Here are my scientific
publication on sting use in scorpions, originally published in
1993 in The Journal of Arachnology 21 (Pp. 60-63).
Scorpions are notorious for their
stinging behavior and powerful venoms. Sting use plays an
important role in prey capture and defense (Vachon 1953;
Cloudsley-Thompson 1958; Stahnke 1966).
As yet, there have been no
controlled and quantitative studies of sting use, but
investigators have
suggested a variety of factors that may be correlated with sting
use. It appears that scorpions with
large, powerful pedipalps seldom use the sting, while species
with small, slender pedipalps readily sting
their prey (Stahnke 1966; Baerg 1961; McCormick and Polis 1990).
Casper (1985) proposed an
ontogentetic change in sting use by Pandinus imperator Koch. Young
individuals stung prey readily, while older and adult individuals
were never observed to employ the sting.
Similar results were reported by Cushing and Matherne (1980) for Paruroctonus
boreus Girard.
Le Berre (1979) noted decreased sting use with smaller prey in Buthus
occitanus Amor., and similar
observations were reported for other species (Pocock 1893; Vachon
1953; Cloudsley-Thompson 1958;
Baerg 1961; Bücherl 1971; Polis 1979).
The purpose of this study is to
examine sting use during prey capture by two East African
buthids,
Parabuthus liosoma
Hemprich & Ehrenberg and Parabuthus pallidus Pocock. Both species used
their stings selectively, depending upon the size, morphology and
resisting behavior of the prey. Results
are discussed in terms of the costs and benefits of venom
injection during prey capture.
MATERIALS AND METHODS
Natural history.-Parabuthus
liosoma and P.
pallidus are found in several
countries in East Africa
(Probst 1973). Adults of the former species are of medium size
for scorpions and have yellow to
yellowish-red body, except for part of the cauda and telson which
are dark red/brown. They have small,
slender pedipalps and a thick, powerful cauda. Similar coloration
and morphology is present in P. pallidus,
but these are slightly smaller and lack the darkened distal part
of the cauda. There are no previous reports
on the life history or behavior of these species.
Materials.-Individuals of P.
liosoma and P.
pallidus were collected in the
vicinity of Isiolo, Kenya in May and
June, 1988. The animals were found in the same semi-arid area
under stones along roadsides, but no more than
one scorpion was ever found beneath a single stone. The substrate
consisted of compacted sand with
occassional grass and bushes.
The scorpions were taken to
Norway, where 11 individuals of P. liosoma and 12 individuals of P.
pallidus
were used in the experiments. The specimens were of unknown age
and ranged in lenght (pro- and mesosoma)
from 18 to 32 mm (X = 25.1 mm, P. liosoma) and 13 to 31 mm (X = 21.3 mm, P.
pallidus).
Specimens were kept individually
in terraria (32 X 20 cm), with a substrate of sand and some
stones. The
temperature was held at 24 to 30° C, and the LD-period was 10:14 hours.
Water was provided weekly by
misting. Animals were not fed except when tested. Only animals
active on the surface in the dark period were
selected for experiments. This appeared to be a useful indication
of hunger, since they usually responded rapidly
when prey were offered.
For testing, the scorpions were
transfered to an observation terrarium (25 X 25 cm) with a
sand-floor. They were
given one hour for acclimation before prey was introduced. Data
on all activities were collected by direct observations
under low intensity red light that is apparently not visible to
scorpions (Machan 1968). All observations were made
during the fall 1988, and spring 1989. Results were tested using
a sign test (Lehner 1979).
Experiment 1.--Sting use
was compared after presentation of three different types of prey
which differed in size and
morphology. These were small (10-18 mm) and large (24-32 mm)
larvae of Tenebrio molitor Linne and a centipede,
Lithobius forficatus
Verhoeff (26-35 mm). Insect larvae and centipedes were seen in
the scorpions' habitat in Kenya,
and thus are probably natural prey for the two Parabuthus species.
After the acclimation period, a
live prey was introduced to the test scorpion, and if accepted,
observations were continued
until ingestion was started. The scorpions were allowed to
complete ingestion before they were transfered back to their
terrarium. If the prey was not accepted by a scorpion, the test
was discontinued, and the animal was returned to its terrarium.
Experiment 2.--Sting use
against non-resistant prey was investigated by introducing
freshly killed Tenebrio larvae (29-35 mm)
to the scorpions. The larvae were presented by moving them with
forceps on the substrate near the scorpion pedipalps.
RESULTS
Prey were subdued in two ways. In
43.3% of the trials (N = 138), scorpions grasped the prey
with one or both pedipalps
and then pulled the prey to the chelicera and began ingestion
without use of the sting. In the remaining trials, the scorpions
used
the sting to subdue the prey. In some of the latter trials,
scorpions did not succeed in penetrating the prey integument;
these
scorpions either attempted to sting again or stopped stinging and
devoured the prey alive. These cases were recorded as sting
use, whereas cases with penetration of the integument were
recorded as successful sting use.
Sting use in P.
liosoma.--In this
species, the sting was used significantly less (P <
0.001) against small larvae than with the
two prey of larger size (Fig. 1). There were no significant differences
in sting use against the large larvae and the centipedes.
Attempts were made to sting both of the large prey types in about
85% of the trials, and the sting use was successful in
58.8% (larvae) and 69.6% (centipedes) of the trials.
Sting use in P.
pallidus.--Individuals
of this species attempted to sting the small larvae significantly
less (P < 0.005) than
the two large prey types (Fig. 2). Small prey were stung in 20.1% of the
trials, whereas the use of the sting against large larvae
and centipedes was observed in all trials. Sting use was
successful in 13.8% of the trials with small larvae, 95.7% with
large larvae
and 78.6% with centipedes.
Assessment of prey.--Prey
were usually not stung immediately after being seized by the
pedipalps. Immediate sting occurred in
14.7% (P. liosoma)
and 26.3% (P. pallidus) of the trials in which the sting was
used. In most trials, the sting was used only after
the prey struggled and resisted capture. In several trials, the
scorpions attempted to subdue the prey with the pedipalps for
several
minutes before finally using the sting.
Sting use against
non-resistant prey.--The scorpions quickly grasped large,
dead Tenebrio larvae which were moved on
the substrate near the pedipalps. Sting use were never observed
in any of these cases. This is significantly different from sting
use
with live prey of the same size (P. liosoma, P < 0.005, N = 9; P.
pallidus, P < 0.001, N
= 13).
DISCUSSION
The results provide evidence that
scorpions restrict use of the sting and thereby conserve venom.
This is supported by the
observations that they displayed decreasing sting use with
decreasing size/resistance of prey (Figs. 1 and 2). In most trials
when the prey were stung, scorpions did not sting the prey
immediately after seizing it (a period of prey assessment
occurred
before use of the sting). Moreover, no scorpion stung
non-resistant prey (dead larvae), even though they were large in
size.
This also supports the notion that the scorpion evaluates the
struggle and resistance activity of the prey before stinging it.
The possibility of restrictive
sting use was suggested from earlier observations of several
scorpion species (Pocock 1893;
Rosin & Shulov 1963; Le Berre 1979; Polis 1979; Cushing &
Matherne 1980), but experimental evidence was lacking before
the present investigation. Williams (1987) suggested that
scorpions more commonly eat their prey alive or crush them by
pedipalps
than inject venom. A similar pattern of restrictive venom use was
reported for some other predators. The ant, Camponotus
maculatus,
uses the venom spray differently for large and small prey (Dejan
1988), and some snakes reportedly vary the quantity of venom
used for different prey (Gennaro et al. 1961; Allon & Kochva
1974).
Sting use in P.
liosoma and P.
pallidus probably depends upon
the size, morphology and resistance of the prey. Large prey
(large larvae) and prey with powerful mouthparts (centipedes)
were stung frequently by both Parabuthus species, whereas small
prey (small larvae) and non-resistant prey (dead larvae) were
seldom stung. The size and resistance activity of the prey was
evaluated by the Parabuthus in an assessment period shortly after
capture.
A restrictive sting use in P.
liosoma and P. pallidus is probably advantageous because the use
of the sting and the following
venom renewal is expensive from an energetic point of view. This
was not examined, but it is a reasonable hypothesis since
the venom contains a mixture of water, salt, proteins and other
complex molecules (Simard & Watt 1990).
AKNOWLEDGMENT
I wish to express apprecitation
to my thesis supervisors, Yngve Espmark and Karl Erik
Zachariassen. I'm also grateful to
Roger Farley, Dieter Mahsberg, David Sissom, Gary Polis and Gary
L. Miller for valuable comments and criticism of
various draft of the manuscript. I thank Inger Andresen for
assistance with the figures.
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