The Quietly Understated Roly Poly
(Photo: Flicker, common use by Frank Carey)
One of the most common invertebrates you encounter when rolling over logs or looking under rocks are pill bugs, also called woodlice or roly polies. But have you ever really considered these harmless creatures? They lead a quiet and secret life, that of an aquatic invertebrate on land.
First of all, they're more closely related to crayfish and shrimp than they are insects like ants because they are members of the class Crustacia. Pill bugs are also in the order Isopoda, and are considered terrestrial isopods. Many of their other relatives are aquatic, and I've always thought they looked like miniature trilobites (see below) which also rolled up as a defense mechanism. The pill bug's largest aquatic cousin (Bathynomus giganteus) lives in deep oceans waters and is 2.5 ft long, weighing in at 3.5 lbs! There are about twelve species of roly poly in North America, but the most prevalent is the common pill bug (Armadillidium vulgare).
Wouldn't you say that pill bugs look like their aquatic cousins the trilobites? (Photo: Flicker, common use Kevin Walsh)
You'll find pill bugs living in moist humid places with lots of vegetation, because that's what they feed on. They are decomposers and herbivores that eat mostly decaying plant matter. This is why you find them under leaves, rocks, and logs. They are not considered major agricultural pests, and their lack of fangs, stingers, or barbs makes them perfect for showing kids and handling.
Let's look at their anatomy:
- Common pill bugs are about 1/4 to 1/2" long and they're usually slate grey to dark blue grey.
- They're insects so they have three body parts, the head, thorax, and abdomen.
- Their head had two sets of antennae, one which you can't really see, and a set of very simple eyes (too tiny to see without a microscope)
- Their body is segmented, with the thorax having seven segments and seven pairs of legs attached to those segments.
- Their abdomen has five segmented plates with two hairy uropods on the end which can sense wind, temperature, and humidity.
- Like all insects, to grow, pill bugs molt or shed the outer skeleton. Unlike other arthropods they molt half of their body at time. When they're molting you'll notice 1/2 of one body section (the newly molted) is pale and the other half a darker greyish color.
- Because they are isopods, and closely related to crabs and crayfish, they breathe through gills on the underside of their bodies. To protect these gills (and keep them moist) they must live in damp places, and they can roll up to keep their body and gills wet. The wetness of their lungs allows oxygen to diffuse across their gill membranes and allows them to breathe.
- Pillbugs are different from their close relations the sow bugs. Pill bugs can roll up to protect themselves but sow bugs can't.
So, while they're quietly trundling along how do they make baby pill bugs?
Female and male pill bugs don't look different on top, their major anatomical differences are on their undersides. Females have several plates that form a shield on their belly, which holds a droplet of water where their young develop. Their reproduction may be sexual or asexual. When a female lays her eggs they are actually tiny plankton floating in the water droplet she carries around (This relates to her ties to aquatic isopods). She produces about 12-24 young and they take about 6-9 weeks to develop. When they are mature, they are ready to crawl out of their brood pouch. The young are white and look like miniature adults. They go through a series of molts (5 or 6) before they reach adulthood. One female can have up to 3 broods each year. Normally pill bugs live about 2-3 years, which is pretty old for a bug!
Who eats pill bugs?
Predators of pill bugs include birds, spiders, centipedes, or basically anything that eats insects.
Pill bugs make great classroom creatures
For your classroom there's a great PDF about how to set up a habitat and make observations from the Bug Man: http://bugs.org/Activities/Activity_M-16%20pillbug.pdf.
There are quite a few books out there on pill bugs, here are several you can find online:
- Pillbugs and Other Isopods: Cultivating Vivarium Clean-Up Crews for Dart Frogs, Arachnids, and Insects, Orin McMonigle (Adults, husbandry and use)
- A Pill Bug's Life, John Himmelman (children's nature book)
- Rolypolyology, Michael Eloshn Ross (lesson plans)
- The Pillbug Project, Robin Burnett (lesson plans)
- Pillbugs & Sowbugs and Other Crustaceans, Elaine Pascoe (children's nature book)
- Pillbugs (Lifecycles, Peebles/Capstone), Schaffer and Donna (children's nature book)
- Pillbugs, Watch It Grow (Pebble Books Paperback), Martha E.H. Rustand (children's nature book)
- The Roly-Poly Princess, Jennifer Lehnertz (children's story book)
When considering pill bugs, I like to think of them as the secret and quiet land trilobites, or crustaceans, quietly grazing away like little cows in the leaves and undergrowth. Hopefully you'll take a closer look at these amazing creatures next time you roll over a log.