There are many different types of earthworms with different ecological niches. The earthworms you typically see in your soil, or after a heavy rain, are either 'Endogeic' or 'Anecic' earthworms.
These kinds of earthworms prefer to live in burrows deep in the soil rather than pure organic matter, and don't live in densely populated conditions. For these reasons, they are not suited for life in a worm bin.
The kinds that are best suited for life in a worm bin are 'Epigeic' earthworms. This is what Red Wigglers are. These earthworms prefer to live in the top few inches of leaf litter/decomposing organic matter. They eat much more rapidly than other types of worms, and can live in densely populated worm-communities. This is what makes them a suitable option for composting bins.
Epigeic earthworms are more pigmented than other worms. Being in just the top inches of soil, they are more likely to be exposed by sunlight. They are also much smaller because they do not need much muscle to travel through their environment as other worms.
Reproduction and Life Cycle
Which came first, the Red Wiggler, or the cocoon?
Without delving into philosophical rabbit holes, life begins for an individual red wiggler worm inside of a cocoon. These cocoons, when laid, are a white color, but quickly darken to a yellow color. When the worms are about ready to hatch it will get even darker, almost a brown color. Just before they hatch, you will be able to see the baby worms inside of them.
Red Wiggler Cocoons
Each cocoon is a capsule for 1 to 7, but usually 2 or 3, fertilized worm eggs.
After an average of 21 days, the baby worms will exit the cocoon through the tapered end of the cocoon.
Baby Red Wigglers are very small, and can be difficult to see. A worm fresh out of the cocoon will be about half the size of this one pictured.
Baby Red Wiggler
For the next 40-60 days, the worms grow bigger and fatter as they eat. After that time frame, they will become adults and start reproducing!
Sexually mature adult red wigglers are easily identified by their clitellum - the band around the earthworm's body towards their head.
Red Wigglers, and all earthworms, are hermaphrodites! They have both male and female reproductive organs. The clitellum is similar to human ovaries as it stores the unfertilized eggs of the worm.
The male reproductive glands are near the underside of the clitellum, but are more difficult to see.
During sexual reproduction, two worms will line up their reproductive organs to be able to exchange reproductive fluids to each other. Both worms will get 'pregnant' from the encounter.
After the exchange of fluids, each worm will form a mucus layer around the clitellum, that will be sloughed off to form a cocoon.
In laboratory conditions, Eisenia Fetida have been found to be able to produce up to 3 cocoons per work per week.
The total lifespan of a Red Wiggler has not been thoroughly researched, but is a matter of years rather than months or weeks.
As previously mentioned, Red Wigglers naturally do not grow much bigger than 3 inches or so. The more nutrient dense, wet, and less crowded their environment is, the bigger they will get. Each worm weighs between a third and a half of a gram. (A little less than 1/1000th of a pound).
Red Wiggler Size
In ideal conditions, Red Wigglers can eat around half their weight in food each day. This will be affected by what kind of food is available to them, temperature, moisture levels, microbial communities, and other factors.
Red Wigglers will do best in the temperature range of 60-80 degrees Fahrenheit - their exact ideal temperature being 77 degrees.
They will begin to die in temperatures above 85 degrees, and at, or near, freezing - depending on how long they are left in that temperature range. 40-80 degrees is a safe temperature range for extended periods of time.
The closer they are to 77 degrees, the faster they will eat and reproduce.
Since Red Wigglers tend to stay in only the top 6 inches or so, square footage (with at least 6 inches depth) is usually the main way to measure how many red wigglers an area can sustain.
1,000 Red Wigglers per square foot is the commonly accepted rate for how densely populated an area can be, but it is possible to keep them more dense than that if you keep them happy.