Life history

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, carefully tapping his front legs on 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 an often complex secondary, sexual organ. Prior to courtship the male secretes sperm from an inconspicuous sperm duct under his abdomen on  to a tiny trianglar 'sperm-web', before drawing it up into the palp. 

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, stroking his legs over her body (Fig.2b). She pulls her legs tightly into her body and allows the male to roll her over onto her back in the water (Fig. 2c). In this position the male can inseminate her, transfering sperm from one or both palps into her genital opening, the epigyne. 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.

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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 have been found to rebuff subsequent approaches by courting males by aggressive flicking their front legs and by rushing towards them. However, there is some 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 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 underwater from a courting male for a protracted period (Fig. 4).

Chemical signals also play an important but under-researched role in courtship. They appear to be very important in helping the male locate the usually passive female. The male pictured in Figure 3 was persistently courting the dead female!

Eggs and egg sacs - Proximity to water is vital to survival of the eggs once they are laid. Sometime after mating, the females lay their eggs into a large silk sac. They weave this, anchored to vegetation, as an inverted cup into which they lay their eggs. The cup is then sealed with more silk, its anchors are cut and a final layer of silk is added by the female as she turns the silk ball round and round with her feet (Fig. xa-c). Although the silk of the freshly spun sac is brilliant white, it quickly turns grey as it oxidises on contact with water. 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 field, it is always a night-time 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 carrys her egg sac under her body for approximately three weeks, holding it firmly in her chelicerae, with her abdomen curled round it and sometimes with an additional silk attachement to her spinners (Fig. 6a). Particularly in dry weather, the sacs are pushed under water every few hours to keep the eggs moist. (Fig. 6b) The females rarely feed during this period. Only a small proportion of captive females offered food would put their egg sac down so that they could eat - this behaviour is also recorded only rarely in the wild. Towards the end of this period they spend increasing amounts of time sitting in emergent vegetation at the location where their nursery web is eventually constructed Fig. 6c). They would appear to be very vulner able to predation in these relatively exposed locations but drop suddenly down 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 hoty 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 young appear to bite their way through the tough silk, emerging through small holes apparently without help from the mother (Fig. 7).

Fig. 7  Spiderlings emerging 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. Their nurseries are large tents of silk attached to emergent vegetation between 10 and 100cm above the water.  The young live in the nursery, guarded by the mother, for around a week, until after their second moult. They then leave the nursery and disperse into the surrounding vegetation. In captivity, the spiderlings start to exhibit cannnibalism 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-covered nursery in Stratiotes aloides

The mother usually sits close to the web, and will rush out and attack anything that threatens the integrity of the web. In captivity, the nurseries are smaller structures than in the wild and are contructed almost entirely by the activity of the spiderlings rather than by the mother. These webs are mobile structures, often migrating upwards as far as their confines allow. In the wild, the relative roles of mother and spiderlings in the contruction of the nurseries are not known - the webs are constructed at night.

Dispersal - There are no recorded observations of dispersal of spiderlings from the nurseries in the wild; this may be a largely nocturnal process to avoid predation. 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. However, 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 the spider into the air like a kite. However, although D. plantarius spiderlings show elements of ballooning behaviour, standing on tiptoe and releasing silk lines in gentle breeze, they usually go on to show 'rigging' rather than true ballooning behaviour, waiting until the silk lines catch in near-by vegetation and then moving along them. 

The

Fig. 8  D. plantarius spiderlings in the rigging position with silk draglines just visible, flying from their spinnerets

rather commoner D. fimbriatus has a higher 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 easily drown by 'sticking' to the meniscus. At this stage they are thought to live and hunt largely in damp vegetation.

Phenology - In the UK, raft spiders usually take two years to mature, becoming adult, breeding and dying in their third summer. In the UK, mature males and females emerge and courtship begins during May. Females swollen with eggs are in evidence during June and the first nurseries start to appear in late June.

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. Canniblaism of the males by the females during mating is rare although it has been observed in captivity, immediatley after mating.

Adult females can live until the 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, usually producing a larger egg sac at the first attempt. By the time the second brood eggs are laid, the males have all died and stored sperm must be used to fertilise them. Second attempts 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 again. After their second brood, females often remain at the nursery web, often for several weeks, until they die. In captivity, females at this stage feed infrequently and usually die by late autumn, even though they appear to be in good condition.  It is likely, however, that a small minority of females remain unmated and can overwinter as adults.

Juveniles hibernate during their first and second winters although little information is available on their hibernation behaviour or requirements. In sites subject to extensive winter flooding they almost certainly spend at least some of this period underwater, perhaps in air pocket under the banks or in the bases of emergent aquatic plants.

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 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 stretching them again and detaching completely from the old skin.

They enter their subadult stage after their penultimate moult, in their second autumn. The final moult takes place the following spring. In captivity, UK D. plantarius undergo a maximum of x moults between dispersing from the nursery and becoming adult. 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 under vegetation so to reduce their vulnerability to predation. 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 excise them by biting them off prior to moult to prevent difficulty in withdrawing them from the skin.  Loss of legs is common in the wild - they are shed easily if grabbed by a predator, breaking between the coxa and trochanter. 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.