Extracardiac haemocoelic pulsations and the autonomic neuroendocrine system (coelopulse) of terrestrial insects

Abstract

Terrestrial insects exhibit extracardiac pulsations (ExP) in haemocoelic pressure, similar in some respect to the human blood pressure pulse. The pulsations are produced by large intersegmental abdominal musculature (abdominal pressure pump). The dorsal vessel of insects is a relatively weak organ which is unable to pump haemolymph against an increased gradient of pressure. The weak cardiac pulsations (myogenic nature) and strong ExP (neurogenic nature) occasionally occur hand in hand during similar periods with similar, but not identical frequencies. This increases the possibility of their mutual confusion. ExP can be recorded directly from haemocoelic cavity by means of hydraulic transducers or, indirectly from the body surface by recording movements of some flexible segments. In most cases, we recorded pulsations in haemocoelic pressure indirectly by recording movements of the terminal abdominal segments in immobile pupal stages. The movements caused by ExP are generally very small and invisible, only in the μm range. However, the corresponding abdominal movements or changes in haemocoelic pressure associated with the heartbeat are 30- to 500-fold smaller, in the range of nanometers. During the past three decades we have recorded cardiac and extracardiac pulsations in haemocoelic pressure in a number of insects and ticks. Practical examples of extracardiac pulsation patterns and their distinction from the heartbeat is described here for all major groups of terrestrial insects. The results obtained with monitoring of haemocoelic pulsations have revealed that terrestrial insects and possibly other arthropods posses a brain-independent, neuroendocrine system, called coelopulse. This type of newly discovered, autonomic, cholinergic system of insects shows apparent structural and functional analogy with the parasympathetic system of vertebrate animals. It regulates a number of homeostatic physiological and developmental functions, using pulsations in haemocoelic pressure for controlling circulatory and respiratory functions. The regulatory nervous center of the coelopulse system is located within thoracic ganglia of the ventral nerve cord (in analogy with parasympathetic centers in the spinal cord). Nerve impulses are dispatched from neurons of the thoracic ganglia through connectives and abdominal ganglia into large intersegmental abdominal muscles, whose contractions cause large peaks in haemocoelic pressure. The described coelopulse system controls a number of important physiological functions. For instance: 1) ExP in haemocoelic pressure cause rapid circulatory inflow and outflow of haemolymph between thoracic and abdominal parts of the body; 2) The relatively strong pressure changes caused by ExP can vigorously move tissue and organs against each other, thus preventing occlusion of haemolymph among densely packed organs; 3) Large extracardiac peaks in haemocoelic pressure open or close passively, one-way valves or tissue fold and promote circulation of haemolymph to destinations that cannot be reached by the heartbeat, i.e. ventral perineural sinus, appendages; 4) Strong ExP in haemocoelic pressure produce rhythmic, up and down compressions of tracheal tubes and air sacs, resulting in actively regulated inspirations or expirations of air through individual spiracles, i.e. actual insect breathing; 5) ExP controlled by the coelopulse neuroendocrine system causes unidirectional ventilation of the determined spiracles during emergency hypoxia, or during enzymatically produced outbursts of CO2; 6) The coelopulse system effectively controls various homeostatic physiological functions, like respiratory water loss, water retention, isoosmosis, optimum body volume, or economic gaseous exchange; 7) ExP in haemocoelic pressure plays an important roles in execution of special developmental events, like ecdysis, oviposition or pupariation. I am convinced that knowledge of the autonomic, parasympathetic-like neuroendocrine system in terrestrial arthropods may open new avenues for comparative animal physiology and pharmacology.