XXII CNIE - Accademia nazionale italiana di Entomologia

homolateral; axons extend forwards only on the side of the brain from which they
originate. The higher Malacostraca, including the stomatopods, have projections that are
heterolateral; axons from each olfactory lobe branch to both sides of the protocerebrum.
Thus, whereas in higher malacostracans, and indeed in dicondylic insects, pathways
from the olfactory lobes show many elaborations, when comparing basal taxa the ground
plan of the insect olfactory system is almost identical to that of basal malacostracans.
The third center to be considered is called the central complex. This is a system of
interconnected neuropils present in the brains of all arthropods that reaches great
elaboration and complexity in the neopteran insects. Common to all arthropods, the
central complex is associated with, and in orthopterous insects is known to develop
from, a specific set of pioneer axons. These fasciculate to cross the midline via a rostral
commissure, and then shift their locations to a more posterior commissure and in so
doing set up a system of chiasmata that lay down the basic framework for a neuropil that
in the Neoptera consists of sixteen modules or subunits (Boyan et al., 2008). This switch
of an axon trajectory from one commissure to another behind it is unique to the
protocerebrum and has been termed by Boyan et al. (2008) as “fascicle switching”.
Modules that result from this axonal rerouting can develop in two neuropils: the one that
is associated with the rostral commissure, called the protocerebral bridge, and one that is
associated with the more posterior commissure to which fascicle-switching axons
project, called the central body. In the Neoptera, the central body comprises two major
divisions, the fan-shaped body and the ellipsoid body, connected to a pair of small,
layered neuropils called the noduli (Loesel et al., 2002).
This mid-line central complex, which is now known to be associated with the control of
limb movements (see Poeck et al., 2008), has reached its greatest elaboration in those
insects and crustaceans that show high levels of dexterity, but is much simpler in
organization in species that have simpler modes of locomotion. In the Machilidae, the
central complex consists of two layers of neuropil supplied by rostral axons that
decussate into the central body neuropil through a system of chiasmata. There is no
protocerebral bridge neuropil, nor noduli. Instead, the neuropil of the central body has
the appearance of an elongated spindle, with two distinctive strata. The same
organization typifies non-reptantian decapod crustaceans, such as caridids (shrimps).
Their central complexes and those of machilids appear identical (Strausfeld, 2009).
Examination of entomostracan brains also reveals a midline neuropil, but this is both
extremely small, has a single chiasmatal element, and is entirely unstratified.
Critics will certainly claim that the three examples reviewed above are insufficient to
show that malacostracans and insects might derive from a common ancestor. And indeed
there is no way of demonstrating conclusively that they have or have not. However,
would the insects have arisen from an entomostracan-like ancestor (see Glenner et al.,
2006), and would another entomostracan-like antecedent have also given rise to the
malacostracans, then each of the features discussed above would have had to evolve
independently in the Malacostraca and Insecta. The organization of the olfactory lobes
and their central projections would then have to be interpreted as examples of
convergence, as would the evolution of four nested optic neuropils and their chiasmata.
Likewise, central complexes that appear to have identical architectures in machilids and
malacostracans would have had to have evolved convergently. On the other hand, the
most parsimonious explanation for all these commonalities is that these features
common to the brains of insects and malacostracans were also present on a common
malacostracan-like ancestor. There are many other features of insect brains, which have
not been considered here, that speak to a malacostracan ancestry. One is the connection
between the labral neuropils and the dorsal protocerebrum. Another is the organization
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