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Abstract
The defense glands in the dorsal prothorax are an important autapomorphic trait of stick insects (Phasmatodea). Here, we study the functional anatomy and neuronal innervation of the defense glands in Anisomorpha paromalus (Westwood, 1859) (Pseudophasmatinae), a species which sprays its defense secretions when disturbed or attacked. We use a neuroanatomical approach to identify the nerves innervating the gland muscles and the motoneurons with axons in the different nerves. The defense gland is innervated by nerves originating from two segments, the suboesophageal ganglion and the prothoracic ganglion. Axonal tracing confirms the gland innervation via the anterior suboesophageal nerve, and two intersegmental nerves, the posterior suboesophageal nerve and the anterior prothoracic nerve. Axonal tracing of individual nerves reveals eight identified neuron types in the suboesophageal or prothoracic ganglion. The strongest innervating nerve of the gland is the anterior suboesophageal nerve, which also supplies dorsal longitudinal thorax muscles (neck muscles) by separate nerve branches. Tracing of individual nerve branches reveals different sets of motoneurons innervating the defense gland (one ipsilateral and one contralateral suboesophageal neuron) or the neck muscle (ventral median neurons). The ipsilateral and contralateral suboesophageal neurons have no homologs in related taxa like locusts and crickets, and thus evolved within stick insects with the differentiation of the defense glands. The overall innervation pattern suggests that the longitudinal gland muscles derived from dorsal longitudinal neck muscles. In sum, the innervating nerves for dorsal longitudinal muscles are conserved in stick insects, while the neuronal control system was specialized with conserved motoneurons for the persisting neck muscles, and evolutionarily novel suboesophageal and prothoracic motoneurons innervating the defense gland.
Highlights
In stick insects (Phasmatodea), paired exocrine defense glands are found in the prothorax (Bedford, 1978; Chow, 2008; Bradler, 2009)
Close to the defense gland, the nerve splits into two nerve branches which contact the proximal gland surface
In C. morosus, the more posterior muscles are termed 3a and 3b (Jeziorski, 1918) or D3 and D4 (Marquardt, 1940). These muscles are more difficult to homologize, since in C. morosus some dorsal longitudinal muscles are absent in the prothorax (Idlm3, Idlm6; Jeziorski, 1918; Leubner et al, 2016)
Summary
In stick insects (Phasmatodea), paired exocrine defense glands are found in the prothorax (Bedford, 1978; Chow, 2008; Bradler, 2009). The defense secretions have been studied for the biochemical components in detail in different species of stick insects (e.g., Meinwald et al, 1962; Smith et al, 1979; Chow and Lin, 1986; Ho and Chow, 1993; Bouchard et al, 1997; Eisner et al, 1997; Dossey et al, 2006, 2008; Schmeda-Hirschmann, 2006; summarized in Dettner, 2015). In Anisomorpha species, the principle component of the defense spray is anisomorphal, a monoterpene dialdehyde (Meinwald et al, 1962) which occurs in three diastereomers in Anisomorpha buprestoides (Dossey et al, 2006)
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