INTRODUCTION TO SALTICIDAE
[From J. Proszynski: Salticidae of Levant. Annales Zoologici,57:1-180, figs 1-741] Fragments from the book by Foelix 1982 and papers of R. R. Jackson by courtesy of Dr. R. F. Foelix and Dr. R.R. Jackson
EXTERNAL MORPHOLOGY AND ANATOMY OF SALTICIDAE
The general structure of Salticidae does not differ
significantly from the structure of other spiders . The body
of a spider is divided into two parts: the cephalothorax (prosoma), specialised
for nervous, sensory, locomotion and food taking functions; and the abdomen
(opisthosoma) with digestive, reproductive, respiratory and circulatory organs
and spinning glands.
The following description of the general biology of Salticidae is largely based on the book "Biology of Spiders" (Foelix, 1982).
The cephalothorax bears the appendages and eyes, and a variety of other sense organs; and houses the central nervous system, the pumping part of the stomach and a large mass of different muscles; it is covered dorsally by a hard, sclerotized shield called the carapace and ventrally by the sternum; the legs protrude through a slit between both shields, each segment of the leg being covered by its own tubular shield. Narrow pedicel connects the cephalothorax and the abdomen. The appendages consist of a pair of chelicerae, specialised for prey capture; a pair of the pedipalps, sensory and specialised for communication with other spiders, and, in males, having reproductive functions; and four pairs of locomotory appendages, the legs. The chelicerae consist of a robust basal segment and a movable fang bearing the terminal opening of the venom gland; the fang rests in a groove armoured on one or both edges with a tooth or teeth, which assist in grasping prey. The legs consist of 7 segments of different shapes and function: coxa, trochanter, femur, patella, tibia, metatarsus, and tarsus. The pedipalps have no metatarsus and in males the tarsus (or cymbium) is specialised for reproductive functions and houses the male copulatory organ (palpal organ); the coxae of the pedipalps bear large maxillary plates (or endites), which constitute part of the mouth parts. The appendages are bent and stretched at the joints by muscles, sometimes extended by tendons. Stretching at two important joints: between the femur-patella and tibia-metatarsus, where there are no extensor muscles, is effected by a rise in the hydraulic pressure of the haemolymph, produced by the pumping contraction of huge muscles inside the cephalothorax. This mechanism is used during jumps, executed by either third or fourth pairs of legs, which are developed accordingly. Legs I and II are used during landing, walking and catching the prey, the latter activity being assisted by an adhesive brush of setae (scopula) on the tarsi, occurring in some genera. The legs and pedipalps, as well as various areas of the cephalothorax, have a large number of various sensory hairs, slits and pits, serving as receptors of touch, vibration (air and silk threads) and as chemoreceptors, the latter consisting of thin membranes, always humid, on which particles from the air stimulate sensory cells. The location of sensory organs on movable parts of the body provides a better reception of weak stimuli in the air, probing of the ground and probing of closely approached insects and specimens of the same species. Intraspecific communication may occur by exchange of chemical and vibration stimuli, but in the Salticidae it is predominantly visual signals: movement of contrasting coloured parts of the body in a specific rhythm and timing of these movements.
Spiders digest caught prey externally, pumping digestive enzymes inside its body and sucking out the liquidified soft tissues of the prey. The liquid food is drawn through special filters into the digestive section of midgut, which gives off numerous diverticula entangled around the internal organs of the abdomen and cephalothorax, penetrating even into the coxae of the legs.
Internal transport of digested food takes place through the walls of the midgut diverticula. Haemolymph flushing the surfaces of the diverticula, dilutes and takes up nutritive substances and carries them to cells throughout the body; part of nutritive substances are also stored in various cells for periods of food deprivation. The haemolymph also carries oxygen, taken up in thin membranous lamellae of the book lungs and bound to the copper containing respiratory pigment haemocyanin. The haemolymph then flows through pulmonary veins into the pericardial chamber, from which it is sucked into the elongate heart along the dorsal wall of the abdomen and subsequently pumped out through various arteries into the open body cavity in the proximity of the major organs, where the exchange of oxygen and food substances with the cells and the removal of metabolic wastes from the cells takes place. The necessity of direct access of haemolymph to each cell to enable such exchanges influences the structure of organs; they are either built of a single layer of cells, or a few layers only, or of loose tissues allowing the seepage of haemolymph between cells, as in bundles of muscles. A second respiratory system consists of tubular tracheae carrying atmospheric air from a pair of stigmata on the ventral surface of the abdomen into the proximity of the tissues; in Salticidae the tracheae are well developed, branched, and penetrate from the abdomen into the cephalothorax and appendages.
The removal of the metabolic wastes carried by the haemolymph takes place using two excretory systems. The extracellular one, consisting here mainly of endodermal malpighian tubules, open to the stercolar pocket where dehydration of wastes takes place and dry wastes are pushed through the anus; there are also vestigial coxal glands in cephalothorax. The intracellular excretory system deposits wastes in some specialised cells, thus removing them from the general circulation in the body.
In spiders the central nervous system consists of about 30,000 neurones and is highly condensed into two compact ganglia in the cephalothorax, the supraoesophageal (called also the brain) and the suboesophageal, while vestiges of the abdominal ganglia appearing only during embryonic development. The brain of active hunting spiders differs from the web spinning ones in the much stronger development of visual centres.
SPECIAL FEATURES OF THE SALTICIDAE
The anterior end of the cephalothorax in the Salticidae
is very characteristic: broadly truncated, almost rectangular, with large Anterior
Median Eyes (AME) and smaller Anterior Lateral Eyes (ALE), usually about
half the size of the first, arranged in a single row; Posterior Median Eyes
(PME or eyes of the III row), also relatively large, are located far behind
along the dorsal edges of the cephalothorax; and the much reduced Posterior
Lateral Eyes (PLE or eyes of the II row), positioned between ALE and PME. The
space between all eight eyes is called the eye field and its outline can be
rectangular or trapezoidal, narrowing or broadening posteriorly, and is often
a useful key character. The specialisation of eyes in Salticidae goes far beyond
mere geometrical arrangement. The prey is spotted from long distance, often
40 cm or more, by secondary eyes (PME, PLE, ALE) which have broader field of
vision and serve as movement detectors, but do not make a sharp image of the
object. When the moving object approaches to within 4-10 cm, the spider turns
in its direction and catches it in the narrow angle of vision of the Anterior
Median Eyes whose large lenses permit telescopic sighting. The retina of the
AME is "boomerang shaped", and consists of about 1000 visual cells
arranged into four differently specialised layers, most dense at the fovea.
It is screened posteriorly by a dark pigment layer and can be moved by special
muscles, which permit change of focus. The AME can form a high-resolution picture
of the prey from a distance of 10 cm or closer; when the prey moves the muscles
shift the retina to maintain its picture at the foveal area. During the "recognition"
process the retina is in constant motion, the shape of the object is quickly
scanned horizontally and at the same time the main eye is rotated along its
main axis, a procedure apparently necessary for identification of moving objects.
The spider begins pursuit of the prey from a distance of about 10 cm, stalking
it from 4 cm and leaping at it from 2 to 1 cm when the prey image is most precise
(Foelix 1982). It is supposed that Salticidae have colour vision. The acuity
of vision of the AME is apparently even higher than in the compound eyes of
insects and is comparable with the eyes of higher vertebrates.
The variety of shape and proportion of the cephalothorax is usually due to length of the eye field and height at eyes III relative to length of cephalothorax, but this is partially illusory. Actually such parameters as the relative length of the eye field vary in the majority of Salticidae from about 40 to 50%, rarely less than 30 % or more than 60 %, and the variation in height of the cephalothorax is rarely less than 40 % or more than 60 %. The illusion is caused by the shape of cephalothorax: a long posterior slope hidden under the bulging anterior part of the abdomen makes the cephalothorax appear short and the eye field relatively long; the impression of height is influenced by the presence of a contrasting belt along the ventral margin of the carapace.
The height of the cephalothorax may possibly be partially correlated with the location of the main locomotory muscles. If the muscles moving particular pairs of legs are developed predominantly within the legs themselves, the cephalothorax may be lower. If, however, the major movements of a pair of legs are mainly executed by muscles in the cephalothorax (acting by pressuring the haemolymph) the bulk of these muscles is larger and hence the cephalothorax higher.
The variety in length of legs I-IV has a functional correlation. Good, rapid runners among the Salticidae have robust legs of equal length; jumpers have legs III-IV more strongly developed. This pattern may be modified in some genera by a strong development of legs I, which may be strikingly longer and more robust, fulfilling important tactile and catching functions; in some genera the development of legs I may be sex linked, with legs I longest in males, while legs IV may be distinctly longer in females.
The shape of the abdomen varies within the Salticidae from long to round, narrow to broad, tapering to truncate and high to low. The dorsal integument in some genera may be strongly sclerotized, forming a hard, light-reflecting, protective shield, called the scutum, usually occurring only in males.
The coloration of the body and its parts varies among genera and species from dull to strikingly contrasting or colourful iridescent patterns. It is assumed that the contrasting colour pattern serves as recognition characters to other spiders. Dull coloured species presumably rely more on chemical signals. A contrasting pattern on the dorsal surface may have a cryptic camouflaging function by breaking the outline of the spider into pieces as seen by predators. The colour pattern is due either to pigmentation, distribution of coloured or iridescent setae and scales, or both.
A special sort of protective adaptation, developed in some 400 species of Salticidae world wide (and some other families of spiders as well) is ant mimicking; this presumably would not deceive ants, which rely mainly on chemical signals, but may deter some visual predators among the vertebrates.
The genital openings in both sexes are located anteriorly on the ventral surface of the abdomen, near the epigastric furrow, and open after the final moult. In female Salticidae the opening is surrounded by a heavily sclerotized plate called the epigynum, which often has complicated, species-specific relief sculpture. The epigynum contains a pair of copulatory openings connected by copulatory channels, usually sclerotized, to sperm storing spermathecae, from which spermatozoa are extruded through a soft-walled channel into the uterus to fertilise the eggs. Sperm may be stored alive for as long as one year (Wild 1969a) (but in ants, stored spermatozoa were found alive after 19 years). There are two kinds of openings in the walls of the spermathecae, provisionally called accessory gland openings: the distal nutritive pores are supposedly used for nutrition of stored spermatozoa, and the proximal scent openings exude sex pheromones to the outside. The eggs are laid through the prominent opening in the middle of the epigynum, or through the soft-walled opening beneath it.
To inseminate the female, the male must first transfer a droplet of seminal fluid from the genital opening into the copulatory organ located on the apical segments of the pedipalps. That organ develops during the last moult. It operates as a pump by changing the pressure of the haemolymph: a rise in pressure inflates the organ and squeezes its internal sperm reservoir; by a partial reduction of pressure the seminal fluid is sucked into the reservoir through a thin spine-like inserting tube called the embolus; a further reduction of pressure contracts the copulatory organ to its normal resting size and position. During copulation, the rising pressure of haemolymph inflates the organ and when the embolus is introduced into the copulatory opening of the female, it squeezes the reservoir and injects seminal fluid into the spermathecae of female. The male copulatory organ contains various sclerotized structures which engage corresponding structures of the female's epigynum during copulation. In some Salticidae, the distal part of the embolus may break off after copulation and remain in the copulatory opening of the female, blocking it. A hardening waxy secretion, presumably preventing copulation with other males may also seal the female opening. The males die shortly after copulation; however the proverbial post-mating cannibalism by the female has been found only exceptionally in Salticidae. Females build cocoons and lay eggs a few weeks after mating, often guarding the eggs and sometimes the young spiderlings, also possibly feeding them as well. Like other arthropods, growing spiders have to shed their sclerotized integument several times before they reach adulthood. The life cycle lasts from one to several years. The most common pattern found in temperate zones is: egg laying in late spring; hatching of young a few weeks later, growing during the summer and autumn; overwintering, reaching maturity and reproducing the following spring, after which females may survive for another year. There is considerable variation in this pattern; in many species of Salticidae the adults of one or both sexes are also found in autumn and winter. There is much to be learned still about the life cycle of many species.
(The following description of the behaviour of Salticidae is largely based on the papers of R.R. Jackson and his collaborators . See REFERENCES.).
The behaviour of the Salticidae is still insufficiently
known, although the past two decades have brought about considerable progress
in this area.
With the exception of a single tropical genus Portia, Salticidae do not spin webs but catch prey by stalking, chasing or ambushing. A special adaptation of the Salticidae is the acquisition of superior vision as their leading sense, with the senses of smell and vibration being accessory; the opposite is the case in other families.
The majority of advanced Salticidae are cursorial predators
of insects; they are agile and fast moving, using stalking and chasing behaviour
to capture insects. This is facilitated by the acute vision of their main eyes,
with secondary eyes functioning as movement detectors in a large peripheral
area (Blest, 1983). They have lost the ability to build webs, but have retained
nest spinning, which is appropriate to a more nomadic life.
Their usual walking is characterised by a specific stepping pattern; of their 8 legs, two sets always move alternately. Left legs I and III move together with right II and IV, and the remaining legs are at rest. During slow walking there may be a small delay in the movement of each leg, but during fast running all the legs of a set move more synchronously. When jumping spiders stop, they draw their legs symmetrically around the body, which allows for a sudden leap.
The legs are anatomically equal, but may differ in length; equal length facilitates faster running. Legs II and I are usually directed forward and pull, whereas legs III and IV point backward and push. The moving of a leg consists of a lifting phase (forward motion without touching the ground) and a pushing or pulling phases. When the animal moves ahead the joints of the anterior pairs of legs must bend and the posterior pairs stretch. Precise co-ordination of movements of the legs is necessary for turning the body; jumping spiders can also move sideways and backwards. The quick extension and torsion of the third or fourth pair of legs (or both) perform jumps. Foelix (1982) describes the ability of Salticidae to jump a distance from a few centimetres to about 25 body lengths during escape (i. e. 1 cm spiders may jump up to 25 cm). However I have observed repeated jumps of about 1.5-2.0 m (about 150 to 200 body lengths) by Asianellus festivus while collecting it in Poland.
We might expect that the behaviour of over 4000 species of Salticidae would be as different as their niches, ways of life and morphology. Until now only a few species have been studied, including two occurring in the Mediterranean: Cyrba algerina and Plexippus paykulli (see Jackson, Hallas, 1986c; Jackson, MacNab, 1989). Apart from these there are only scattered observations on a few other species, so this is a broad field for further research.
It was suggested (Jackson, Blest, 1982) that ancestral
Salticidae were web builders, but also web-invaders, both as kleptoparasitic
predators of insects on alien webs and on other spiders and their eggs. They
may have used web vibration signals to communicate with specimens of the opposite
sex and may have also mimicked vibration of ensnared prey to lure the spider
host. Special adaptations for that way of life have been lost in the majority
of Salticidae and retained only by the tropical genus Portia, a relatively slow
moving, cryptic spider, which feeds predominantly on other spiders on their
webs. Similar araneophagic vibrator web-invasion behaviour has been documented
in only three genera belonging to primitive subfamily Spartaeinae, represented
throughout the Mediterranean by Cyrba algerina. Cyrba is behaviourally
intermediate; like Portia it is an araneophagic web-invader that uses vibratory
stimuli and can pursue web spiders for up to several hours to catch and eat
them; but simultaneously it is also similar in many ways to typical Salticidae:
building only a rudimentary nest; using visual displays and active, agile movements
during intraspecific interactions (dancing is performed by Cyrba, which
Portia is unable to do); and requiring most of the motor and sensory
abilities of effective cursorial predators of insects as in typical Salticidae
Cyrba algerina moves on firm substratum in a rapid, stop-and-go walking, suddenly running several centimetres and then resumes walking, jumping over gaps. Cyrba does not build its own webs, but walks with apparent ease on any type of alien web, and does not stick, even when dropped on its back. On webs with widely spaced threads, Cyrba uses its forelegs as probes. However, movements on an alien web are very different from those on firm substratum: they include very slow walking or shifting position of the legs; it may take 4-10 seconds to lift a leg, move it forward 2 mm and place it down; then it pauses for several minutes, and then completes the step by retracting the leg and easing forward, again taking 4-10 seconds. Often the spider moves forward one leg at a time in succession, until all or most legs have been shifted, before easing the body forward. Finding an alien web with some prey on it (a host, its eggs, or ensnared insects) Cyrba readily invades it, but in a very complicated and careful manner. It approaches the web slowly, ready to run away if attacked by a stronger host spider, moving increasingly slowly as it gets closer. It does not readily move completely onto the web, often remaining for prolonged period (2-7 hours) at the edge of the web, keeping at least the fourth pair of legs on a non-silk substratum. It touches the silk very slowly, usually with the anterior pairs of legs only, and remains inactive there for several minutes before beginning to pluck with its palps. Plucking causes the web to vibrate as if an ensnared insect is trying to liberate itself and this lures the host spider. The palps move up and down, forward and backward, also with rotary twists forward and backward, with some difference in the velocity and amplitude of movements. Bouts of plucking usually last 2-4 seconds, but sometimes for 10 min., followed by a pause of variable length. If the lured host spider begins to approach, plucking becomes more stereotyped. When the spider comes within a few millimetres of the "vibrating" predator, Cyrba slowly lifts legs I and II, extends them forward over the prey and stabs it with its fangs, piercing the cuticle and injecting venom, after which the fangs are immediately withdrawn. The prey becomes paralysed within about 20 seconds. If stabbed spiders try to run away, Cyrba slowly approaches and retrieve its paralysed victim. If the web host spider fails to approach, Cyrba sometimes moves slowly further out onto the web, advancing very slowly, eventually coming to within a few millimetres of its victim where it then makes an attack. Sometimes Cyrba leaps into webs onto prey, usually no more than a few body lengths anyway. At the end of the leap, the web spider was usually been grasped, but if the predator missed or contacted but failed to secure the prey, it moves slowly back to the edge of the web. Leaps are most common when a web spider becomes inactive after being lured within 10-30 mm following a long period (1 h.) of intermittent plucking. The predator slowly raises and extends legs I and II, and then leaps. Pursuit times with web spiders as prey are usually about 1 h. Cyrba occasionally feeds on insects in the sticky webs of other spiders, usually walking upto them as slowly as it does to the spider; they occasionally eat eggs of theridiid and amaurobiid spiders in the web. Under experimental conditions it did not respond to the eggs of cursorial spiders.
In other families of spiders even more complicated behaviour has been observed: Ero furcata (Mimetidae) has been observed in Poland to mimic sexual vibration signals of male Meta segmentata (Araneidae), which deceived the female of that species (Czajka 1963). In Florida, the nocturnal Dinopis (Dinopidae) lured male moths by emitting sex pheromones of their females (Mark Stowe - personal communication).
On the other hand Cyrba has been observed readily stalking cursorial insects, like typical cursorial Salticidae, approaching them rapidly from 50-150 mm, then slowing down as it got to within 20 mm. Legs are raised, a drag line usually fastened, and Cyrba attacks, either after a short pause (2-10 sec.), or immediately. The spider leaps from 1-3 body lengths away or approaches within a few mm and attacks by stabbing. If the spider misses, it never chases the insect. Pursuit times during cursorial predation on insects were usually 1 min. or less. Cursorial spiders were readily stalked and occasionally captured, but other Salticidae usually detected approaching Cyrba and decamped. Cyrba did not respond to the nests of cursorial spiders.
Males of Cyrba algerina detect the presence of
females by scent. Aerial and contact pheromones (for instance scented silk of
a drag line on the substratum), which are detected by sensory organs on the
pedipalps and legs, stimulates pronounced palpating (palps touching) and "quiver
swim waving" of the legs. Facing the female from about 100 mm away, the
male lifts his legs and begins to approach. Then in rapid succession he twitches
the abdomen (.25 s.), flutters palps (.25 s.) waves erect legs (.25 s.), and
steps to the side and dances briefly. There is an interval of 1-2 seconds of
erect posturing while standing, before the sequence of display is repeated.
The female usually simply watches the male leaving intermittently, the male
following with legs raised. Occasionally, the female advances quickly toward
the male which then usually backs away, the female running away about 7 cm,
then turns and faces the male. If female persists in leaving, the male usually
stops approaching and the interaction ends. While the female remains stationary,
the male advances toward her, stepping in a special way, either posturing, or
waving his erect legs. There are five types of different dances
performed by male Cyrba (zigzag, linear, semicircular, elliptical and
complex dance). In the most forward position of a dance the male brings his
erect legs over the female. If she remains inactive, the male mounts her and
copulates. Male-female interactions usually last 1-5 minutes, the copulation
itself only a few seconds (in other Salticidae it may take several hours).
The meeting of two males results in displaying erect leg posture when they are distant and crunched legs posture when they are close, ending finally in an embrace - both specimens facing each other with legs I-II erect, contacting each other with tarsi touching. Finally they separate and run away; the interaction usually lasting 1-30 sec. The meeting of two females is shorter (usually only 5-15 sec.) with the display of erect legs when they are distant, and crunched ones when closer, with propulsive advances intermittently. Generally, the display in Cyrba consists of over 30 different movements and postures.
Under natural conditions, the oviposition sites of Cyrba are presumably on surfaces of rocks; in laboratory conditions it spins a thick sheet of silk against the wall of the cage, oviposite in the centre and covers the eggs with a flimsy layer of silk. Occasionally it encloses the nest with crudely woven external silk layers, tubular in shape and open at both ends. Moulting is completed on a small platform of spun silk (the diameter being twice the body length), with the spider hanging upside down from the platform while moulting, often sliding down 10-20 mm on a drag line before completing the moult.
Plexippus paykulli is a large, cursorial salticid spider, belonging to the hoppers group (as opposed to the runners), which frequently leap about during normal locomotion and move in a characteristic stop-go fashion: rapidly advancing a few steps, pausing briefly to reorient, then advancing again. It is a fierce predator, attacking and capturing large and dangerous prey, overcoming them by bold attack and brute strength rather than by their relatively weak venom. Like Cyrba, it may also feed on insects ensnared in alien webs as well as the host spiders, but it gets them in a completely different manner. It leaps onto the prey on the web (unlike the slow, careful movements of Cyrba); if it misses the prey, it immediately struggles to free itself from the web.
The nests built by Plexippus, similar in both males and females, is a complex structure. It consists of large (30 x 20 cm to 50 x 30 cm) tube with a natural elastic opening at each end; the tube is surrounded by a dense tangled array of silk forming a thick layer (up to 20 mm) over the tube. Apart from making a safe resting place and protecting the eggs, the nest also helps in capturing insects if these become entangled in the silk. Nests play an important part in the behaviour of the spider, the males developing a special way of communicating with females, which they "smell " inside the nest, by vibrating the silk walls. In some instances, copulation takes place inside the nest; if the female is subadult, the male builds a contiguous nest and waits until the female matures, and then copulates with her.
When meeting females outside the nest, male Plexippus resort to a visual display, as rich as that of Cyrba consisting of a number of postures and dances, including a zigzag dance. Thirty five elements of behaviour (movements and postures) have been described by Jackson, MacNab (1989). Copulation may be accomplished in several ways, depending on the situation in which the mates meet, whether in a nest or outside. Sometimes spiders mate suspending themselves on the female's drag line, away from the substratum. Meeting a specimen of the same sex produces visual displays of a rich variety.
The way Plexippus responds to active insects
is presumably typical for the majority of Salticidae. After spotting an insect,
the spider orients itself towards the prey from as far away as 10 cm, then walks
rapidly towards the insect. On coming close it slows down, lowers the body,
pulls its legs towards the body (crouched posture) and, after a brief pause,
leaps 1 - 5 cm onto the prey, readily attacking and capturing mobile insects,
even those twice its own size. Locusts and hornets often jumped or flew away
with P. paykulli hanging on until its venom took effect, which sometimes
took several minutes. If an insect coming into contact with nests of P. paykulli
becomes stuck for several minutes or even seconds, P. paykulli responds
by coming out of the nest and either leaps onto prey from 1 -3 cm away or walks
across the nest over to the insect and attacks it; P. paykulli can capture
ensnared insects twice its own size.
Whilst we can draw some conclusions about other Salticidae from the above observations, there are many problems awaiting behavioural study. What is the functional significance of structures like the hardened integument (an abdominal scutum), elongate or overgrown chelicerae, or special setae brushes? What is functional significance of a contrasting colour pattern and to what degree does it assist in recognition of a member of the same species? And finally what degree of morphological variation corresponds with the biological limits of the species?