Diplopoda — Nervous and Neuroendocrine Systems

Home , Chordeumatida, Glomerida, Neurilemma

CHAPTER 8

DIPLOPODA — NERVOUS AND NEUROENDOCRINE SYSTEMS

ANDY SOMBKE AND JÖRG ROSENBERG

NERVOUS SYSTEM

No detailed and comparative analysis of the diplopod nervous system is currently avail- able and our present knowledge of the anatomy is anything but complete. The first descriptions were given by Treviranus & Treviranus (1817), Newport (1843), and Owen (1855). Detailed anatomical information were firstly given by Saint-Remy (1890), Holm- gren (1916) and Hanström (1928). Sahli (1966) described in detail the nervous system of representatives of the Julida. In parallel, Seifert (1966) provided a comparative overview on the nervous system of representatives of Penicillata, Chordeumatida, Glomerida, Julida, and Spirostreptida. In mandibulate arthropods, the nervous system can be separated into a central nervous system, composed of brain and ventral nerve cord, and a peripheral nervous system. The brain is a syncerebrum formed by the close association as well as structural and functional transformation of segmental ganglia (Richter et al., 2010; Loesel et al., 2013). Thus, the brain is tripartite and composed of the neuromeres proto-, deuto-, and tritocerebrum. Each neuromere is usually compartmentalized to some degree into definable clusters of neurons in the periphery that surround central neuropils (Richter et al., 2010). In the Myriapoda, the neuraxis (the axis of the nervous system) of the brain is commonly arcuated along the anterior-posterior body axis resulting in a dorsal or even posteriodorsal position of the protocerebrum (Fahlander, 1938; Sombke et al., 2012). Brain neuromeres are not separated by clear borders, but can be distinguished externally by their associated nerves. The neurilemma is thin and composed of two parts: an internal perilemma and an external neural lamella (Joly & Descamps, 1984). The perilemma is composed of different cells types including glial cells. In Gymnostreptus olivaceus, Francicso et al. (2015) described an additional external discontinuous and irregular cell sheath whose function appears to be linked to nutrition and protection.

Protocerebrum

The protocerebrum is the largest part of the brain (Fig. 8.1A-D). It consists of the frontal lobes and extends laterally to include the optic lobes (missing in blind species). Diplopoda possess two optic neuropils commonly termed lamina and medulla (e.g. Holmgren, 1916; Hanström, 1928; Francisco et al., 2015; Sombke & Harzsch, 2015). The second-order optic neuropil (medulla) is also termed ‘visual tectum’ because it matches the characteristics of the hexapod lobula plate (Strausfeld, 2005). In Orthoporus ornatus, Strausfeld (2012) described that photoreceptor axons project into a single planar lamina, from which relays extend centrally to target a group of spindle-shaped neuropils via uncrossed axons. However, older descriptions of ‘Julus sp.’ (e.g., Hanström, 1928) mention a layered arrangement of this second-order optic neuropil. The mediodorsal part of the protocerebrum is termed pars intercerebralis (Holmgren, 1916). In ‘Julus sp.’, Holmgren (1916) distinguished three globuli (I-III) associated with the mushroom bodies that correspond to the ‘masse ganglionnaire antérieur’ and the two ‘masses ganglionnaire externe et interne’ of Saint-Remy (1890). The globuli are accumulations of small neurons poor of cytoplasm termed globuli- or Kenyon cells. Globulus II corresponds to the corpora pedunculata of Hexapoda (Saint-Remy, 1890). According to Sahli (1966), all three globuli belong to the protocerebrum. In general, arthropod mushroom bodies are centers for sensory integration and memory formation, and represent the neuronal basis for associative and flexible behaviors (Loesel et al., 2013). The most prominent input originates in the olfactory lobes through the projection neuron tract which in diplopods was only described for ‘Julus sp.’ (Holmgren, 1916) and Orthoporus ornatus (Strausfeld et al., 1995; Strausfeld, 2012). Dendritic arborizations of globuli cells form calyces or stalked glomeruli (‘Stielglomeruli’ sensu Hanström, 1928), axons provide a system of parallel fibers called pedunculi or stalks (‘Stiele’ sensu Holmgren, 1916). In Orthoporus ornatus, Strausfeld (2012) depicted microglomeruli that conjoin the mushroom bodies. A short pedunculus extends laterally, dividing into several parallel lobes and reaching the midline but not crossing it (Strausfeld et al., 1995). Another pedunculus crosses the midline forming a contralateral connection (Strausfeld, 2012). In addition, medial (or median) bodies occur (Holmgren, 1916; Horridge, 1965; Sombke & Rosenberg, in press) (Fig. 8.2C). Here, axons converge and thus establish a contralateral connection of the pedunculi. Histologically, Sahli (1966) described intracerebral structures (‘îlot intra-globulaire’ or ‘organes intra-globulaires’) in Julida, Polydesmida and Nematophora situated anterior-dorsal within globulus I. Similar structures (‘organes neuraux’) were described by Juberthie-Jupeau (1967a) within globulus I of different species of Glomerida (Glomeridae, incl. Trachysphaera; Glomeridellidae), and by Nguyen Duy-Jacquemin (1974) in Polyxenus lagurus. Jamault-Navarro (1992) interpreted such structures as intracerebral sensory structures (‘structures sensorielles intracérébrales’). An unpaired midline neuropil or central body, common in arthropods, seems to be absent (Loesel et al., 2002) or ‘insignificant’ (Holmgren, 1916; Hanström, 1928) in Diplopoda (compare Fig. 8.2C). According to Holmgren (1916) it is completely embedded in fibers. Strausfeld (2012) interpreted this reduction as expressing a redundant function of leg motor circuits;