XXII CNIE - Accademia nazionale italiana di Entomologia

The optic lobes of Entomostraca consist of two neuropils only. The outer neuropil, called
the lamina, derives from an outer cell proliferation zone. The lamina is linked by
uncrossed axons to a plate- or tectum-like inner neuropil, whose neurons have very large
dendritic fields and connect directly to premotor neurons in the brain. In contrast, the
compound eyes of malacostracans and insects are served by four nested retinotopic
neuropils. Three of these are linked by two successive chiasmata, whereas the fourth
receives a system of uncrossed fibers. The first chiasma horizontally reverses in the
second optic neuropil (the medulla) the order of retinotopic columns in the first optic
neuropil (the lamina). The second chiasma carries retinotopic neurons from the medulla
to a third neuropil (the lobula) where the horizontal order of columns is again reversed.
Uncrossed axons from the medulla extend to a fourth neuropil, the plate-like lobula plate
or, in some insects, a neuropil immediately under the lobula. Although the laminas of
Entomostracan crustaceans (e.g. Artemia, Triops) have a retinotopic organization of
photoreceptor endings and second-order relay neurons that is similar to the laminas of
other crustaceans and insects, at deeper levels the optic lobes of malacostracans and
insects share multiple characters that are entirely absent from entomostracans. For
example, many morphological types of neurons in the medullas of malacostracans have
the same shapes and relative dispositions as in the medulla of insects (Strausfeld &
Nässel 1980). Studiess of the growth of the lamina and medulla in insects describe both
centers as arising orthogonally from two adjacent sets of precursor cells, called the outer
optic anlagen (Meinertzhagen & Hanson 1993). In entomostracans, there is only one
such set of precursors. In insects and malacostracans the medulla is likely to have
originated from an ancestral duplication of the cell lineage originally providing the
entomostracan lamina (Meinertzhagen 1991). This means that the medulla is essentially
the evolutionary progeny of the entomostracan lamina (Strausfeld 2005) and that the
uncrossed axons extending from it to the plate like fourth optic neuropil are homologous
to the uncrossed axons extending from the entomostracan lamina into its second optic
neuropil. Thus, if insects have arisen from an entomostracan-like ancestor, then they and
the malacostracans have independently evolved four optic neuropils and two chiasmata,
as well as many cell types whose morphologies are almost indistinguishable in these two
groups.
Not only do the Entomostraca lack two optic neuropils that are common to the
malacostracans, but they also entirely lack any evidence of olfactory lobes. In contrast,
both malacostracans and insects possess olfactory lobes served by homologous
appendages, the antennules. These centers in the deutocerebrum are characterized by
discrete subunits: glomeruli in insects, and columns in most malacostracans (the
exceptions are the stomatopods and phyllocarids, both basal to the Malacostraca, in
which olfactory lobes are also glomerular). A further commonality between basal
malacostracans and insects is revealed by second-order projections from the olfactory
lobes into the most rostral segment of the brain, the protocerebrum. In phyllocarids, relay
neurons from the olfactory lobes project out to the lateral protocerebrum. The same
occurs in the Archaeognatha, flightless ‘bristletails’ that are considered Mid-Devonian
relics (Labandeira et al. 1988) and whose mandibles have only one point of articulation
with the head capsule. These basal monocondylic insects, typified by the Machilidae,
have projections from the olfactory lobes that similarly terminate in the lateral
protocerebrum. In the Machilidae, as in phyllocarids and indeed all Malacostraca, these
projections are direct: they do not involve the mushroom bodies because these centers
are absent both in Malacostraca and the Machilidae (Strausfeld, 2009). Another likely
ancestrally shared character (synapomorphy) of malacostracans and insects is that in the
phyllocarids and machilids projections from the olfactory lobes to the protocerebrum are
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