"Nerve" - what is it, definition of the term
A neural conduit, consisting of bundled axons surrounded by supportive glial tissue, carries electrical signals between sensory receptors and effectors, serving as the principal communication pathway in the nervous systems of arthropods such as ticks, bugs, lice, and fleas; it encloses myelinated or unmyelinated fibers within a protective sheath and integrates sensory input to coordinate locomotion, feeding, and physiological regulation.
Detailed information
The peripheral neural network of arthropods consists of a ventral longitudinal cord composed of serially arranged ganglia, each containing clusters of neuronal cell bodies and associated axonal tracts. Sensory input is processed by paired supra‑esophageal and sub‑esophageal ganglia, while motor output is coordinated through thoracic and abdominal ganglia. Neurotransmission relies on acetylcholine, glutamate and biogenic amines, with synaptic ultrastructure comparable to that of other invertebrates.
In ticks (order Acari), the ventral cord is relatively short, terminating near the posterior opisthosoma. Two fused supra‑esophageal ganglia form a compact brain that integrates chemosensory information from Haller’s organ. Four thoracic ganglia control leg movement, and a series of abdominal ganglia regulate feeding apparatuses. Axonal projections from the brain extend to the salivary glands, enabling rapid secretion during blood ingestion.
True bugs (order Hemiptera) possess a more elongated cord with distinct thoracic ganglia corresponding to each segment of the thorax. The brain includes a protocerebrum, deutocerebrum and tritocerebrum, each receiving input from compound eyes, antennae and mechanoreceptors. Sub‑esophageal ganglia coordinate proboscis extension, while paired thoracic ganglia manage wing articulation and leg locomotion. The abdominal ganglia are reduced but retain control over reproductive organs.
Lice (order Phthiraptera) display a highly condensed neural system. The brain and sub‑esophageal ganglion are fused into a single mass situated beneath the head capsule. Only three thoracic ganglia exist, each innervating a pair of legs. The ventral cord terminates early, reflecting the parasite’s limited mobility. Sensory neurons are concentrated in the antennae and leg setae, providing rapid response to host cues.
Fleas (order Siphonaptera) retain a compact ventral cord with four thoracic ganglia and a reduced abdominal segment. The brain is small, with prominent optic lobes despite the insect’s reduced vision. Sensory neurons in the antennae detect host vibrations and temperature gradients. Motor neurons in the thoracic ganglia drive powerful jumping muscles, a hallmark of the order.
Comparative points
- Length of ventral cord: ticks (short) → bugs (elongated) → lice (condensed) → fleas (moderate).
- Brain complexity: bugs (tripartite) > ticks (fused) > fleas (small) > lice (single mass).
- Number of thoracic ganglia: bugs (four) > fleas (four) > ticks (four) > lice (three).
- Sensory specializations: ticks (Haller’s organ), bugs (compound eyes), lice (antenna setae), fleas (vibration receptors).
These structural variations reflect adaptations to distinct ecological niches, ranging from hematophagy in ticks to rapid locomotion in fleas. The conserved ventral cord architecture underlies coordinated behavior across all four groups.