
2 - 10mm in body length, which were kept in the
laboratory for observation and experiment. I attempted
to feed these nightly between the hours of 2200 - 0200.
P. scaber of weight 20-35mg were introduced either by
dropping them into the mainsheet of the web in the
vicinity of the spider, or by placing them in peripheral
parts, or wholly outside the web.
Records of prey taken were kept for 19 S. grossa
(fig. 9). Predation is relatively infrequent and the
spiders averaged about five days between meals. Some
spiders endured at least three weeks without feeding
despite the presence of food and there is no reason
to believe they could not starve for longer periods.
The spiders were observed to have a poor sense
of vision. Isopods walking even within 5cm of the
spider never elicited a response unless the web was
touched indicating that detection of prey relies
Spider Predation in Porcellio
primarily on vibratory stimulus. The isopods can alert
the spider to their presence in several ways. They
can fall directly into the web, trip over the scaffolding
lines and fall into the web, or become entangled in
drop lines or outlying guy lines. Subsequent struggles
alert the spider to a stereotyped response. Isopods
contacting the web are not necessarily trapped. It
was observed that P. scaber can walk upside down
either up or down a single thread of the web without
much difficulty.
The habit of S. grossa of discarding its old
prey in a tangled heap below the mainsheet of the web
made it easy to determine longterm diets for
representative spiders. Accumulated carcasses from
ten webs were taken from the garage study site and
analyzed for contents. Similar collections were made
from the webs of S. grossa in two other locations, an
open carport and a closed woodbox from a home on
Jack's Point in New Monterey. The results are shown
in figs.10 and 11. The same animals were found fairly
consistently in the webs, but by far the primary prey
of S. grossa as determined from the webs is P. scaber
with a frequency of 84%.
Moths form the second most commonly taken prey.
but only because of two very exceptional webs, occurring
at heights of 100 and 170cm above the ground in the
main garage study site. Both of these (and no others)
were taken from the window ledges; the primary prey
found here were moths and other flying insects,
undoubtedly attracted to lights shining through the
windows. Most of the animals classified under the
category "Other" or "Others", figs.10 and 11, occurred
in these two webs. They were mostly flying insects
10
Spider Predation in Porcellio
such as horse flies, beach flies, dragon flies, etc.
but there were also a few earwigs and other spiders.
Aside from these two webs, the main prey of
S. orossa clearly consists of crawling arthropods,
which blunder into the web on foot. I expected to
find dietary differences between spiders with webs
close to the floor and those high on the walls, but
no such differences were found. Webs were very abundant
occupying almost every available corner or crevice
in the study area, and everywhère except in the
window webs, P. scaber is the major prey. Two points
are of special interest in this regard. First,
P. scaber is probably the main food here because it
is most available rather than the most attractive prey.
In feeding experiments in the laboratory I concluded
that S. grossa does have a relative distaste for
P. scaber. On several occasions when an isopod had
been refused, a fly was introduced and elicited a complete
predatory response resulting in consumption.
A second point of interest is that P. scaber
seems equally available to spiders on the floor and
those near the ceiling. When P. scaber was released
on the garage floor they were observed to have no
difficulties climbing the garage walls, and on several
occasions marked isopods released on the floor were
captured as early as four hours Iater in webs halfway
up the walls. Climbing is probably part of the normal
nocturnal foraging activity of P. scaber. Bristowe (1941)
found that specimens in captivity would eat spiders
eggs. They also are observed to consume dead and
decaying matter inclüding dead members of the species.
The largest number found climbing on the walls were
observed around 0400 when the humidity was reaching a
11
Spider Predation in Porcellio
Maximum. It is reported that P. scaber prefers vertical
surfaces where moisture collects but does not become
excessive (Heeley, 1941).
The spiders are reported to have a six-year
lifespan (Kaston, 1953). The sizes of the middens
of old prey below webs suggests that they remain in
the same web as long as there is a good food supply
and they are not disturbed. Should they be disturbed,
they readily spin a new one in a suitable location
as seen by those collected for the laboratory. The
webs are discreet units separated from neighboring
webs (fig. 1). Two spiders do not share the same web.
When 10 hungry spiders were placed together they were
found to be extremely cannibalistic, killing off each
other until one remained. It then sat down to consume
the dead losers. In view of the large number of annual
offspring this activity would function to limit the
population size to that suitable for the amount of
available prey in the environment.
. .*


12
Spider Predation i
Porcellio
ACKNOVLÉDGMEN
I am deeply grateful to Dr. Donald F. Abbott
and his wife Dr. Isabella A. Abbott for their guidance,
inspiration, and use of Querida del Mar in my research
as well as for sustenance during my field work.
For their positive influences on my personal growth
I will always be indebted.
For help in my field studies, special thanks
go to Dr. Eugene C. Haderlie for access to his home
as a study site and to S. Randall Pratt for assistance
with Porcellio scaber.
..
Spider Predation in Porcellio
TURE CITED
LITERA
Dristowe, W.S. 1941. The comity of spiders. 2 vols.
Ray society, London.
Cloudsley-Thompson, J.L. 1958. Spiders, scorpions,
centipedes and mites: The ecology and natural
history of woodlice, myriapods, and arachnids.
Pergamon Press, New York, London, Paris, Los
Angeles. 228 pp.
Dendry, A. 1895. The cryptozoic fauna of Australasia.
Rept. sixth meeting Aust. Assn. Adv. Sci. 6:99-119.
Gorvett, H. 1951. The tegumental glands in the land
Isopoda. B. The lobed glands: structure and
distribution. Quart. J. micr. Sci. 92:275.
1956. Tegumental glands and terrestrial
life in woodlice. Proc. Zool. Soc. Lond. 126(2):291-314
Heeley, W. 1941. Observations of life histories of
terrestrial Isopods. Proc. Zool. Soc. Lond.
111(B):79-149.
Kaston, B. 1953. The. spiders. Wm. C. Brown Co.,
Dubuque. 289 pp.
Kullman, E.J. 1972. The convergent development of
orb webs in cribellate and ecribellate spiders.
A. Zool. 12(3):395-405
Levi, H.W. 1968. A guide to spiders and their kin.
Golden Press, New York. 168 pp.
Miller, M.A. 1938. Comparitive ecological studies
on the terrestrial isopod crustaceans of the
San Francisco Bay region. Univ. Cal. Pub. Zool.
43(7):113-142.
14
Spider Predation in Porcellio
IGURE CAPTIONS
Fig. 1. Blueprint diagram of the garage study area
near Asilomar Beach, Pacific Grove, California,
showing exposed studs, horizontal supports, and
windows. Shaded black areas represent main
sheets of webs; cross hatched areas represent
the outlying drag lines; arrows point to
locations of spiders in predatory stance.
Graph showing number of P. scaber caught
Fig. 2.
in traps as an indication of relative activity
level during a 24 hour period on April 26 to
27, 1973.
Fig. 3. Graph showing number of P. scaber caught
in traps as an indication of relative activity
level on three successive nights, April 30 to
May 3, 1973. Lines above show the relative
humidity (RH), temperature 5cm above the ground
(T), and temperature of the iceplant
(Mesembryanthemum) habitat (T,).
Graph showing periods during which individual
Fig. 4.
spiders in the garage wall study area (fig. 1)
were active in the characteristic predatory
stance over a 24 hour period, May 28-29, 1973.
Blackened horizontal bars indicate activity.
Fig. 5. Graph showing periods during which the population
of spiders in the garage wall study area (fig. 1)
were active in the characteristic predatory
stance over a 24 hour period, May 28-29, 1973.
Blackened vertical bars indicate the number
of spiders active in an hourly period.
Photograph of S. grossa in process of spinning
Fig. 6.
eg ocoon
15
redation in Porcellio
Spider
Steatoda grossa in its predatory
Fig. 7. Photograph of
stance, hanging inverted from its web of irregular
threads.
Fig. 8. A) Photograph of web of Steatoda grossa in
garage wall study årea (fig. 1) showing ain
sheet, scaffolding lines, drop lines, and
daytime retreat.
B) Photograph - closeup view of same web
freshly misted.
Fig. 9.
Rates of predation in 19 specimens of Steatoda
grossa maintained in the laboratory.
Fig.10. Tabulated results of examination of discarded
prey collected from ten webs in the garage
wall study area (fig. 1) and four other webs
from Jack's Point for comparison. Numbers
indicate number of individuals of prey species.
Fig.11. Graph showing percentage composition of prey
according to numbers of prey individuals, in
ten webs from the garage wall study area (fig. 1).

16
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