Courtship - Water plays a vital role in courtship and breeding as well as in hunting and locomotion. The water surface sensory system is used in mate finding. Courtship is often very protracted, usually involving the male in numerous careful approaches across the water surface towards the female, vibrating his front legs in arcs across the meniscus, often moving his abdomen from side-to-side and stopping frequently to groom his palps. In all spiders the palps are the intromittent organ. The end 'tarsal' segment of the palp becomes swollen in the immature males. After the final moult it is fully developed as a structurally complex secondary sexual organ. Much of the complex structure is internal but external projections - apophyses - also help to guide and facilitate the process of mating. The detailed structure of one of these projections is the key feature used to distinguish male D. plantarius from D. fimbriatus.
Prior to courtship the male secretes sperm from inconspicuous sperm ducts under his abdomen on to a tiny triangular 'sperm-web', before drawing it up into a reservoir in the palpal organ.
If receptive, the female usually allows the male's approach. When the male and female are very close, they may both bob their bodies slowly up and down. The male usually flicks his front legs rapidly (Fig. 2a) as he moves in to touch the female, vibrating his legs across her body (Fig. 2b). She pulls her legs tightly in against her body to allow the male to roll her over onto her back in the water (Fig. 2c). In this position the male can inseminate her, transferring sperm from one or both palps into her genital opening, the epigyne (click here to watch Jame's Dunbar's video footage of courtship). If only one palp is discharged, the male will usually approach the female again for a second copulation. Inside the female, the sperm move via ducts to paired sacs, the spermathecae, where they are stored. The eggs are fertilised only later, as they are laid.
|Fig. 2a Male 'flicking' his front legs as he approaches the female||Fig. 2b Male approaching the female and touching her with his legs||Fig. 2c The female pulls in her legs to allow the male to roll her onto her back before copulation|
In captivity, mated females, particularly when very gravid, have been found to rebuff approaches from other courting males by aggressive flicking their front legs and by rushing towards them. However, there is evidence from DNA studies that multiple paternity can occur in broods in the wild.
|Fig. 3 Male D. plantarius courting an immature adult female - she is hiding, upside-down, underwater.||Fig. 4 Male D. plantarius courting a dead female|
The males will also court sub-adult females, even though they cannot mate with them: the females do not develop an opening to their reproductive tract until after their final moult. In captivity, an immature female has been recorded hiding from a courting male by remaining underwater for a protracted period (Fig. 4).
Chemical signals also play an important but under-researched role in courtship and appear to be very important in helping the male locate the usually passive female. The males often follow the draglines left by receptive females, repeatedly touching them with their palps as they move across the water. The male pictured in Figure 4 was persistently courting the dead female!
Eggs and egg sacs - Proximity to water is vital to survival of the eggs once they are laid. The females lay their eggs into a large silk sac. They spin this as an inverted cup, anchored to vegetation. The female pivots in circle beneath the growing cup, streaimg silk upwards around its rim. The eggs are laid upwards into the cup which is then sealed with more silk. The anchoring threads are then cut and a final layer of silk is added by the female as she turns the ball round and round with her feet (Fig. 5a-c). The silk of the freshly spun sac is brilliant white and crinkled - the final layer of crossing thread that encase it appear to be of straighter silk threads. On contact with the water, the white silk quickly turns grey as it oxidises. The female is clearly very vulnerable to predators immediately prior to and during construction of the silk sac and egg laying. In the wild, the sacs are spun in dark spaces deep in amongst damp vegetation, close to the water. In captivity, and probably also in the w, this is a nocturnal or crepuscular activity.
|Fig.5a Spinning an egg sac - the female lays her eggs into an inverted cup of silk and then continues spinning to close the opening.||Fig. 5b The sac is detached & the female turns it with her legs adding a final coating of silk.||Fig. 5c The completed sac is detached - the silk turns grey when it oxidises on contact with water|
The female carries her egg sac under her body for three to four weeks, holding it firmly in her chelicerae, with her abdomen curled round it (Fig. 6a), sometimes with an additional silk attachment to her spinners at the end of her abdomen. She usually puts her sac down only when she needs to groom - an essential activity for maintaining her waterproofing. Even then she will remain in physical contact with the sac, ready to grasp it again at the slightest sign of danger. The females rarely feed during this period. In captivity, only a small proportion of females offered food will put their egg sacs down and eat. In the wild, there are very few records of females with eggs sacs taking food. Particularly in dry weather, the female must push her egg sac under the water surface every few hours to keep the eggs moist (Fig. 6b).
The young spiders hatch and undergo their first moult whilst still within the sac. As the time approaches for the young to emerge, the female spends increasing amounts of time sitting in emergent vegetation at the location where her nursery web will eventually be constructed, up to a metre above the water (Fig. 6c). The females appear to be very vulnerable to predation, holding their large sacs in these relatively exposed locations, but are able to drop suddenly into the water if disturbed.
|Fig. 6a Adult female D. plantarius carrying her egg sac||Fig. 6b Female dipping her egg sac in water in hot weather||Fig. 6c Female with egg sac 1m up in Cladium mariscus, seen from below|
When the spiderlings are ready to hatch, the egg sac becomes softer and starts to sag. The mother assists their emergence. During the night, about 24 hours before emergence, she turns the sac with her legs, biting through the tough silk. By morning holes though which the young eventually emerge are clearly visible (Fig. 7). Although a few young may emerge during the day most do so in the early hours of morning, before dawn. Captive females have also been observed interspersing tearing at thier sac with their chelicerae with spinning the framework of the nursery as the bulk of the spiderlings start to emerge.
Fig. 7 Spiderlings starting to emerge from their egg sac. The red arrow shows the position of the hole through which they are emerging. The photograph is of a female D. plantarius held in captivity for research under license.
Nurseries - The spiders of the family Pisauridae, including D. plantarius, are also known as nursery web spiders because they guard their young in large, tent-like silk nurseries. The nurseries of D. plantarius are attached to emergent vegetation between 10 and 100cm above the water. The mother usually sits close to the structure, in contact with the anchoring threads. and will rush out and attack anything that threatens the integrity of the web. The webs are constructed at night and so it is difficult distinguish the relative roles of mother and spiderlings in their construction. In captivity the mother constructs the basic framework of the web, anchoring long silk strands to the surrounding vegetation. Much of the dense internal silk structure appears to be contributed by the activity of the spiderlings, dragging silk lines behind them. The structure of the web is more dense after 24h than when first constructed and starts to disintegrate about 24h after dispersal.
During the day the spiderlings cluster in a tight ball, exploding from it to disperse down the anchoring silk lines if danger threatens. At dusk they become active, moving around the web and feeding on tiny flies caught within its structure. The young remain in the nursery for around a week before they disperse and begin an independent life. In captivity, some spiderlings start to exhibit cannibalism if left in the nursery in a confined space from more than about 10 days.
|Fig. 7a D. plantarius nursery in Cladium mariscus||Fig. 7b Dew-filled nursery in Stratiotes aloides|
Dispersal - There are no recorded observations of dispersal of D. plantarius spiderlings from nurseries in the wild; this appears to be a largely nocturnal process. Many of the spiderlings are likely to leave the nursery by moving out into the surrounding damp vegetation. High concentrations of them are found in the vicinity of deserted nursery webs. D. plantarius spiderlings also have the potential to disperse by so-called ballooning - the process by which small spiders release silk lines that catch in the breeze, lifting them into the air like a kite. However, although the spiderlings show elements of ballooning behaviour - moving upwards through the vegetation above the nursery, standing on tiptoe and releasing silk lines in gentle breezes - they usually wait until their silk lines catch in near-by vegetation and then move along them (so-called 'rigging'), rather exhibiting true ballooning behaviour and free-flying in the breeze over greater distances.
Fig. 8 D. plantarius spiderlings in the rigging position with silk draglines just visible, flying from their spinnerets
The rather commoner D. fimbriatus has a greater tendency to balloon under experimental conditions and, in the field, its spiderlings are frequently recorded in trees. The reduced tendency to balloon in D. plantarius may represent an adaptation to living in small, isolated patches of habitat. Ballooning spiders have no control over where they land and, when suitable habitat is in very short supply, there is likely to be heavy selection against individuals dispersing by this means.
The tiny spiderlings rarely re-appear on the water surface until they are 3-4mm long. If blown onto a water surface, or if the surface is disturbed by the wind, they can drown by 'sticking' to the meniscus. At this stage they are thought to live and hunt largely in damp vegetation.
Phenology - At Redgrave & Lopham Fen in the UK, raft spiders usually take two years to mature, becoming adult, breeding and dying two years after hatching. However, there is increasing evidence that the life span can be quite variable; from one to three years. Most of the spiders are thought to spend their final winter as sub-adults, undergoing their final moult and emerging as adults in April or May. Courtship begins immediately. Females swollen with eggs are in evidence during June and the first nurseries start to appear in late June.
Adult male D. plantarius are rarely found in the wild after the end of July. They are exhausted by the end of the mating season, during which they feed infrequently and move relatively long distances in search of females. Cannibalism of the males by the females during mating is rare although it has occasionally been observed in captivity, immediately after mating.
Adult females can survive until autumn. Although most nursery web building is concentrated in July and August, the breeding season is protracted. Most adult females are thought to make two breeding attempts during the summer in the UK, usually producing a larger egg sac at the first attempt. On the south coast of England there is some evidence that a small minority of females produce three egg sacs during the summer. By the time the second brood eggs are laid, the males have all died and stored sperm must be used to fertilise them. At Redgrave & Lopham Fen, second broods have a much higher failure rate: by late summer the females are usually in poor condition, often with one or more legs missing, and weather conditions are usually less favourable to successful completion of breeding.
After their first brood of young disperse, the females usually move back to the water surface to resume hunting as their bodies swell with eggs once again. After their second brood has dispersed, they often remain at the nursery, sometimes for several weeks, until they die. In captivity, females at this stage feed infrequently and die by late autumn, even though they appear to be in good condition.
Immature D. plantarius are relatively inactive during their first and second winters; there are relative few records of them during the winter months and little is known about their requirements during this period. In sites subject to extensive winter flooding they almost certainly spend at least part of it underwater, perhaps in air pockets under the banks or in the bases of emergent aquatic plants. At Redgrave & Lopham Fen they have been found in dipwells - perforated metal tubes used for the measurement of water levels - in severe winter weather.
|Fig. 9a Moult - the old carapace of this unbanded D. plantarius has split off and the legs are just starting to be withdraw from the old skin (photos. of captive animals held under license for research).||Fig. 9b Moult - the legs are being pulling out of the old skin. The old carapace can be seen at the base of the abdomen||Fig. 9c Moult - the last two legs legs are almost free||Fig. 9d The spider is free of its old skin (exuviae), attached only by the spinners. It pulls its legs in tightly for several minutes before flexing them again and detaching completely from the old skin.|
The spiders enter their sub-adult stage after their penultimate moult, usually in their second autumn. The final moult takes place the following spring. In captivity, UK D. plantarius undergo a maximum of 13 moults between dispersing from the nursery and becoming adult nearly two years later. However, when reared under consistently warm condidions they can mature in one year and the number of moults is reduced. Moult is an intrinsically risky process (Fig. 9 a-d). The spiders loose interest in food for a few days prior to moulting. They suspend themselves hidden under vegetation to reduce their vulnerability to predators. In captivity moult is one of the commonest causes of death. If the process is not completed rapidly, limbs beome stuck in the old skin or, if withdrawn successfully, are curled, preventing normal movement. Spiders with injuries to their limbs can excise them by biting them off prior to moult. This avoids the difficulty in withdrawing feformed limbs from the old skin. Loss of legs is common in the wild - they are shed easily if grabbed by a predator, breaking at the weakest point, between the coxa and trochanter, close to the body. When a limb is lost, a new one develops within the coxa and uncoils as a normal, but atypically short limb after the next moult. The new limb grows with each succeeding moult, taking 3-4 moults before it regains normal size.