XXII CNIE - Accademia nazionale italiana di Entomologia

WHERE DID INSECTS COME FROM? NEUROPHYLOGENETICS, AND THE
ORGANIZATION OF INSECT BRAINS
Nicholas Strausfeld
Center for Insect Science and Division of Neurobiology, University of Arizona, Tucson
This lecture will consider the pedigree of a specific class of brains: namely, the brains of
insects in relation to those of other arthropods, in the context of asking from what
ancestral lineage might the insects have emerged. This question is not yet resolved even
though molecular phylogenetics using nuclear DNA proposes a close affinity between
the Insecta and Entomostraca, a basal group of crustaceans (Mallatt and Giribet, 2006).
However, this view gives rise to some awkward enigmas, not the least of which is that a
group of crustaceans known as the Remipedia emerge as a sister group of the
Entomostraca (Regier et al., 2008). The remipedes are fascinating animals. Each
segment is homonomous, equipped with identical appendages except those of the head.
Remipedes look primitive, and because of this they have been classified as a basal group.
The problem with this interpretation is that neuroanatomical studies by Fahnenbruck and
Harzsch (205) on one species of Remipedia, demonstrate beyond any doubt that these
animals are equipped with brains that have all the features expected of a higher
malacostracan crustacean, apart from the absence of optic neuropils. This deficit is
hardly surprising because these crustaceans are troglodytic, living in pitch dark in caves.
That the thorax and abdomen of these strange crustaceans are organized almost
homonomously is likely to be a secondary adaptation to an environment in which
specialized limbs for feeding, locomotion, or defense are redundant.
More recent neuroanatomical analyses that compare insects and crustaceans further
challenge the now widely held opinion that the Insecta are closer to the Entomostraca
(also referred to as the Entomostraca) than they are to the Malacostraca. Phylogenetic
analysis using neural characters suggests that insects are more closely related to the
Malacostraca than to any other group of arthropods and that a common ancestor of the
Malacostraca and Insecta is likely to have possessed features typical of extant basal
malacostracans, such as the Phyllocarida. However, before summarizing the evidence as
to why this is a plausible relationship, I must first outline how brain characters provide
useful and stable indicators of phylogenetic relationships and how such characters are
used for cladistics, the reconstruction of phylogenetic relationships.
The first attempts at using brain structures to infer evolutionary relationships was by two
Swedish scientists, Nils Holmgren, in 1916 and Bertil Hanström (his student) in 1926.
These authors compared brain regions across the arthropods, focusing on such wellknown
structures as the optic lobes, mushroom bodies, and ocelli. Although insufficient
by today’s standards, these authors came to the conclusion that insects are the sister
group of the crustaceans, a view that was revolutionary at the time and which has only
come back into vogue in the last ten to fifteen years.
Today, many more characters are used for phylogenetic analysis. These relate to six
types of morphological entities. These are, first, architectural criteria that can be applied
to circumscribed neuropils; next, the morphologies of neurons themselves, such as
whether nerve cells within a delineated neuropil are uni- or multistratified, whether they
exhibit polarities, whether they have axons, or are anaxonal, whether their axons cross
segmental domains, whether their axons are homo- or heterolateral, and many other
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