Saturday, October 22, 2011

Porcellio spinicornis Say: The Spine-horned Little Pigbug that didn’t belong

A cluster of Edmontonian Sowbugs aka Woodlice when they aren't in a yard

Swans were winging their way overhead this morning, but the chill and the wind have kept what winged-insects are still about well hidden. It’s that time of the year for raking leaves, cutting down frost withered vines, composting rotting green tomatoes, and emptying pots of browned flowers. Moving pots, though, does give one the chance to observe some of the less obtrusive arthropods that skulk under them and scatter to any convenient crevice when disturbed. Among the skittering is one I have been meaning to write about for awhile, but never had the time to sit down and work out its proper name.
A freshly moulted Sowbug in an Edmonton backyard
What I did know was that the small, flat arthropods that were so common underneath things in my backyard were members of the crustacean order Isopoda (they have 7 pairs of legs). In Australia, I would have called these animals 'slaters', but when I was a kid in another land, I called them ‘so-bugs’. I got the common name, sowbugs, from a book and assumed it had something to do with them being scattered across the ground like sown grain. When you turned over a rock or board or moved a pot, they did sort of look like scattering grains, so the name seemed to make sense. But much to my embarrassment, as a young entomology student I learned that everyone else in North America called them ‘saughbugs’ after  fancied resemblance to female pigs! Some sowbugs can roll into a ball (conglobation) when disturbed, and these are called pillbugs for a reason that was obvious even to me.
Conglobated Pillbug (probably Armadillidium vulgare) from BC
Well, whatever you call them, sowbugs are one of the more successful lineages of land animals. We usually think of crustaceans (crabs, lobsters, crayfish, shrimp, prawns, and the like) as being inhabitants of the oceans, but most of the 3600 described species of the suborder Oniscidea are fully terrestrial.  An exception are members of the family Ligiidae that live an amphibious existence along the coast (see Ted MacRae’s great post). The earliest known fully terrestrial isopods are Eocene fossils from after the great extinction event that removed the dinosaurs inter alia at the end of the Cretaceous Period. Perhaps one or more ancestral amphibious sowbug found life on land easy in the brave new Eocene world, but in any case, their descendants have colonized most of the landmasses on earth (Antarctica and a few small islands excepted). Mostly they all do the same thing – eat decomposing plant  matter and so are ‘good bugs’, at least to any gardener that likes a healthy garden.
Sowbug lurking in a crevice - something they do very well
Until about 9-10,000 years ago, Alberta was pretty much free of any animal life, let alone sowbugs. Any that once lived here would have been ground to dust by the glaciers and this state of affairs seems to have held until recently. My wife, who grew up in Edmonton and enjoyed turning over rocks to see what lived underneath, claims there were no sowbugs here when she was a child. Our friend John Acorn, a dedicated Albertan naturalist, supports her claim. Jass & Klausmeier (2000) list no records, whatsoever, of terrestrial isopods from Alberta. So, what is this slater living in my backyard?
Antennal spines (arrows), black head, dark midline stripe, yellow spots, arrangement of lungs, and other characters support Porcellio spinicornis Say, 1818
Answering that question is what took so much time; however, thanks to an excellent key by Stephen Hopkin (1991), the invaluable BugGuide, the University of Alberta library, and a certain terrier-like attitude to unnamed bugs by both Mr & Mrs HBG, we now have an answer. Back in 1818, Thomas Say, the great American naturalist, conchologist, entomologist, and nascent explorer, published a paper in the Journal of the Academy of Natural Science (Philadelphia) on new species of Crustacea he had discovered in the United States. Among these hard shellfish was one whose ancestors left the oceans long ago and had spread far and wide on land: Porcellio spinicornis Say, 1818. Apparently, Say was impressed by the dorsal ridges on the second and third antennal segments that ended in stout spines (hence spini [L. spine] corn- [L. horn, or in this case, antenna]) and no doubt the attractive colours in the living animals. Presumably, Pierre André Latreille, who in 1804 named the genus with the Latin for a small pig was channeling the sowbug meme. However, this Spine-horned Little Pigbug should not be confused with another widely distributed sowbug, the Common Woodlouse Oniscus asellus L., 1758, a species that I have yet to see in Alberta.
Common Woodlouse from BC
Say appears to be wrong about the origin of his woodlouse. Today Porcellio spinicornis Say is considered one of about three dozen species of terrestrial isopods that have been introduced into the Americas from the ‘Old World’. This must have happened very early on, but now this species can be found in much of North America including Canada (although not officially in Alberta). Some pest control operators like sowbugs, because fussy homeowners find them distressing and are willing to spend money to have them sprayed into oblivion. I don’t see why. Although they do scurry about and hide under pots and the like, they appear to eat nothing except plant matter that has died and been overgrown with fungi and bacteria. This seems fine if they keep to cities and suburbia and so far, that is what they seem to do.


Hopkin SP. 1991. A key to the woodlice of Britain and Ireland. Field Studies Council AIDGAP Guides 204

Jass J & B Klausmeier. 2000. Endemics and immigrants: North American terrestrial isopods (Isopoda, Oniscidea) north of Mexico. Crustaceana 73 (7): 771-799.

Leistikow A & Wägele JW 1999. Checklist of the terrestrial isopods of the new world. (Crustacea, Isopoda, Oniscidea). Revta bras. Zool. 16 (1): 1 - 72,1 999

Lindroth CH. 1957. The Faunal connections between Europe and North America. Biodiversity Heritage Library

McQueen DJ. 1976. Porcellio spinicornis Say (Isopoda) demography. II. A comparison between field and laboratory data. Can. J. Zool. 54: 825-842.

McQueen DJ & JS Carnio. 1974. A laboratory study of the effects of some climatic factors on the demography of the terrestrial isopod Porcellio spinicornis Say. Can. J . Zool. 52: 599-611.

Say, T., 1818. An account of the Crustacea of the United States. Journal of the Academy of Natural Science Philadelphia, 1: 235-253, 313-319, 374-401, 423-458.

Schmalfuss H. (2003): World catalog of terrestrial isopods (Isopoda: Oniscidea). Stuttgarter Beiträge zur Naturkunde, Serie A, Nr. 654: 341 pp.

Schmidt C. 2008. Phylogeny of the Terrestrial Isopoda (Oniscidea): a Review. Arthropod Systematics & Phylogeny 66(2): 191-226.


  1. Interesting article. Despite how common they are, this is one denizen of the garden that I haven't photographed yet. I'll see if I can round up some to keep in a terrarium for the winter, they might provide some photo-ops when I start getting cabin fever!

    And thanks for adding another word I won't be able to shake form my mind -- "conglobated". Most appropriate, although "conglobulated" would sound even better...

  2. It's very cool to imagine those ancient isopods alongside their descendants. I had no idea that some were members of an introduced species.

  3. Hi Heather

    Firstly, thanks for the comments on the Ig-Nobels. It was a real hoot as you could see from the video-something real different for both Darryl and me.

    Your slaters et al. at wonderful shots. We have a similar one in the orchid shadehouse, with yellow spots. It seems quite small compared to yours. The questions is what are they eating. The orchid people would swear that they eat the orchid roots but maybe they are feeding on fungi and decomposing plant material as you suggest and cause no problems.

  4. Nice post. I caught an eyeful of these yesterday when moving firewood around - the yellow markings on their back are quite striking!