Monthly Archives: August 2019

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.