Category Archives: Fen Raft Spider Behavioural studies 2019

Study 009

Fen Raft Spider observational study record 009

Location:                                           Carlton Marshes: Dyke 5a, just south of bridge

Date:                                                  23/08/19

Time start:                                        10.00

Time finish:                                      11.15

Weather (from BBC weather app):

  • General:                              Sunny and light winds
  • Humidity:                            74%
  • Pressure:                            1025
  • Temp:                                  20C
  • Temp feels like:                21C
  • Wind:                                  6mph SW
  • Cloud cover/sun:              Intermittent cloud, mostly sunny

Observational subject:

One nursery web, one adult, two nearby juveniles

Vegetation/macro location:

Web in channel of dyke, on water soldier. Juveniles on water soldier and frogbit


10.00: One nursery web containing spiderlings. A banded adult female is on the far (North) side of web -in deep shade and hard to see initially.

The web is dead centre in the picture

10.20: A little movement amongst spiderlings, which seem to be in two groups. The group on the left begins to spread. There is an egg sac to the right of this group with a group of spiderlings immediately beneath it. The guarding female is behind and to the left of the web. She is in the shade and looks very dark in colour in this light.

From left to right, guarding female, spiderlings, and egg sac with spiderlings beneath
Closer view

Mum was very hard to get a good look at. Moving to a different viewpoint to the left, and knocking up the EV compensation to lighten her up, resulted in a better view of her but with overexposure of the surroundings.

She looks quite different here -the cephalothorax is lighter than the abdomen, and she has light brown bands. The legs are transilluminated in the bright backlighting sunlight -a daylight radiograph!

Below is a video of the web showing a burst of spiderling activity. The dark flickering to the left and below the adult female is just the playing of light and shadow in the background. My videography skills are in their infancy so bear with me!

10.21.The banded juvenile just north of the web and facing north, sitting on water soldier leaf. It’s in hunting pose, with forelegs on the water meniscus. Body length is estimated at around 5mm.

The banded juvenile is at the meniscus, perched on a water soldier leaf. You can see if just above the frogbit pad at the bottom of the pic, about 3/4 of the way from left to right.
Closer up
Small but perfectly formed hunter. The forelegs are dipping the water meniscus. Tiny vibration-sensitive hairs on the ends of the leg will pick up water movement that could indicate prey nearby.

10.40 The juvenile on the water soldier has moved behind and up the leaf and is now almost hidden from view.

10.55. A second banded juvenile is seen a metre further away, on a frogbit pad. Body length estimated at 5mm long. The first juvenile has returned to the water surface.

The second juvenile is on the frogbit pad in the upper left quarter of the pic. The first juvenile is on left of the bottom right quarter, out of focus, on the water soldier leaf.
Second juvenile
First juvenile now in focus, second out of focus.

No unusual or notable behaviour was noted today. But it’s worth commenting that the juveniles are relatively close together and look almost identical. There is a good possibility that they are siblings who hatched in this location and are growing up there.

Study 008

Fen Raft Spider observational study record 008

Location:                                           Carlton Marshes: Dyke 2b, South end, opposite end of 2a

Date:                                                  14.08.2019

Time start:                                        10.00

Time finish:                                      11.30

Weather (from BBC weather app):

  • General:                              Light cloud and a moderate breeze
  • Humidity:                            77%
  • Pressure:                            1014 mb
  • Temp:                                  17C
  • Temp feels like:                18C
  • Wind:                                  14 mph, SSW
  • Cloud cover/sun:              Intermittent sunshine.

Observational subject:

The same webs as Study 007 done yesterday.

Vegetation/macro location:

All the webs are built on water soldier in the channel of the dyke


Web 1: As yesterday: old, empty egg sac and spiderling exuviae present, no adult nearby

Web 2: As yesterday. Spiderlings still present, one female seen behind. Banded, dark in colour, with white hazy appearance on backs of abdomen and thorax and parts of legs. I think this is the same individual as yesterday. She remained partly hidden for most of the study period.

Web 2
The abdomen of the dark female is just visible about 1/4 of the way along the horizontal centre line of the pic, in a gap between two leaves.
Closer up and now in the centre of the pic. You can see several of her legs too. Just a slight shift in angle of view and she’s lost from sight.

Web 3: Old web with empty egg sac, only one adult female with egg sac seen today beneath web. From back markings, identifiable as F3:2 from yesterday, with confluent white patches.

The female is below the left-most quarter of the web, midway between the web and the bottom of the pic.
Great view of F3:2

Web 3a: Behind and to right of Web 3. Spiderlings present and female (without egg sac) seen below web and on water soldier. Hazy appearance to back. Or is this in fact a male? There are “boxing glove” palps. * * * See footnote * * *

Web 4: As yesterday, spiderlings present, no adult seen

Web 4a: Behind and to left of Web 4. As yesterday, spiderlings easier to see today. No adult seen.

Webs 4 and 4a with spiderlings.

No significant movement or activity to report today.


On the difficulty of sexing a Fen Raft Spider -advice from Dr Helen Smith.

” There are a couple of images where you have identified the spiders as males and I wasn’t convinced. Adult males have disproportionately long legs and the cephalothorax always looks relatively bigger than the abdomen. If the palps are curled under, at some angles this makes them looked clubbed. We had issues with this on some images we were sent for the Wildguide to spiders – it could be really difficult to be sure which sex we were looking at. Numbers of adult males around will be very low by now but a bit later in the season we’ll start to see sub-adult males which usually overwinter before their final moult. Their leg and body proportions are ‘female’ and the palps start to become clubbed, although the palpal organ is not fully developed. I’ve only once seen a newly emerged adult male in autumn – at Castle Marshes in mid-October “

Addendum 1:

I returned to these webs 6 days later, after a period of very high winds, storms and rain. Most of the webs were severely degraded though one remained with spiderlings. I took photos but in all honesty they don’t add to what is already in this post. I was unable to make any written notes as I was surrounded by grazing cows -not too bad, until a curious calf approached. Mum was watching and if she had perceived a threat to junior she might have decided to take issue with me. Not keen on dealing with over half a ton of beef charging me, I politely retreated.

Addendum 2:

I returned one more time on the 30th August, 17 days after the start of the study (13th August, Study 007). The area looks desert-like compared with the lush vegetation at the start. Quite a dramatic change in appearance with the water soldiers sinking underwater and very little green, new growth in the channel. There’s evidence of cow damage on the far bank with a new web a metre up in the marginal vegetation empty and probably damaged. Only two of the documented webs in the channel can be seen.

The marshes know winter s approaching. the dykes are starting to change, preparing for their long winter sleep.

Study 007

Fen Raft Spider observational study record 007

Location:                                          Carlton Marshes: Dyke 2b, South end, opposite end of 2a

Date:                                                  13.08.2019

Time start:                                       10.30                   

Time finish:                                      12.15

Weather (from BBC weather app):

  • General:                              Sunny intervals and a moderate breeze. Cooler than recently
  • Humidity:                            67%
  • Pressure:                            1016 mb
  • Temp:                                  16C
  • Temp feels like:                16C
  • Wind:                                  14 mph, westerly
  • Cloud cover/sun:              Intermittent sunshine. Cool when the clouds cover the sun

Observational subject:

A number of webs and adults seen today so numbering system devised. Webs numbered 1 to 4  from left to right as seen (South to North), and adults numbered as (=gender + web number): (adult number) eg F2 would be the sole female at web 2, but  F3:1 and F3:2 would be the first and second females seen at Web 3.

Today’s site. The webs look very hard to find in this picture, but with close-focus binoculars on site it’s not so hard.

Vegetation/macro location:

All webs on water solder and in the channel of the dyke


Web 1: Old, empty egg sac and spiderling exuviae present, no adult nearby

The empty egg sac is near the bottom of the web.

Web 2: Spiderlings present, one female seen behind. Not banded, very dark in colour, with white hazy appearance on abdomen, cephalothorax and parts of legs.

Web 2 -the dark female is above, behind and to the left, slightly out of focus

Dark female

Web 3: Old web with empty egg sac, two adult females (F3:1 and F3:2) beneath web, both banded and carrying egg sacs. 3:1 has white markings on back of abdomen and cephalothorax and is tan in colour, 3:2 is dark and has distinctively patterned confluent white patches on back of abdomen and cephalothorax.

Old web with two egg-sac carrying females beneath
Distinctively patterned banded females carrying egg sacs

F3:2 with a clear view of her eyes, which are glinting in reflected sunlight, and her disctinctive white dorsal markings.

Web 3a: Behind and to right of Web 3. Spiderlings present.

Web 4: Spiderlings present. Unbanded brown female beneath web.

The web is above and right of the spider. A few spiderlings are visible

Web 4a: Behind and to left of Web 4. Spiderlings present though hard to see initially.

10.50: F2 moved away from web and disappeared from view for a while, then reappeared later on.

11.00: Spiderlings in Web 2 began to make dish formation facing the sunlight during the study period.

Dishing of spiderlings causing increased sunlight exposure

11.00: F3:1 moved away from web and disappears under water soldier leaf

11.40: F3:1 has reappeared about 50cm to left of previous position

11.50. Spiderlings in Web 2 becoming quite active. No sign of female with hazy abdomen and thorax. She has wandered off again.

Footnote: I asked Dr Helen Smith for her advice on the white markings on the females. This is her response. Utterly fascinating:

“The white staining on the bodies of the adult females only happens this time on year, and (I’m pretty sure) only occurs on females that have second broods and are wearing out. It’s a surface deposit and I’m pretty sure it’s excreted guanine. When they go underwater they excrete a white cloud of guanine to ‘cover their tracks’ against attack by visual predators. When they’re young, they groom their body hair assiduously to maintain waterproofing but at this stage in life they neglect this. You may also have noticed that they are now much more likely to be seen sitting up in quite vulnerable positions guarding the nursery. I suspect they can afford to risk all at a stage when they’re no longer in the market for any further reproduction – and there’s also no advantage in spending time down on the water finding food.”

Study 6.

Fen Raft Spider observational study record 006


Location:                                           Carlton Marshes: Dyke 2b (at end where it meets 2a)

Date:                                                  30.07.2019

Time start:                                        11.00

Time finish:                                      12.35

Weather (from BBC weather app):

  • General:                  Sunny and a moderate breeze
  • Humidity:               59%
  • Pressure:                1008 mb
  • Temp:                      22C
  • Temp feels like:     25C
  • Wind:                      17 – 19 mph SSE
  • Cloud cover/sun: Intermittent cloud and hot sunshine when present

Observational subject:

A web with ball of spiderlings and egg sac, nearby female adult

Vegetation/macro location:

In middle of dyke on water soldier, 20cm above surface


11.00: Spiderlings emerging slowly from egg sac with inferior opening. Female, dark and banded, on water soldier slightly below web and about 20cm from it, and about the same height above water. Sac and spiderling huddle about the same size.


Spiderlings emerging from egg sac top to the right of centre, female can be partly seen in shadow at bottom, just to the left of centre. She is facing left. Her left band and a few legs can be seen. 

11.40: Female moved down from sedge to water surface. Hard to see as partly hidden by vegetation. Moved back up and onto web itself but continued downwards again and out of sight for rest of study in spite of viewing from a range of angles

12.00: Egg sac now smaller, and ball of spiderlings now about twice the size of the sac.


Spiderlings continue to emerge.

12.05: Spiderlings appear to have completely emerged from sac and spreading out

12.10: Spiderlings have taken up a concave disc formation, directly facing direction of noonday sun due south.


Forming a disc facing the sun.

12.35: No further activity seen. End.


It is probably safe to assume that the female had very recently opened the egg sac. If the emergence rate of the spiderlings was roughly constant (making a reasonable assumption) then they would have taken roughly three hours to emerge.

The formation of a concave disc facing the sunlight direction supports the idea that the spiderlings are exhibiting basking behaviour. The female, on the other hand, seemed to have retreated into shade though this could not be confirmed.

Study 5

Fen Raft Spider observational study record 005

Location:                                           Carlton Marshes: several dykes

Date:                                                  23.7.19

Time start:                                        11.30

Time finish:                                      15.00

Weather (from BBC weather app):

  • General:                              Sunny and light winds. Feels hot
  • Humidity:                            58%
  • Pressure:                            1018 mb
  • Temp:                                  22C
  • Temp feels like:                25C
  • Wind:                                  N 6mph
  • Cloud cover/sun:              clear sky

Observational subject:

My aim today was to observe adult spiders. Only 4 days ago on a routine survey we found 38 webs and maybe a dozen adults. Today on a very hot day, circumnavigating most of the survey-allocated Dykes at Carlton across 3 hours,  I saw only maybe 8 of those webs, most looking tattered, and only one adult, in a deeply shaded recess in the margin, under vegetation. Too dark and too distant even for a decent photo.

3 days ago we had storms and brief but torrential downpours. These would have damaged or destroyed many of the webs we had seen on the routine survey. After speaking to a biology graduate friend, I understand that spiders tend to shelter in the cool in hot weather, and are prone to desiccation in such conditions. Maybe the FRS adults had all gone into the shade, or perhaps underwater. There was no sign of guarding adults in the very few webs with spiderlings present that I saw today.

Vegetation/macro location:



It seems reasonable to conclude that the storms followed by hot weather affected my ability to find viable nursery webs and provides explanation for the apparent absence of adults.

Courtship and Defence

Here is a selection of fascinating photos taken while on a routine nursery web survey. Whilst not a formal behavioural study in the sense of the others written up here, it is worth including here as the most appropriate section overall.

I can’t take credit for the interpretation of the pics and videos -I sent them to Dr Helen Smith who kindly interpreted them for me. These are mostly her words:


Though this individual is not behaving in any particular way, we were interested to know more about the brightness of its bands which we found striking. He appears to have lost his second left leg. There is also a recorded conversation about its age in a courtship video below -Helen comments on this too.

“Band colour and brightness varies between individuals and is not particularly age-related. The exceptions to this are (1) that tiny juveniles tend to have lighter ground colours and (2) adult females tend to have more subdued band colours, although you do see some brightly banded ones. I’d be interested in trying to quantify the latter!”

“Re the overheard conversation backing this video, this year’s juveniles will only be a max of 3 weeks old and a very few mm long – you’re unlikely even to notice them and they’ll mainly be in the marginal vegetation rather than on the water. Anything vaguely ‘middle sized at the moment will be from last year’s broods.”



“This couple are indeed a courting pair – the female is already quite gravid. The bobbing in the video is part and parcel of the courtship communication – both sexes do it. In the male it only signals intention but in the female, done (I guess) at a particular frequency or intensity. it can also signal lack of interest! Subtle stuff.”

Here is a nursery web (top right) and below and to the left is a spider. Try and spot it. If you can’t the answer is at the end of this post.



Here he is -and it’s a male. He has a slim abdomen and clubbed palps like boxing gloves -these contain sperm, ready to inseminate the female, facing him. You can see her legs between the two sedge leaves forming a triangle.

About the above videos (of the same pair of spiders as in the stills), Helen says: “This shows another courtship sequence with the male doing the classic leg-tapping to create concentric waves that signal intention to the female. This can be a very protracted business and the fact that she moved away doesn’t necessarily mean that she’s not interested and won’t accept him in the end.” Apologies for the extra noise -no one was aware I was recording!


This shot shows a female guarding her nursery. Her body is quite thin because she’s hardly eaten for weeks while carrying her egg sac but her palps are straight and not clubbed as a male’s would be.


Here the same female is being defensive and guarding her nursery. You can see one of her palps very clearly – and it’s straight, not clubbed at the end. Apologies for the focus -not a lot of control with the apparatus -my camera battery had run out and I used a camera phone held only about 18 inches away, so she had detected my presence. She is holding her right first two legs in a V-sign -very appropriate!

Answer to the spotting question: He’s at the bottom of the photo, to the right of the triangle formed by the two most right hand sedge leaves. In the apex of that triangle you can partly see a female facing him.

Study 4

Fen Raft Spider behavioural study record 004


Location:                                           Carlton Marshes: Dyke 5b, 8m South of bridge

Date:                                                   16.07.19

Time start:                                        14.00

Time finish:                                      15.45

Weather (from BBC weather app):

  • General:                  Sunny intervals and light winds. Feeling very warm.
  • Humidity:               58%
  • Pressure:                 1018 mb
  • Temp:                       21C
  • Temp feels like:      25C
  • Wind:                       N, 6mph
  • Cloud cover/sun:    V sunny with occasional cloud cover

Observational subject:

2 Balls of spiderlings in a nursery web with egg sac, adult beneath


Vegetation/macro location:

Web and spiderlings about 30cm above water surface in water soldier about 1m from east bank. Adult sitting on frogbit on water surface immediately beneath web.


14.00. Dark unbanded adult female present


Initially 2 balls of spiderlings seen in web. Little activity at this time.



14.20. Adult has moved away from me to next frogbit pad. Can only see back of abdomen and spinnarets. Contrasting light and obscuring leaf cover prevent photo.

14.40 has turned around and is facing me


15.00. Spiderlings have coalesced into a lens shape roughly facing the direction of the sunlight


15.40. Spiderlings noted to be very active in full sun


Lens formation and increased activity consistent with increased exposure to warmth from sunlight. Suggests active basking behaviour in spiderlings. No active movement of adult into sunlight does not support active basking behaviour in this individual during this study.

14.45 End.

Study 3

Fen Raft Spider behavioural study record 003


Location:                                           Carlton Marshes: Dyke 10. Same as in Study 002.

Date:                                                   09.07.19

Time start:                                        11.30

Time finish:                                      12.15

Weather (from BBC weather app):

  • General:                   Cool slight breeze
  • Humidity:                74%
  • Pressure:                 1022 KPa
  • Temp:                       15C
  • Temp feels like:       –
  • Wind:                        S, 7mph
  • Cloud cover/sun:    Cloudy

Observational subject:

Same web as in 002.

Vegetation/macro location:

As before


11.30. Web looks thinner and scantier since seen 18 hours previously. Volume taken by spiderlings seems less, suggesting either less spiderlings or that they are huddling more tightly. Two formations seen, a vertical sausage shape and a separate small ball. Both have a reduced surface area to volume ratio compared with the more diffuse shape of yesterday. No adult seen, very little activity indeed compared with yesterday.

Formation consistent with heat conservation.

12.15: No further activity noted and as I was getting cold (being unable to rapidly reduce my surface area to volume ratio), study ended.


Study 2

Fen Raft Spider behavioural study record 002


Location:  Carlton Marshes: Dyke 10, immediately by northernmost red and white verge marker post

Date:                                                  08.07.19

Time start:                                        14.00

Time finish:                                      16.10

Weather (from BBC weather app):

  • General:                Mild day, breezy
  • Humidity:              –
  • Pressure:                –
  • Temp:                     17.5C
  • Temp feels like:     –
  • Wind:                      Moderate breeze
  • Cloud cover/sun:   Intermittent cloud, mostly sunny


Nursery web with spiderlings. No adult seen.

Vegetation/macro location:

Web in sedge, about 12 inches above water, in margin.


The web in study 01 can no longer be seen. This web is further south along the same dyke.

14.00. Spiderlings in a torus formation, the right-hand half shaded, the left in sunlight. Bursts of activity noted in two forms: one as a cascade spreading over the surface of the huddle of spiderlings, the other as a simultaneous burst of activity all over the surface, with individuals at separate points around the huddle simultaneously starting movement. Does this suggest different stimuli for activity? There was no visible trigger for the activity, which always settled down within a few seconds.

14.45. Longer spells of sunlight on web and increased frequency of bursts of activity noted during this period.

14.50. Several bursts of activity occurring within the space of one minute but lasting only a few seconds. These bursts were in the simultaneous pattern.

15.00. It is possible to see now from a different viewpoint that the torus is in fact two separate balls of spiderlings. Learning point: get several viewpoints where possible.

15.30. There are in fact three balls of spiderlings, with one to the left of the others, shaded and well hidden under a broad sedge leaf. The other two are now both unshaded and bursts of activity are noted in both, when the sun emerges. The balls are beginning to join up.

15.55. Sunlight now continuous as clouds have broken up. The two right hand balls have coalesced to form a crescent concave upwards, approximately facing the direction of the sunlight. More a radio telescope shape than a lens shape.

Observed formation shape change consistent with basking behaviour to increase body temperature. This would increase metabolic rate and consequently growth rate.

Increased activity during periods of sunlight consistent with increased metabolic rate and muscular activity with increased body temperature.

Stimuli for activity bursts not known.

16.10 End.


The hidden third noted ball of spiderlings on the left, shaded by a sedge leaf. The next two are visible and partly obscured.


Same view closer in and a little later.


Crescent formation

Study 1

Fen Raft Spider behavioural study record 001

NB This is the first session and as such was experimental to assess what to record and how to record it. The template on which this record is based was developed subsequently, hence not all fields are complete for the first few records

Location:                                                         Carlton Marshes: Dyke 10

Date:                                                                 02/07/19

Time start:                                                      11.00

Time finish:                                                    12.50

Weather (from BBC weather app):

  • General:                            Fair mild day
  • Humidity:                         –
  • Pressure:                           –
  • Temp:                                18C
  • Temp feels like:                –
  • Wind:                                 Gentle breeze
  • Cloud cover/sun:              Intermittent sunshine and broken cumulus cloud

Observational subject:

Nursery web


Web amongst sedge, 18 inches off water in margin. 5m south of 10mph sign. A recently opened egg sac was reported seen on 29/06/19 -three days before this observation. No adult seen. Egg sac present, only two spiderlings seen crawling about web and on sac. Became more active when sun came out.

Impression is that the web is looking tatty -there were some strong breezes in in the area in the three days before this record.

12.05: Little activity

12.50: No further observations to make. Ended.


About the Behavioural Studies

I’ve been asked by Dr Helen Smith, the UK’s leading authority on the Fen Raft Spider, to help her answer questions she has about the Fen Raft Spider. Our starting point is observing behaviour. What this means for me is to sit in the blissful peace and tranquillity of Carlton Marshes in a chair, with binoculars, camera and notebook, and simply watch a nursery web and/or and adult spider, and observe, and note. My main job is to report the “how” of Fen Raft Spider behaviour, and where I can, I may make a suggestion or two along the way about the “why”.

The following posts will attempt to be factual and objective reporting, so I’m not attempting to provide a literary read. But some of you may find the information interesting.

Neither of us have any clear idea of where this will lead, or indeed if I’m able to contribute at all to the sum of knowledge around this beautiful and sophisticated animal. But I’m going to have a go!

Thank you for getting this far with me. Please stay on the journey.