(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.
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!
Predators of pill bugs include birds, spiders, centipedes, or basically anything that eats insects.
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:
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.
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