XXII CNIE - Accademia nazionale italiana di Entomologia

attributes. The distribution, size and clustering of neuronal somata are useful characters
as are the recognition of specific geometries of neuropil subunits, such as columns,
glomeruli, and other discrete structures such as wedge-like domains (Strausfeld, 1998).
For any taxon, characters have been scored as either being present (1) or absent (0) for
more than thirty species representing the major arthropod groups. It is important to avoid
biasing the data set by assuming specific functions or identities. For example, with
regard to mid-line neuropils that appear to be present across the crustaceans and insects,
all that can be scored is whether they are modular, the numbers of modules, and whether
their contributing neurons derive from remotely positioned somata or somata that are
immediately adjacent to the center (Strausfeld et al., 2006). Thus far, over 130 characters
have been identified and are used to construct a matrix in which their presence of
absence is scored for 35 or so species (including one outgroup species, either the
polychaete annelid Arctonoe fragilis or Nereis bicolor) representing many of the major
arthropod groups. To minimize bias, characters are treated as unlinked, unordered and
equally weighted. They are not assigned character states and nor functional affiliations.
Once such a large matrix is assembled, it is then analyzed using special statistical
programs designed to resolve the degree to which the species investigated share
structural affinities, thus providing the most parsimonious relationships amongst them
(Swofford 2002). Species used for such reconstructions represent their wider taxonomic
classes. For example, for the enigmatic Onychophora (velvet worms), a lobopod group
distantly related to lobopods that characterize lower Cambrian fossil beds, three species
have been used, two peripatopsids, Euperipatoides rowelli and Phallocephale
tallagandensis and an un-named peripatid species from the Mazatlan peninsula.
Although from two continents, Australia and S. America, their central nervous systems
show no detectable differences in organization, suggesting extreme phyletic stability
over a period of 237 million years, since the break up of the super continent of
Gondwanaland. Likewise, the central nervous systems of Old World and New World
scorpions (from the Sahara and from the Sonoran Desert) show comparable stability.
Neural phylogenies sample both scutigeromorph and scolopendromorph Chilopoda
(centipedes), and Diplopoda (millipedes) from Asia and America. Likewise, species of
araneans (spiders) include primitive Liphistiidae from Asia, wandering spiders, orb
weavers, and salticids or jumping spiders. Included too are other chelicerates, such as the
vinageroon and whip spiders. Species include several apterygote, palaeopteran and
neopteran insects along with the entomostracan crustaceans Triops and Artemia,
reptantian species, such as crabs, and non-reptantian malacostracans. The latter include
members of the Isopoda, Stomatopoda, and Phyllocarida, the first representing a highly
derived group of Eumalacostraca, the last representing what is thought to be the most
basal malacostracan taxon.
Once assembled, a heuristic search of the data matrix, employing about a thousand
random stepwise addition replicates, provides unrooted relational trees, the most
parsimonious of which provide a comprehensive arthropod phylogeny. What is striking
about this is that in most respects an optimal tree is congruent with phylogenies resolved
by molecular phylogenies. There are, however, two notable exceptions to this. The first
is that the Onychophora, long assumed to be related to, but not included in the
Euarthropoda, is revealed by neural cladistics to be within the Euarthropoda and basal to
the chelicerates (e.g. Limulus, araneans, scorpions, Pycnogonidae). The second
exception refers to the position of the insects. For some years insects have been admitted
as a sister group of the crustaceans on the basis of shared ommatidial structures (Dohle
2001), insects and crustaceans providing the clade Tetraconata, also termed the
Pancrustacea. Molecular phylogenies support this affinity, but indicate that the Insecta
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