Wood lice: Why crustaceans and not myriapods?

[Earl_Snake-Hips_Tucker]

A cursory glance at your basic wood louse would probably lead most people to believe that they’re more closely related to millipedes than crabs. Instead the opposite is true.

I’m not arguing the taxonomy–I just don’t want to do a whole lot of digging on the web. Simply: What are the features that they have in common with crustaceans, and what are some differences between them and millipedes?

[Giles]

The big thing that distinguishes crustaceansd is that they have ten legs – hence the other name for them, Decapoda.

The crustacean head is fairly uniform, having five pairs of appendages comprising two pairs of antennae, a pair of mandibles and two pairs of maxillae.

[Quoted from here]
(http://www-biol.paisley.ac.uk/courses/Tatner/biomedia/units/crus2.htm)

Next time you see a crab, a lobster or a slater, count the legs.

[astro]

The Woodlice

I imagine this is one “from the sea” trait

small quote from much longer article

The order Isopoda contains more than 10,000 species worldwide, most of which are marine. About 3,500 species of woodlice (Suborder Oniscidea) have been described but there are probably at least as many waiting to be discovered (Schmalfuss, personal communication), especially in tropical rain forests. Isopods evolved in the sea and are thought to have made the transition to land via the seashore. Species such as the common sea slater, Ligia oceanica, are still restricted to the coast. Successive species became increasingly resistant to the rigours of a terrestrial existence and probably colonised the land via moist leaf litter of pteridophytes (horsetails and ferns) during the Carboniferous (Piearce, 1989). Two key features ‘pre-adapted’ marine isopods to colonise the land. First, the dorso-ventral flattening of the body gave them an extremely stable ‘squat’ posture when walking. Second, retention of the young in a fluid-filled brood pouch protected them from drying out in the first few weeks after hatching from the egg. All isopods have seven pairs of legs which are usually of similar structure (hence the name iso-pod from the Greek isos meaning ‘equal’ and podes meaning feet).

Traditionally, woodlice have been regarded as poorly adapted to life on land. However, research conducted in recent years has shown that woodlice have evolved several sophisticated physiological and behavioural mechanisms to control water loss. These include the modification of the pleopods to form lungs and a capillary conducting system which allows nitrogenous waste to be excreted as ammonia gas, without significant loss of water (Hoese, 1984). Indeed the Dutch word for woodlice,‘pissebed’, is probably derived from the smell of urine that large aggregations of woodlice give off.

[Polycarp]

Decapods constitute one Order or IIRC Suborder of Crustacea, albeit the one with almost all the “popular” crustaceans – shrimp, lobsters, crabs, hermit crabs, etc. (Isopods and barnacles constitute two other orders, and there are a bunch of additional ones that will not be vamiliar unless you’re a plankton geek.

But while Giles’s point about Decapods was in error, the five appendage head structure he describes is indeed the defining characteristic of Crustacea. (There’s a fair amount of discussion on this from an interested layman’s perspective in Gould’s book on the Burgess Shale, Wonderful Life, if anyone wants to read further on it without wading through unexplained technical jargon – the reason being that much of the Burgess Shale fauna was erroneously classified as crustaceans without the defining characters.)

[Blake]

The simplest way that a layperson, or anyone else for that matter, can tell that a slater isn’t a millipede is by flipping on on its back. Millipedes have one pair of legs on each body segment, crustaceans don’t. Flip a slater on its back and you will see that the last several segments quite clearly do not have any legs and thus they can’t be millipedes. It’s a fast, simple accurate technique that cna be readily used in the field without an specialised equipment like microscopes for counting the mouthparts.

[Gunslinger]

See also these benthic horrors (Snopes thread; scroll down for pictures). Wood lice as big as a breadbox. :eek:

[The_Asbestos_Mango]

I, for one, look forward to welcoming our isopodic overlords.