1. Which Isopods Roll Into a Ball: Pill Bugs or Woodlice?
If you gently poke a pill bug with a finger, it pulls in its antennae and forms a seamless sphere. The woodlouse, however, does not roll; it is said to run away. [1]
Researchers in the United States who studied land-dwelling isopods (the group that includes pill bugs and woodlice) wrote that only the pill bug family can form a complete ball. [1] However, other sources note that other families include species that can roll partially, though most cannot roll completely. [4]
Therefore, the ability to roll is a minority trait among isopods. It is safer to say that the pill bug family is the one where the ball closes completely. [1] So, what does rolling actually protect?
2. How Does Rolling Up Protect a Pill Bug's Body?
Rolling up does not change the size of the body. What changes is the surface area exposed to the outside air. When the belly side faces inward, the exposed area decreases significantly. [1]
In a 2008 experiment in the United States, it was reported that when the common pill bug (*Armadillidium vulgare*) was rolled up, the rate of water loss decreased by 34.8%. [1] This means the rate of loss dropped to about 65% of normal (100 − 34.8 = 65.2). It does not stop water loss entirely. [1]
Furthermore, this effect only occurred when relative humidity was between 6% and 53%. At 75% humidity, there was no difference. This means that when the air is already moist, rolling up does not change water loss. [1]
On the belly side, some isopods have "pleopodal lungs" visible as white spots, which allow them to breathe air. The number varies by species; *Porcellio laevis* (a type of woodlouse) has 2 pairs, while some species have none. [5] When an isopod rolls up, this belly side becomes hidden from the outside. [5]
So, rolling is a way to conserve water in dry places. But how do those who do not roll protect themselves? [1]
3. Are Woodlice That Cannot Roll Built to Run Away?
In 1984, a researcher named Schmalfuss categorized land isopods by body shape and escape style. There are "runners" with thin bodies and long legs, "clingers" with flat, wide bodies and short, strong legs that grip the ground, and "rollers" that curl up. [3]
The woodlouse species *Porcellio laevis* is described as having a body shape suited for quick movement. [2] In Japanese explanations, woodlice are said to have flatter bodies and be much faster than pill bugs, running away when danger approaches. However, the sources used here do not specify exactly which of the three types woodlice fall into, so we do not assign them a specific category. [3]
Conversely, the rolling body has a special structure. The pill bug family's body is made of overlapping plates (tergites) connected by joints. To roll, the plates must curve, abdominal muscles must work, and there must be a groove-like structure in the head to store the antennae. [3] The ability to roll seems to be a shape gained by changing the entire body structure. [3]
So, which wins in a real encounter with an enemy: the runner or the roller? [3]
4. Is Running Always Better Than Rolling Against Ants?
In a 2005 study in the United States, pill bugs (*Armadillidium vulgare*) and woodlice (*Porcellio laevis*) were exposed to predation stress from ants. [2] *A. vulgare* can roll into a closed ball, while *P. laevis* has a body suited for quick movement. [2] The result showed that the death rate was higher for *P. laevis*. [2]
This suggests that *P. laevis* had a higher death rate in this ant experiment. [2] However, this is just one experiment with one type of predator. It is not known if the same result would occur with spiders or birds. It also does not mean rolling always wins. [2]
Still, a mystery remains. If rolling is stronger in some situations, why did not all isopods evolve to roll? A hint lies in the bodies of female pill bugs. [2]
5. Why Can Some Female Pill Bugs Not Close Up Completely?
Female pill bugs raise their eggs in a brood pouch (marsupium) on their belly. Researchers wrote that in the later stages of carrying eggs, the swollen brood pouch can prevent the body from closing into a perfect ball. [2]
In the same study, females that were touched by ants had shorter egg-carrying periods. The authors interpret this as an adaptive trade-off (an exchange of gains and losses). By shortening the period when they cannot roll, they make it easier to protect themselves from ants. [2] This is one research result for *A. vulgare* and the authors' interpretation. [2]
The rolling body conserves water and resists ants, but it is a special shape supported by overlapping plates and muscles. During the time of carrying young, this shape can break down. Woodlice can be seen as relatives with a different body shape. [2]
6. How Can You Compare a Pill Bug and a Woodlouse Under a Stone?
If you find one pill bug and one woodlouse under a stone or flowerpot, place them on white paper. Gently touch them with a small twig or leaf to see if they roll, run, and how far they run. [1]
Do not apply strong force to the insects. Return them to their original place after you are done. Wash your hands after touching them. Note how long it takes for the rolled pill bug to start moving again, and which way the woodlouse runs. [1]
Writing these observations down allows you to verify the gains and losses discussed here with your own eyes. [1]