Every gardener has had the gruesome experience of plunging a spade into the ground, only to find that he has sliced anearthworm in half.

Will it die? Regenerate the lost part? Become two earthworms?

The answer depends on a variety of factors, including the type of earthworm and the location and tidiness of the amputation.

Scientists studying earthworms get mixed results even when using anesthesia and a scalpel, so sloppy surgery from a rusty trowel won’t do much for a worm’s chances for regeneration. However, worms can rebound from sacrificing some of their hundred-plus segments to a hoe or to a hungry robin or mole.

Regeneration of heads and tails commonly occurs when an injury activates stem cells that differentiate into replacement parts. Another transformation occurs when tissue suddenly finds itself closer to the front or back of a regenerating worm. Through a process of cellular reorganization, the tissue conforms to its new role in the worm.

The rules of regeneration

• Most earthworms can lose several segments from their head and grow them back. With the red wiggler, a worm often used in composting, the more head segments lost, the less likely they will be fully regenerated. The marsh-loving blackworm, however, always generates eight replacement head segments no matter where the worm has been bisected.

• The ability to generate a new tail is almost universal among segmented worms.

• An amputation between head and tail can sometimes result in two worms, with the front section growing a new tail and the severed tail growing a new head.

• Sometimes a severed tail generates new tail segments instead of a head. Like the rest of the worm, the twin-tailed creature absorbs oxygen from the soil and can stay alive for a while, but it’s unable to feed itself and will eventually perish.

• Severed red wiggler tails especially “have trouble mounting productive head regeneration and thus die of starvation and brainlessness, if you will,” says Mark Zoran, who studies nervous system regeneration at Texas A&M.

• A severed head made up of fewer than 20 segments can heal, but the animal tends to develop a dysfunctional lower digestive tract. Would it die of constipation? “I guess it is possible,” Zoran says, “but I doubt that little head fragment would be doing much eating while in such a state of disrepair.”

• If sexual organs are lost in an amputation,night crawlers can regenerate them, but red wigglers can’t.

The self-amputators

Temperature shifts can cause blackworms to develop a fissure between the head and tail, roughly at the 48th of its 150 segments. Each fragment develops a new head or tail, with each part forming a full set of gonads.

A not-too-distant relative of earthworms is the white worm, a tiny translucent worm that some people grow to feed to their aquarium fish.

One species of white worm relies exclusively on fragmentation to reproduce. It spontaneously fragments into five to 10 pieces, each of which grows a new head and tail. Sometimes, a fragment will grow heads at both ends, resulting in what scientists call a bipolar worm.