Neurons of self-defence: neuronal innervation of the exocrine defence glands in stick insects

High disparity in repellent gland anatomy across major lineages of stick and leaf insects (Insecta: Phasmatodea)

BACKGROUND

Phasmatodea are well known for their ability to disguise themselves by mimicking twigs, leaves, or bark, and are therefore commonly referred to as stick and leaf insects. In addition to this and other defensive strategies, many phasmatodean species use paired prothoracic repellent glands to release defensive chemicals when disturbed by predators or parasites. These glands are considered as an autapomorphic trait of the Phasmatodea. However, detailed knowledge of the gland anatomy and chemical compounds is scarce and only a few species were studied until now. We investigated the repellent glands for a global sampling of stick and leaf insects that represents all major phasmatodean lineages morphologically via µCT scans and analyzed the anatomical traits in a phylogenetic context.

RESULTS

All twelve investigated species possess prothoracic repellent glands that we classify into four distinct gland types. 1: lobe-like glands, 2: sac-like glands without ejaculatory duct, 3: sac-like glands with ejaculatory duct and 4: tube-like glands. Lobe-like glands are exclusively present in Timema, sac-like glands without ejaculatory duct are only found in Orthomeria, whereas the other two types are distributed across all other taxa (= Neophasmatodea). The relative size differences of these glands vary significantly between species, with some glands not exceeding in length the anterior quarter of the prothorax, and other glands extending to the end of the metathorax.

CONCLUSIONS

We could not detect any strong correlation between aposematic or cryptic coloration of the examined phasmatodeans and gland type or size. We hypothesize that a comparatively small gland was present in the last common ancestor of Phasmatodea and Euphasmatodea, and that the gland volume increased independently in subordinate lineages of the Occidophasmata and Oriophasmata. Alternatively, the stem species of Neophasmatodea already developed large glands that were reduced in size several times independently. In any case, our results indicate a convergent evolution of the gland types, which was probably closely linked to properties of the chemical components and different predator selection pressures. Our study is the first showing the great anatomical variability of repellent glands in stick and leaf insects.

The neuronal innervation pattern of the subgenual organ complex in Peruphasma schultei (Insecta: Phasmatodea)

The proximal tibia of orthopteroid insects contains sensory organs, the subgenual organ complex, detecting mechanical stimuli including substrate vibration. In stick insects, two chordotonal organs occur in close proximity, the subgenual organ and the distal organ, which likely detect substrate vibrations. In most stick insects, both organs are innervated by separate nerve branches. To obtain more data on the neuroanatomy of the subgenual organ complex from the New World phasmids (Occidophasmata), the present study documents the neuronal innervation of sensory organs in the subgenual organ complex of Peruphasma schultei, the first species from Pseudophasmatinae investigated for this sensory complex. The innervation pattern shows a distinct nerve branch for the subgenual organ and for the distal organ in most cases. Some variability in the innervation, which generally occurs for these chordotonal organs, was noted for both organs in P. schultei. The most common innervation for both organs was by a single nerve branch for each organ. The innervation of the subgenual organ resembled the nerve pattern of another New World phasmid, but was simpler than in the Old World phasmids (Oriophasmata) studied so far. Therefore, the peripheral neuronal innervation of sensory organs could reflect phylogenetic relationships and provide phylogenetic information, while the overall neuroanatomy of the subgenual organ complex is similar in stick insects.

Independent suboesophageal neuronal innervation of the defense gland and longitudinal muscles in the stick insect (Peruphasma schultei) prothorax

This study investigates the neuroanatomy of the defense gland and a related muscle in the stick insect Peruphasma schultei with axonal tracing and histological sections. The gland is innervated by three neurons through the Nervus anterior of the suboesophageal ganglion (SOG), the ipsilateral neuron (ILN), the contralateral neuron (CLN) and the prothoracic intersegmental neuron (PIN). The ILN has a large soma which is typical for motoneurons that cause fast contraction of large muscles and its dendrites are located in motor-sensory and sensory neuropile areas of the SOG. The CLN might be involved in the coordination of bilateral or unilateral discharge as its neurites are closely associated to the ILN of the contralateral gland. Close to the ejaculatory duct of the gland lies a dorsal longitudinal neck muscle, musculus pronoto-occipitalis (Idlm2), which is likely indirectly involved in gland discharge by controlling neck movements and, therefore, the direction of discharge. This muscle is innervated by three ventral median neurons (VMN). Thus, three neuron types (ILN, CLN, and PIN) innervate the gland muscle directly, and the VMNs could aid secretion indirectly. The cytoanatomy of motorneurons innervating the defense gland and neck muscle are discussed regarding the structure and functions of the neuropile in the SOG. As a basis for the neuroanatomical study on the defense gland we assembled a map of the SOG in Phasmatodea.