What’s that Woodlouse?

My 3 year-old son and I spent some quality time in the garden looking under pieces of wood and brick left on the ground from last fall. We discovered all sorts of “bugs” (Alden’s word for them) and watched them scurry from the newly exposed surface for the safety and moisture of the dark underside of the wood and other debris. It was quite fascinating to watch these crowds of “bugs” hurrying for cover. A few rolled up into balls in an attempt to protect themselves – unfortunately these were the ones that Alden wanted me to move to the edge of the paver and roll off the edge (I managed to talk him out of it!).

Sowbugs (Oniscus asellus and Porcellio scaber)
A collection of sowbugs (Oniscus asellus and Porcellio scaber) and one Common Pill-bug (Armadillidium vulgare) – can you see three different looking woodbugs in this image?

I noticed that there seemed to be at least two different kinds of woodlice under the old boards lying around our garden. Later, I took a closer look at my photographs and am reasonably certain that I’ve identified the three species correctly – all are introduced species from Europe. Woodlice have a whole host of common names including: sowbugs, woodbugs, bibble bugs, pill bugs, roly polies, sink-lice, slaters, tiggyhogs, and monkey peas.

You can just say – “Wow, what a lot of woodlice.” and be reasonably correct in your identification. Or you could take the plunge and take a closer look. Check out this web page by Louise Kulzer for a quick visual identification of common sowbugs and pillbugs of the Pacific Northwest.

Here’s what we’ve got in our garden:

The European Sowbug (Oniscus asellus) was by far the most common sowbug underneath the wood. It has a smooth texture and has a wide flat oval like shape. When disturbed it tends to freeze before moving off. This sowbug is usually brown to black in colour and is spotted with yellow or white. There also seems to be bigger gaps at the edges of the body segments creating a feathery looking edge. The European Sowbug is in the Order Isopoda (Isopods), Suborder Oniscidea (Woodlice), and Family Oniscidae (Sowbugs).

European Sowbug (Oniscus asellus)
A closeup of a European Sowbug (Oniscus asellus). Note the more textured Common Rough Woodlouse (Porcellio scaber) underneath, center.

The second most common woodbug was the Common Rough Woodlouse (Porcellio scaber). The most obvious difference with this woodlouse is the textured surface and colouration. This species is a dark flat-gray with a bumpy, tubercle covered back. Looking a little closer you’ll notice that it has three distinct lobes at the front of the head instead of two that you’d find on the European Sowbug. You may be able to see this detail by clicking on the first image and looking carefully at the heads of the textured woodlice. The Common Rough Woodlouse is in the Order Isopoda (Isopods), Suborder Oniscidea (Woodlice), and Family Porcellionidae.

There were also a few Common Pill-bug (Armadillidium vulgare). You can separate the pill-bug from the sowbugs and woodlice by the fact that it curls up into a perfect ball when threatened. I’m reasonably sure that this is the Common Pill-bug, another introduced species from Europe. The colouration is somewhat similar to the European Sowbug in that it is slate gray with some mottled yellow markings. However, when unrolled it has a smoother, rounded look to it and lacks the prominent tails (uropods) that make it different from the European Sowbug. The Common Pill-bug is in the Order Isopoda (Isopods), Suborder Oniscidea (Woodlice) and Family Armadillidiidae (Pillbugs).

Common Pill-bug (Armadillidium vulgare)
Unlike the sowbugs, the Common Pill-bug (Armadillidium vulgare) is able to roll into a ball.

Woodlice are fascinating creatures. They’ve got three major body parts – a head, thorax and abdomen – but that’s where their similarity with insects ends. The head is typically small and has two pairs of antennae. The pereion or thorax is covered with seven overlapping plates or segments. Each segment has one pair of legs so the animal has fourteen legs.

The pleon or abdomen has six segments although they all may not be completely visible. At the end of the woodlouse is a triangular plate called the telson – the shape of the telson is sometimes used to separate similar looking species. Each of these segments also has one pair of legs but five pairs are modified for breathing. They consist of leaf-like flats that, when moist, allow oxygen to dissolve and be absorbed by a gill-like structure. Some species, like the Common Rough Woodlouse, have pseudotracheae which are moist tubes that allow oxygen to pass directly into the bloodstream. The last pair of legs are called uropods and are used as sensory organs. They often stick out behind the woodlouse like twin tails.

Mating sounds like an intriguing process – here’s how it is described on Gordon’s Woodlice Care Sheet:

Mating occurs at night, and is therefore hard to see. The male climbs onto a receptive female, licks her head and drums on her back with his legs for about five minutes. He then shifts to a diagonal position on the females back and passes sperm to her left side genital opening from his right hand stylets. He then changes his position to the opposite diagonal and deposits sperm in her right hand genital opening from his left hand stylet. Sperm transfer takes about 5 minutes for each side.

The fact that woodlice excrete waste in the form of ammonia gas, have blue blood because they use haemocycanin (copper based) rather than haemogobin (iron based) to transport oxygen in their blood, and eat their own feces in order to reabsorb the copper makes them worthy of a second look. Since they’re mostly detritus eaters, feeding on dead leaves and decaying material (and sometimes each other) means that they’re not really harming garden plants. They’re definitely very cool creatures!

More marvelous macro photography can be found weekly at Macro Monday.

Macro Monday
Macro Monday


  1. great Macros but….. I’m more comfortable a bit more far away from those 😉

  2. I always wondered what they were. Nice macro shots of them. I must make a point to visit your blog more often.

  3. You can add “potato bug” to your list of common names. My entry hall seems to be where they go to die. I haven’t figured out why or how they get in and I don’t see them anywhere else in the house. Good to see your blog up and running again.

  4. Wow! Fascinating info – especially the procedure for mating. I haven’t been a “bug” person, but your blog and Wanderin’ Weeta’s pull me in. Thanks.

  5. Thanks Carol – I enjoy popping over to your blog as well and keeping up to date on what’s happening on the Lower Mainland!

  6. Thanks Dave – we get them under out house in the crawl space entrance but they don’t do so well where it’s dry. The fact that they’re dying in your house is probably a good sign!

    I’ll definitely be looking for a new host when my contract comes up – Netfirms has been nothing but a hassle as far as downtime goes.

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  8. Thanks Carol – it does sound like they get quite intimate! Glad you could stop by for a visit!

  9. I’ve always been aware of different species (since I was a kid and tried to roll up the species that wasn’t designed to roll up), but haven’t ever read about it. Thx for sharing.

    I have seen one other species…it’s probably the least common, but it is typically smaller than the others.

    I took a look at the woodlouse id guide that you linked to, and I don’t think it’s any of the ones listed. It’s most similar to “Trichoniscus demivirgo”…but I’d have to see a photograph to be sure (I don’t think the legs stuck out like it does in the illustration).

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