Pholcus phalangioides

Family : Pholcidae

Text © Prof. Giorgio Venturini

 

English translation by Mario Beltramini

 

The Skull spider or Cellar spider (Pholcus phalangioides Fuesslin, 1775) is an arachnid belonging to the order Araneae, family Pholcidae.

The name of the genus Pholcus comes from the Greek “pholkos”(φολκος) , that means crooked, rickety, lame or snub-nosed: in the Iliad (Book 2) Homer thus defines Thersites, ugly and troublesome Greek warrior who is insolent with the leaders.

The species Phalangioides comes from Phalangium, a genus of opilionids with the long legs (whose name in turn comes from the Greek “phalanx” (φαλαγξ) meaning joint of the finger, stick, but it is also the name of a spider) and from the Greek “oides” (οιδης), suffix that means “similar to”, hence similar to a Phalangium.

We meet Phalanx also as character of a little known version of the Greek myth of Arachne, the girl turned in spider by the goddess Athena because having too much vaunted her own weaving skills (Athena piqued herself to be the best weaver in the world). Phalanx should have been Arachne’s brother, also transformed in opilionid.

Zoogeography

The species Pholcus phalangioides is probably native to the subtropical regions of the Old World, its synanthropic characteristics have then determined its diffusion in most of the temperate countries of all continents, with a greater diffusion in South America and in Europe.

Habitat

Being unable to survive in cold environments, in many regions of its distribution range its habitat is limited to the houses, especially the warmed up ones.

It loves calm and with poor lighting locations. It builds up its cobwebs, usually oriented horizontally, especially in humid locations and poorly lit or in little disturbed zones of buildings, cellars or garages. Often they are met also in warm and dry places, such as spaces of windows or the attics, on garrets and in the corners of the rooms. In many parts of the world its presence is accepted or even appreciated, thanks to its fame of killer of dangerous spiders, even if the diffused and stupid arachnophobia may render problematic the cohabitation with the man who, also for aesthetic reasons, tends to sweep away the webs. Besides in the houses, in the mild climate zones, it builds its irregular web in the trees cavities or among the rocks.

Morphology

Like the other members of the family, Pholcus phalangioides has very long and thin legs, and for this reason is often mistaken with an opilionid; however it is easily distinguished due to the body subdivided in two parts, prosoma and opisthosoma, whilst in the opilionids the body is not subdivided.

With its 7-10 mm long body, it is the greatest species of the family. The females are slightly bigger than the males. The colour is usually grey-pale brown, the back of the prosoma has a dark spot with poorly defined borders and also the opisthosoma shows dorsally slightly dark and blurred spots. The sternum is usually grey, without dark areas. The shape of the body, roundish, has been compared to a human skull, hence the English name of skull spider; others do describe it as similar to a peanut. The most striking feature is given by the very long legs, up to 5 or 6 times the length of the body and covered by thin grey bristles. The eight eyes are arranged in a central couple and two lateral groups of three. The eyes of the central couple are much smaller than the lateral ones. Being the body translucent, under the microscope we can appreciate the cells of the hemolymph that are circulating.

Like the other spiders the Pholcus phalangioides has the body covered by an exoskeleton, that is a not extensible cuticle, consequently for being able to grow up, a moulting must be done, in fact it has to change its exoskeleton. In order to get out from the old one, the spider produces enzymes that detach it from the underlying tissues and produce a dorsal fissure through which the spider gets out, abandoning the old cuticle, called exuvia. The frequency with which the moulting is done varies depending on the age (it’s more frequent in the young spiders) and on the feeding conditions.

Ethology

These spiders build an irregular and quite loose web, usually arranged horizontally. Usually the spider stays in the web with the back facing down. The most evident behaviour feature is that of a particular defensive strategy in which the spider, feeling menaced, oscillates rapidly the body with circular movements, keeping itself clung to the web with its long legs. This causes a very fast vibration of the web that renders the animal practically invisible (this behaviour stands at the origin of the Italian popular name of “ragno ballerino’ = dancing spider). We may easily cause this response by touching lightly the web with a finger. The oscillations usually last a few seconds or tens of seconds, but when the aggressive stimulus is serious, like the attack by another spider, it may last even several minutes. If then the disturbance persists, the spider reaches a corner of the web or falls down and gets away.

Often can be seen intersected webs of different specimens. In fact, the skull spiders normally coexist peacefully among them, but are decidedly aggressive towards spiders of other species. If for instance a Tegenaria enters their web, this is immediately covered by threads that reduce it quite soon to immobility.

Then follows a highly effective venomous bite, given usually to a leg.

At this stage the victim, often much bigger than the predator, predigested due to the action of the poison, is generally emptied thanks to another bite on a leg.

Pholcus phalangioides is predator of other spiders and small insects; moreover, males as well as females may practice the cannibalism in case of absence of preys of other species.

The web has no adhesive properties, but its structure, loose and irregular, traps the insects rendering difficult the escape.

When a prey is caught, the spider rapidly wraps it in the silk using the second and third pair of legs to rotate it and the fourth pair for leading the thread coming out from the spinneret glands. Particularly big preys are secured with tie rods anchored on the highest part of web. The preys are killed with a venomous bite and can be eaten at once or conserved.

Also preys who, while not having come across the web limit to walk along the tie rods, are successfully attacked. The skull spider protrudes from its web, clinging only with two legs, and launches its silk against the prey. In any case it does not seem that it attacks preys staying completely out from the web. Occasionally, it nourishes also of the eggs laid by other spiders or of insects caught from others’ webs.

As predator, Pholcus phalangioides shows a remarkable versatility. Unlike many other spiders, the skull spider frequently leaves the web and goes hunting, not hesitating to invade the web of other spiders for killing and eating them. This activity, however, is not devoid of risks seen the poisonousness of the potential prey.

In such case it implements a strategy of aggressive mimicry doing some specialized vibratory movements that impart to the victim’s web oscillations that mislead the owner, simulating this produced by a normal prey that wriggles on the web trying to get free. The spiders, in fact, do interprete the vibration on the web as mark of successful catch.

When the spider gets close, attracted by the vibrations of the web, the skull spider raises on its long legs and keeps motionless till when the prey comes in contact with a leg and only then it triggers its attack immobilizing it by means of the silk and then poisoning it with a bite.

Thanks to its long legs, Pholcus phalangioides is able to immobilize the prey even if keeping far from it, thus avoiding a possible bite.

The skull spider is well equipped for invading the webs: with its very long legs it can cross even webs with very large meshes and, placing only the tip of the tarsi avoids to be entangled.

Only the adhesive piece of the web, that is, whose threads have the surface covered by very thin curled filaments that render them particularly adhesive, can put it in trouble.

When it walks on the web it’s invading, the legs of the skull spider may occasionally remain entangled. In such case, it bites off the silk and cleans up the leg, before eliminating the adhesive piece of the web and replacing it with its own thread.

Some studies suggest that the eyesight plays a minor role in the predatory behaviour of the Pholcus phalangioides, who instead takes advantage mainly of its sensitivity to vibrations.

Although having the habit to invade the webs of many species of spiders, its efficiency as predator of other spiders varies very much in base of the type of web, and is quite less in case of the cribellate webs as those of the amaurobids.

Reproductive Biology

The adult specimens are present during the whole year: the females can live up to three years, two of which at the adult stage, the males 1-2 years only.

As a consequence of its tropical or subtropical origins and of the domestic habitat, its vital cycle is not influenced by the seasonal changes and can reproduce all year long.

Even if mature animals can be found during the whole year, the mating however occurs more often in early spring.

It appears that the males are more attracted by females of large dimensions than by the smaller ones, this may increase the reproductive success, since the bigger females produces a bigger number of eggs.

The males find the females followings an odorous trace of pheromones, whilst during the mating is a tactile communication prevails.

The male must approach the female cautiously, in order not to be mistaken for a potential prey and then attacked.

The strategy consists in giving the web vibrations of a particular frequency acting as a specific signal.

Quite particular is the method of drawing the sperm in the male: this in fact tends a thread between the legs of the third pair, then rubs it back and forth on the genital orifice till when a drop of sperm gets out that then remains stuck on the thread. The thread of silk is then passed ahead, carrying the drop of sperm up to the chelicerae, from here, finally, it is taken by the very voluminous copulatory organs present on the pedipalps.

During the courting, the male does some vibrating movements in face of the female hanging abdomen up on its own web and gets close to her from above, with the palpal bulbs spread apart outwards at 90°.

The mating may last various hours and the male lays the sperm with the pedipalps into the epigyne of the female. The female can conserve the sperm in a cavity at the beginning of the uterus, called uterus externus, till when the eggs are ready for the fecundation. The moment of the fecundation and of the deposition of the eggs is conditioned by the availability of food.

Since the sperm can be conserved even for long time, it is possible that the female couples again before that the eggs are fecundated and therefore the sperms of two males might mix in the uterus externus. It seems however that usually the last male has the priority in fecundating the eggs, probably thanks to a mechanism of removal of the previous male sperm during the coupling. As a matter of fact, the male, during this, performs rhythmic movements of the pedipalps leading usually to the expulsion of the sperm already present. The about 20-50 eggs, of pale pink colour, united through a sticky secretion, are laid in an ovisac formed by few threads and of the diametre of about 4 mm. After having laid the eggs the female wraps them in the silk and keeps them under the body holding them with the chelicerae.

Shortly before hatching, in the eggs with the semitransparent shell are already recognizable the long little legs still bent. The newborns just out from the shell gather out of the pile of eggs and together with this are placed on the web. The mother watches the newborns (prenymphae) for about 9 days, till when they moult and transform in small spiders morphologically complete. The young now leave the mother’s web and go looking for a suitable site where to build their own web.

Relationship with man

As well as the opilionids, with whom it is often mistaken, the Pholcus phalangioides has the fame of having a very strong venom; however it is said that its dangerousness should be modest due to the small size of its fangs (the sharp extremities of the chelicerae). Actually, the opilionids are not venomous at all and the bite of the skull spider can pierce the human skin (the fangs are about 0,25 mm long, whilst our epidermis has a thickness usually less than this measure), but causes, at most, a feeling of mild burning lasting just a few seconds.

The fame that Pholcus phalangioides has of being a very venomous spider comes may be from the fact that it often kills and eats up really venomous spiders and even potentially fatal for man, like the Australian “redback spiders” (Latrodectus hasselti). Actually, this capacity it has does not come from a bigger poisonousness but from its ability as hunter. As typical and efficient predator of other spiders, the skull spider is very interesting as tool of struggle against venomous spiders. In particular, are under study attempts to use the Pholcus phalangioides in the struggle against Loxosceles reclusa (Sicariidae), the so-called brown recluse spider or violin spider of southern USA, very venomous and of health interest seen its dangerousness for the man.

Other attempts concern the struggle against venomous spiders of the genus Latrodectus, such as the Australian Redback spider (Latrodectus hasselti) or the American Black widow (Latrodectus mactans), considered as standing among the most dangerous. Other studies have shown that the skull spiders are efficient predators of the wood borers. In case of an infestation of borers in old houses the population of Pholcus phalangioides increases and can quickly reduce significantly the numebr of the borers; the decrease of the bores in turn leads to the reduction of the number of the spiders. It is under study the potential use of these spiders for eliminating the borers and a positive outcome might allow to reduce the use of chemical treatments dangerous for the habitat. Besides the struggle against the borers, the skull spiders might be useful against the mosquitos.

Skull spider predators

Predator of spiders, that hunts on their own web, in turn the skull spider is predated by other spiders, in particular of the family of the salticids, some of which utilize its same strategy of aggressive mimicry based on the vibrations of the web. The best studied case is that of the salticids of the genus Portia, such as Portia fimbriata, present in Australia and in South-East Asia, who is one of the most fearsome predators of the Pholcus.

This salticid uses the same predatory strategy of the Pholcus, mimicking the movements of a victim trapped in the web and therefore the consequent vibrations. This salticid is particularly clever in adequating the vibrations it imparts to the web to the characteristics of the spider who is aggressing and goes on in applying them even for long time. Have been observed specimens of Portia continuing to vibrate the web even for three days, till when the intended victim is misled (the spider’s patience!). Moreover this salticid choses for its huntings the slightly windy days that vibrate the web, creating therefore a disturbance that helps in deceiving the victim. To its success as predator of other spiders contributes also the appearance, that simulates a fragment of dried leaf caught in the web.

The skull spider is clearly in condition to distinguish the dangerousness of its aggressors and in fact it responds to the intrusion of the salticid spiders making himself invisible with vibrations of the web much more long than what is the case of other predators.

Synonyms

Aranea phalangoides Fuesslin, 1775; Pholcus dubiomaculatus Mello-Leitão 1918; Pholcus litoralis L. Koch, 1867; Pholcus communis Piza, 1938; Pholcus phalangioides Walckenaer,1805.