In the Field with Spiders: Course Preview

Making Silk

This discussion was selected from Week 2 of the AMNH online course In the Field with Spiders, part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators. This is an excerpt from an actual course discussion, but learner names have been changed. Explore more sample resources...

From the readings this week, describe how a spider's anatomy is well adapted for the production of silk. How is silk produced, and how is this process unique?

Nada 5 Jul 7:01 PM

Wow. This silk production business is complicated. Several types of silk each type for a different job - e.g. egg sacs, trapdoors, various parts of webs. Silk glands in the abdomen produce the proteins in soluble form and push the fluid into ducts toward the spinnerets at the end of the abdomen. Some spinnerets can have 100's or more spigots that release the silk. As it squeezes through the spigot, the pressure changes the silk into a solid. (What happens to the water? Is it kept inside and reused or does it intercalate with the proteins?)
The silk and the glands are different than those of Lepidoptera and there is nothing else like it in other organism. Although, apparently there are enough similarities with goat milk production and extrusion that Canadian's have made a transgenic goat in hopes of producing large quantities of silk for commercial use.

Katie 17 Jul 11:53 AM

Hi, Nada
Being an avid consumer of goat milk products, I thought, "How cool" when I read about the efforts of the Canadian-based "Nexia Biotechnologies" to produce spider silk proteins in goats' milk. I'm sure you read, too, of the drawbacks to the process----mainly that you would need milk produced from 200 goats in one day to make just one bulletproof vest! The difficulty is in finding a more cost-effective way to produce the silk. A research team at the University of Wyoming is now considering introducing the silk gene into alfalfa, a process which they describe as being more technologically challenging than intellectually challenging. Seems like they may be on to something. Isn't science GREAT?

Instructor Luke 6 Jul 2:35 PM

HI Nada
The silk is secreted into the lumen of the spinning gland in tiny droplets. The pear shaped glands have a single layer of epithelium, have a duct attached to the gland.. The silk is stored into gland and then extruded when there is an increase in the hemolymph pressure in the abdomen. One the silk is solid, the process cannot be reversed (back to liquid) -- nothing to do with the air, but merely by the tensioning of the silk protein (fibroin). Reabsorption of the water from the fiber takes place (it is thought) in the lumen. The silk glands most studied are the ampullate glands. Each gland leads to a specific spinneret and each of these connects by a duct to a spigot. Can you see if you can find a web site that will help you with the physiological difference between the silk glands of ancient spiders and modern spiders?

Nada 7 Jul 7:44 AM

I am still looking but
discussed the anatomy of the most primitive spiders, Liphistiomorphae, found on 2 islands in Japan. The spinnerets are in the middle of the abdomen so not very mobile. Silk is used for egg wrapping so it is not sticky.

From our text, scientists think that was the first use of silk in the earliest spiders 350 mya. A Dr. Zschokke (I have to go back and find the website) examined a piece of silk caught in a 120 mya piece of amber from Lebanon. Dr. Z thinks it matches silk made by today's orb weavers. It apparently had sticky droplets on it.

Nada 7 Jul 7:47 AM

I am interested that the silk solidifies as a result of squeezing the water out sort of the way I squeeze the water out of silk blouses. They must be washed by hand and shouldn't be wrung to get excess water out. At the same time the process aligns the protein fibers.

Scientist Martin 7 Jul 1:32 PM

Nada: So glad you found your way to Liphistius. They are THE neatest spiders! I have attached a photo of one. Please also read my response to Tammy about early/fossil silk. Liphistius

Tammy 7 Jul 10:51 AM

I was thinking about presenting some of this information to younger students. The scientific criteria for classification is rather detailed.

It seems to me that younger students could benefit from simply sorting spiders into groups according to whether how/if they use webs to catch their prey .

It seems some spiders are more passive (or patient) in that they lay a web and wait for their meal to come to them. Others like the jumping spiders and wolf spiders seem to have more of a hunting instinct.

Presumably spider webs are designed to catch the most suitable prey in the most efficient manner for a given type of spider. But I wonder about the evolution of spider webs. Is it more efficient to go and look for prey, (even though this probably requires a greater energy expenditure requiring more food)

I wonder if someone would comment on the article in
which discusses the differences between 2 dimensional and 3 dimensional webs as a protective device for spiders.

Scientist Martin 7 Jul 1:25 PM

Hey Tammy: You raise some interesting questions. What evidence would you or your students accept to support the contention of which strategy; sit and wait or go looking, might be selected for?
Classifying spiders by their ecological characters might be useful for an ecologist studying feeding guilds, but can you see it does contain any information about the history of the spider taxa that construct specific web types.
Thanks for the article link. We know that the orb web evolved at least by the Jurassic (from morphological fossils not the presence of silk) and that some 3D webs evolved in spider groups that at one time also spun orb webs (2D). I find the evidence somewhat plausible, but unfortunately sphecid wasps prey on a large variety of spiders, some that make no web at all. All of which leads to the understanding that evolution really does occur in strange ways!

Tammy 9 Jul 6:45 AM

The Challenge
Martin asked What evidence would you or your students accept to support the contention of which strategy; sit and wait or go looking, might be selected for?

I have been thinking about this and the more I consider it the more questions come up. Before evaluating these strategies I think I would need to know a lot more ....Spiders who use webs to collect prey eat what is caught. But do they eat everything that is caught. My initial thought was to start by counting the effectiveness of the webs in catching prey as compared to the number of catches made by the hunters. BUT

1. Are some "catches" not palatable. The book talked about purse spiders netting aside a critter that was not palatable and in another spot talked about wasps with a nasty taste. So gross totals of critters captured in a web may not be valid..

2. How much can a spider eat in one day? There was a reference to spiders surviving without food for a number of days. Is this generally true of genus specific?

3. The web spider has a really good day with lots of catches. Some of them are wrapped and stored. But for how long is a stored prey edible?

4. Do all web spiders store their extra's?
Here is the scenario. Spider A caught 8 critters in its web but only three were edible.
Spider b only caught 4 creatures and 3 were edible.

Do you say they were equally effective. I wondered about the energy spent in housekeeping (getting rid of the useless catches and repairing the web.) With regard to the active hunters, How accurate are they in terms of getting edible prey. Do they spend any energy on non edible prey? My guess would be no so they could be considered the more efficient. And I do realize that the answers may vary among the many types of spiders. A second set of considerations concerns how you would go about making these observations. Spot checking the contents of a web may not give a true indication of results. What if the spider has already eaten and done its housekeeping. What looks like an empty web might be one ready for the next kill. And how do you know what a purse or funnel web spider has inside its web.

Keeping track of the diet of a jumping spider is another problem. If you are lucky to see it inaction that is one thing but how do you know what is happening the rest of the time. Do spiders have particular hunting times -- I know there are nocturnal hunters and daytime hunters but are their certain times of the day (or night ) they are more likely to be hunting.

The book talked about how spiders have split second timing in terms of pouncing. How do you know what it got since there is really no time between its targeting and eating? I am thinking of Diane Fosse spending hours just observing her gorillas. Do arachnologists spend their hours just staring at a piece of territory observing a spider? And how do you observe subterranean spiders... or those that live under the tree bark. Even those that live in leaf litter.... if you move the leaf to see the spider haven't you changed its environment?

I guess controlled experiments could be done in a lab situation but how real a setting can be setup? In terms of field work, is data collected on a rainy or just damp, or cool day significantly different from that collected on a warm or dry day. It seems to me that each condition sets up different results.
There must be spiders that hunt in the rain.... in rainforests at least.
So many things to consider. And I have lots of other ideas along this line.

Bottom line -- I have a deeper appreciation for the work arachnologists (and entimologists) do. It is one thing for me to identify a critter from an enlarged picture. Quite another thing to even see it in the wild!
I would need a web cam in my garden with an option to zoom in and record over a long stretch of time......

Paul 12 Jul 9:09 AM

Tammy, Here's a few more complications to throw into your questions:
How about determining the hunting effectiveness of web building spiders that have more than one spider in them, such as the communal web dwellers, the spiders that keep their young in their webs with them for a period of time and the pirate spider?. Is there a difference in what kind of prey the pirate spider gets versus what the spiders of a single species get? in other words, does the pirate spider get the dregs while the other spiders share more equitably?
I wonder if any of these questions could be feasibly answered in an elementary science classroom. Any suggestions on how to investigate these matters?

Luke 7 Jul 1:29 PM

There seems to be a great variety of silks used for different functions in different spiders. I suppose, after doing the reading, that the genes that carry the code for spider silk have been isolated. How many genes are involved in this process? Has a comparison been done between the sequences of various species?

Scientist Martin 7 Jul 1:41 PM

Luke: There are many genes involved as you might imagine for so complex material. Molecular biologists have isolated genes which code for about 3-4 different silk types. The proteins have, as you know been expressed in several taxa E. coli, goats etc. but we are a long way from synthesizing a composite material we would consider spider silk. I'm not sure how far along whole sequence comparisons between taxa are.