Fascinating. Look forward to reading more in months to come!

Yes! The yellow tail gets me! So interesting.

I think that there are definitely environmental factors as well as genetic factors which affect the colouring of earthworms. I would be interested to learn more about how an earthworm's environment affects its appearance; there doesn't seem to have been much research into this.

I think the yellow pigment you sometimes see in Eisenia worms, particularly in the last few segments of the animal, is a waste product which has collected inside the worm. I am not sure why the amount of yellow varies from worm to worm. I suspect the cause is environmental rather than genetic. The yellow pigment appears to be inside the worm, under the skin, rather than a colouring of the skin itself.

I have seen some Eisenia worms which have a great deal of this yellow pigment inside them, not just restricted to the tail end of the worm. The presence of this yellow pigment can make an unstriped worm appear on casual examination to be striped. The yellow pigment shows through more clearly in the areas between the worm's segments and this gives the appearance of stripes.

I would be interested to hear from anyone who has knowledge of what this yellow pigment is and why the amount of it varies so much between worms.

Eisenia fetida/ andrei worms vary a lot in their colouring. Some are a deep purple verging on dark brown, others ruby red, still others various shades of pink or a creamy colour, with almost no pigment at all.

My observations suggest that despite all these variations in background colour, worms can be neatly divided between those that have the striping pattern and those that don't.

The striping pattern is the result of an unpigmented band in each intersegmental zone. The presence/absence of these unpigmented bands is easy to see when the background colour of the worm is fairly dark.

In worms which lack much pigment, it can be difficult to make out whether the pattern is present or not. The presence of the yellow pigment inside the worm (as mentioned in the previous post) also makes it difficult as it can make an unstriped worm appear striped.

I find that on close inspection all worms can be classified as either striped or non-striped. The non-striped worms still have visible divisions between the segments, but lack the broad unpigmented bands possessed by the striped specimens. When non-striped worms are stretched out, these visible divisions between the segments are more noticeable and can fool the casual observer into thinking the worms bear the striped pattern.

As previously stated some earthworm scientists state that the non-striped worms are E.andrei and the striped worms are E.fetida. Other scientists claim that the situation isn't as straightforward as this. I am hoping my experiements will caste a little light on the situation.

I have attached some photos to illustrate the striped and unstriped worms close-up.

striping pattern present, background colour pinkish

striping pattern present, background colour ruby red

striping pattern absent, background colour pinkish

striping pattern absent, background colour ruby red

striping pattern clearly visible in preserved specimen, note the absence of pigment in the intersegmental zones

Do you think these visual differences are related to age of the worms?