The principal organ of hemolymph propulsion is the dorsal vessel, or at least it is the most visible organ associated with hemolymph movement. By tradition, the abdominal portion of the dorsal vessel is called the "heart" and the thoracic and cephalic (head) portion the "aorta." Both terms are borrowed from better-known vertebrate structures and give an inaccurate impression of the roles of those structures.
The dorsal vessel, especially in the abdomen of insects, is suspended in the pericardial sinus, which is delimited by the dorsal cuticle and the dorsal diaphragm (when present). Contractions of the dorsal vessel operate against the pull of lateral connective tissues, which are responsible for dilating or opening the vessel (in diastole) following each contractile stroke (called systole or systolic contraction). In most larvae of holometabolous insects (insects that possess a pupal stage) and in most primitive insects, the dorsal vessel is a simple tube running from the rear of the abdomen to the head, where it becomes closely associated with the top of the foregut and then travels under and opens just underneath or in front of the brain. This arrangement ensures a constant supply of nutrients and removal of waste products to and from the brain mass. In addition, the dorsal vessel is often intimately associated with the retrocerebral nervous system (including the hypocerebral ganglion, corpora cardiaca, and corpora allata complex) just behind the brain, which may deliver neurohormones and possibly other hormones into the aorta by way of specialized release sites.
The dorsal vessel has openings called ostia along the sides and ventral surfaces of each segment of the abdominal heart. The most common ostia allow the flow of hemolymph into the dorsal vessel and contain valves to prevent backflow. These are called incurrent ostia. Some insects have openings without valves through which the hemolymph moves constantly; these are called excurrent ostia and are common, for example, on the ventral side of the grasshopper heart, which also has a full complement of paired incurrent ostia associated with each segment in the abdomen.
Occasionally, insects have structures that branch out from the dorsal vessel. In the American cockroach (Periplaneta americana) and some other orthopteroid insects (e.g., crickets and mantids) there are paired segmental vessels diverging from the heart laterally. In the cockroach, these vessels are simple sacs of connective tissue and have no inherent musculature, thus providing a simple channel to the lateral aspects of the pericardial sinus in the middle segments of the abdomen. These specialized vessels ensure lateral perfusion of the pericardial sinus in moderate to large insects. Lateral tubes and vessels are not known in small insects.
The dorsal vessel is composed of muscle cells (collectively called the myocardium) that lie sometimes as opposed pairs and sometimes as spiral bands to form the cylinder of the dorsal vessel. The myocardium in all insects is spontaneously active, usually beginning in the embryonic stages. This type of heart is termed myogenic because the electrical activity underlying contractions arises in the myocardium itself. This is in contrast to a neurogenic heart present in, for example, crustaceans such as crabs and lobsters, in which a barrage of nervous impulses drives the heartbeat from a discrete cardiac ganglion center.
In the pupae (and sometimes in resting adults) of holometabolous insects, the heartbeat exhibits reversal during which peristaltic contractile waves first push hemolymph from back to front (anterograde peristalsis) then at other times exclusively from front to back (retrograde peristalsis). Because heartbeat reversal is characteristic of even highly mobile mosquito pupae, reversal of hemolymph flow is thought to be an adaptation to an insect body that is rigid (the front end of a mosquito pupa is a rigid structure in which internal tissue and organs are undergoing drastic changes in shape to form adult structures, including the wings).
Nervous stimulation or mechanical disturbance causes the anterograde pulsations to revert to retrograde peristalsis. Because substances that block nerve impulses can cause the anterograde peristalsis to disappear, nervous signals (possibly inhibitory signals) are assumed to be responsible for alternating between the two peristalsis conditions, with retrograde being the basic condition.
Until recently, little was known about the innervation and control of heartbeat activity. Although the basal heartbeat rate of most insects is around 60 beats min-1 at room temperature and at rest (American cockroach and the locust, Locusta migratoria), the heartbeat of adult house flies (Musca domestical) is extremely unusual in that it fluctuates seemingly at random from over 300 beats min-1 to zero regardless of activity of the insect, flying or at rest.
The central nervous system of the adult house fly is composed of the brain and a thoracic ganglion mass. No ganglia are present in the abdomen. Because of this unusual anatomy, the dorsal vessel in the abdomen can be separated from all innervation from the central nervous system simply by cutting between the thorax and abdomen. After this operation, the heartbeat of the fly becomes quite regular at around 60 beats min-1. This indicates that the house fly heart is innervated by both inhibitory and excitatory motor neurons from the central nervous system. Recently Ruthann
Nichols demonstrated inhibition caused by one or more neuropeptides in the fruit fly, Drosophila melanogaster.
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