Pillbug Parenting

The secret lives of roly polies and how they intersect with human beings

Assuming roly poly position

They slowly amble along on seven pairs of legs.  Disturb them, and they roll into a ball.  Perhaps you were fascinated in your youth, as I was, by what we used to call roly polies or pillbugs.  I’m not sure what the attraction was exactly, but they have that armored, vaguely prehistoric look, and, of course, have the fascinating habit of rolling into a ball. Roly polies, sowbugs, and pillbugs are different names for the same group of animals known as woodlice, or to scientists as isopods.  Though related, sowbugs and pillbugs differ in a number of important respects. Notably, sowbugs cannot roll into a ball, and pillbugs can.  I fear that as a youth I may have tormented sowbugs, probing them repeatedly with the expectation that they would roll into a ball.  I hope I didn't do lasting harm. Pillbugs are ancient creatures, accompanying human beings around the world and acting as recyclers in nature.  They deserve our respect.

Sowbug

Isopods, like pillbugs and sowbugs, are a kind of crustacean, the group of animals that also contains shrimp and crabs, and are found living both on land and in the water.  Isopods evolved about 300 million years ago, and likely began to colonize terrestrial environments more than 50 million years ago.  Note, by contrast, that primates evolved roughly 75 million years ago and Homo sapiens evolved only about 200 thousand years ago.  Certainly, pillbugs and their ancestors have demonstrated evolutionary staying power well beyond what we humans have shown – the jury is out on whether we’ll achieve anything like one million, much less 300 million years on the planet.  Pillbugs retain characteristics of their aquatic ancestors, most notably utilizing gill-like organs for respiration.  Pillbugs thus require moist conditions to survive and are often found under stones, logs, and leaf litter where the microenvironment retains higher humidity.  They also find their way into homes, where basements may provide sufficient humidity for their survival. 

Breaking and entering into homes leads to trouble for pillbugs.   Human homes are generally too dry for pillbugs to survive.  This leads to the sad scene in which dead pillbugs litter the floor in a curled-up state.  Pillbugs roll into a ball not only for defensive purposes, but also to conserve water.  When pillbugs do find a location in the home that is moist enough for survival, they face retribution from homeowners who take a dim view of their presence, and go so far as to call this ancient creature a pest.  The presence of pillbugs indoors may occasion the spraying of insecticides both inside and around the home.  Or, dead and live pillbugs may be vacuumed up and discarded in the trash – an undignified end for a venerable arthropod.  But why kill them?  Pillbugs are not a threat.  They don’t damage carpets or furniture, and they don’t cause injury to people.  Some people even keep pillbugs as pets.  Apparently they are easy to rear and, while not particularly affectionate, they don’t bark or bite.   Gently returning pillbugs to a suitably moist place outdoors is doing a favor for the environment.  Pillbugs are important decomposers of dead leaves, contributing to the recycling of vegetation.  Their efforts help insure that we are not knee deep in leaf litter, and that the soil is replenished with nutrients.

Pillbug parenting is a fascinating aspect of their life history.  Pillbug females prepare for motherhood by molting.  In contrast to insects, which shed their outer skin or exoskeleton between developmental stages, pillbugs continue to grow and molt as adults.  Biologists call the molt that readies the female for mating and bearing pillbug brood, the nuptial molt.  Males sense the readiness of females for this molt, and court them.  In the final phase of their courtship, males clamber onto the female’s back and hold tight, waiting for the female to reach a physiological state that is optimal for breeding success.  This behavior, called the nuptial ride, may last for seconds or even days, depending on the species.  Inseminated females carry up to 200 eggs in a pouch on their underside, called the marsupium, not unlike a kangaroo.  Marsupial liquid sustains the eggs and newly hatched pillbugs, which may spend their first few days in the pouch.  The juvenile pillbugs in the marsupium also obtain the minerals required to harden their exoskeleton from the marsupial liquid provided by the mother.  Once the pillbugs leave the safety of their pouch, they benefit from the mother’s feces.  Not mother’s milk, but an important source of nutrition just the same for the small and vulnerable young pillbugs. Maternal care goes only so far for pillbugs though.  The mother pillbug may even eat her young if given the opportunity! 

Pillbugs are thought to have originated in Europe, and then were transported to the rest of the world by human beings.  Therefore, in their new locations pillbugs are strongly associated with human homes, parks and gardens, and have only later spread beyond into undisturbed habitats.  They are a beneficiary of the Anthropocene, taking advantage of the global movement of humans and cultivated plants.  Most people are quite familiar with pillbugs, but are unaware of the intriguing lives they lead in and around human environments.  Pillbugs are but one example of nature that accompanies human beings around the planet, finding its way through cracks and crevices into our carefully constructed habitats.  Knowing a little more about them may encourage us to see Pillbugs in a new, more respectful light – a prime example of the nature that is found underfoot.