Opiliones or Phalangida - what's in a name?

In line with many modern texts of invertebrate zoology, I have used the name "Opiliones" to refer to the harvest-spiders, for example in the title for this set of web pages: Opiliones Notes.

But in other places I have used the name "Phalangida" for these creatures; thus my Ph.D. thesis was called Fine Structural Studies on the Eyes of Phalangida.

So just what is the difference between Opiliones and Phalangida - or are they the same?

Do you want to go straight to the bottom line?

I must confess that I tend to use the terms interchangeably, regarding them as being synonymous. One of the most typical of harvestmen is the common European species Phalangium opilio, which it is tempting to regard as the "type genus" for these animal and so on a sort of emotional, intuitive level I tend to think of them as Phalangida, rather than Opiliones. But is this justified - or are they different groups of arachnids?

If you look it up in a textbook of invertebrate zoology, you are likely to (a) get the impression that Opiliones and Phalangida are synonymous terms and (b) learn that the order comprises three sub-orders:

Cyphophthalmi, a few species of small, mite-like animals which live in soil and leaf mould. Their pedipalps are like miniature legs. Note that some of these opilionids have no eyes (hence their name?).
Laniatores

Laniatores , which usually live in leaf mould in the tropics. Larger in size than the previous sub-order, these arachnids have their pedipalps modified into raptorial appendages which can grasp the prey by folding like a jack-knife; quite often these palps are armed with strong spines. Their legs may be short or long.

Palpatores, the "common or garden" daddy-long-legs, so-called because of their extremely long legs which feature prominently in their biology (as locomotion, prey capture, sensory organs).
Our favourite, Mitopus morio, belongs to this group and is portrayed at the top of this page as the "Back to Opiliones Notes" button, but the drawing here shows a little more detail of a generalised palpatoran with its legs removed so that you can see the shape of its body. Note the prominent eyes on the ocularium sticking up from the animal's back and the pincer-like chelicerae pointing downwards at the front end, with he leg-like palp just outside.
Palpatores body (legs removed)

So far, so good - but read on...

Over the years, the relative positions of these three sub-orders in relation to each other have shifted according to different authors and how they have interpreted comparative data, often anatomical.

So, to cut a long story short let's just say that in more recent times, phylogenetic studies have shown that the Opiliones is best considered to contain two sister groups - the Cyphophthalmi and the Phalangida, with the latter group comprising the Laniatores and Palpatores.
A good coverage of these taxonomic aspects in terms of modern assessments based on analysis of nucleotide sequence can be found in the paper by Jeffrey W. Shultz & Jerome C. Regier: Phylogenetic analysis of Phalangida (Arachnida, Opiliones) using two nuclear protein-encoding genes supports monophyly of Palpatores (2001 Journal of Arachnology 29: 189-200). As the title indicates, the Palpatores are shown to be a monophyletic group; this means that the members of the group can be traced back to a common ancestor. If you read their paper, you'll find some interesting details about the relationships between the various families within the Palpatores and how they might relate to the Laniatores. Their analysis is based on examination of two nuclear protein-encoding genes from 27 species in seven superfamilies and using Cyphophthalmi as an outgroup. Very interesting reading, if you like the precise technical details!

If you want to read an account of the taxonomy of this order of Arachnids, you could do far worse than the classic text by Theodore Savory: Arachnida, published in 1964 by Academic Press (London).

To conclude:

The Opiliones and Phalangida are almost the same thing - but not quite...
as you can see in this dendrogram, which shows the division of the Opiliones into its sub-groups, with the three sub-orders highlighted in red.
Thus, Opiliones = Phalangida + Cyphophthalmi
and Phalangida = Palpatores + Laniatores,
where Palpatores = Eupnoi + Dyspnoi, in which Eupnoi are the familiar harvestmen (harvest-spiders) of temperate Europe and North America.

Classificatory dendrogram of Opiliones
The Cyphophthalmi have just a few species, with a scattered distribution over the planet (which could be regarded as characteristic of a primitive group).
There are about 900 species of Laniatores, which are perhaps the most specialised of the phalangids. This sub-order comprises six families: Oncopodidae, Triaenonychidae, Assamidae, Cosmetidae, Gonyleptidae and Phalangodidae. Of these families, only the last lives in Europe in contrast to the others which are found more in the southern hemisphere (south-east Asia, central and south America, Africa and Australia).
There are almost as many species of Palpatores, which contains the harvestmen of northern temperate zones, both America and Europe as well as Asia. Some species extend down to the tropics, but mainly their distribution is in contrast to that of the Laniatores. The Palpatores are divided into two groups; the Dyspnoi which comprises four families - Acropsopilionidae, Trogulidae, Nemastomatidae and Ischyropsalidae - and the Eupnoi with just one family - the Phalangiidae. The typical, characteristic and familiar harvestmen belong to the Phalangiidae (such as Phalangium opilio and our starring species Mitopus morio).

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