Lice taxonomy

Please take a look at the new taxonomy for Psocodea and see what you think. I followed Tree of Life, and based on the discussion below it seemed like everyone would either support these changes or be neutral about them. We don't have much in the way of images of parasitic lice, so these are easily shuffled around if another arrangement seems better.

I just came upon this discussion and figured that I would put in my two cents. I’m not a taxonomist/systematist/cladistics person, but I do loosely follow the louse literature. The issue is trying to rectify cladistics with traditional taxonomy.

As a background, the Order-level names for the lice have flopped around since the early 1800’s with Anoplura + Mallophaga vs. Phthiraptera being the major area of contention. Traditional taxonomy gave way to systematics, and this did not stop the flopping. But now we’ve entered the era of cladistics, and things are becoming clearer. The use of cladistic analysis has transformed the resolution with which we can view the relationships between organisms. Cladistics is very good at showing you groups of monophyletic organisms. (And when you see which groups are monophyletic, you also see which groups are not, such as Mallophaga, Phthiraptera, and Psocoptera.) As mentioned by others here, we have good cladistic analyses for lice.

But how do you decide which monophyletic group deserves the taxonomic level of Order, and what will you call that group? Because of the amount of detail provided by cladistics, many of the people who use it prefer abandoning the traditional taxonomic ranking system that constrains the number of named monophyletic ranks for an organism. Why bother having an Order, Phylum, Class, etc.? There are good alternatives to traditional taxonomic ranks, but you could argue that they are not as easy to understand for the beginner.

Unfortunately for the fans of traditional taxonomy, many of the people who do higher-level cladistics (e.g. at the Order level) don’t make the judgments about which monophyletic clade gets which taxonomic rank. Higher-level taxonomy is outside the rules of the International Code of Zoological Nomenclature, so without some trusted expert to bridge the cladistics-taxonomy gap, we risk chaos.

One challenge of resolving these insects into traditionally ranked monophyletic groups comes with the flexibility in defining members of groups. Phthiraptera, as currently defined (containing Anoplura and the three groups of chewing lice) makes Psocoptera polyphyletic. But if you split Psocoptera into a couple of orders and added some of the Psocoptera groups into Phthiraptera, you could make everything monophyletic. Order names are flexible taxonomic labels- there are no real rules that define them. In that case, Phthiraptera will remain the appropriate Order, not Psocodae or Psocoptera. Or you could lump them all together under Order Psocoptera. You wouldn’t necessarily need to change the name to Psocodae. Or you could change the name of this new combined order Crazyoptera because so many crazy things have happened to those wings. There are no real rules, and there is no organization out there that polices this kind of thing. Chaos is avoided only because most entomologists defer to a few respected sources, even if we see flaws in the names or groups used. That requires respect for scientists that have a deep understanding of evolutionary relationships so that all of the orders are of equivalent weight.

The most tried and true “expert” reference would probably be Borror and DeLong’s Introduction to the Study of Insects. They use Phthiraptera. The Tree-of-life puts primary focus on unranked cladistics, but also uses traditional taxonomy in places. For example, even though they do explain that the aforementioned groups are not monophyletic, they have a page on Phthiraptera. So even though it doesn’t resolve all insects into monophyletic groups (as currently defined), Phthiraptera is really the Order name to use. I suspect it will be a number of years before someone really bridges the gap between cladistics and taxonomy for the entire Psocoptera/Psocodae group. We can’t leapfrog into the future because we don’t know exactly where we’re going. So I think we’re safe going with Phthiraptera for the foreseeable future. But if anyone knows the authors of An Introduction to the Study of Insects, they could go right to the source.

In most, even recent texts, the gold standards mallophaga and anoplura are still applied. However, I think bugguide might stay ahead of the curve if the old taxonomy is changed to the newer system. It looks to me that the order Phthiraptera is here to stay, at least for a while. I do however, have concerns of confusion caused by the switch, but I have complete faith that bugguide users will catch on fast. I would support either decision, either switching or keeping the old standard. I myself still refer to anoplura and mallophaga as orders and will take some time for me to get used to it. Thanks for reading my opinion, for whatever it is worth.

They are old and discredited names as they do not correspond to our best theories of how lice evolved. In fact, Phthiraptera is inappropriate as an order as lice arose (at least 3 separate times) from within Psocoptera. Lice are related to booklice (Liposcelididae). If we wanted to be in line with the most current reseach, book lice, bark lice, and parasitic lice would all be in the order (superorder?) Psocodea. I personally would prefer this scheme, but I think some other people on bugguide prefer to adhere to certain texts as standards.

Reference: Murrell, A. & Barker, S. 2005. Multiple origins of parasitism in lice: phylogenetic analysis of SSU rDNA indicates that the Phthiraptera and Psocoptera are not monophyletic. Parasitol. Res. 97:274-80