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520 and perineural sinuses. Undulations of the ventral diaphragm aid the backward flow of hemolymph. Relaxation of the heart muscle results in an increase in heart volume, and, by negative pressure, hemolymph is sucked in via incurrent ostia. As noted earlier, circulation through appendages is aided by accessory pulsatile organs. In most insects hemolymph enters the wings via the anterior veins and returns to the thorax via the anal veins. Though the structure of wing pulsatile organs is varied, they always operate by sucking hemolymph out of the posterior wing veins (Pass, 1998, 2000). In some Coleoptera and Lepidoptera, tidal flow of hemolymph occurs in the wings; that is, hemolymph flows into or out of all veins simultaneously.

In apterygotes and mayflies hemolymph flow is bidirectional (Figure 17.1B). Anterior to a valve located in the heart at about the level of the eighth abdominal segment, hemolymph flows forward toward the head, while behind the valve the hemolymph is pushed backward along arteries that terminate at the tips of the cerci and median filament (Gereben-Krenn and Pass, 2000). Reversal of heartbeat may also occur and is characteristically seen in pupae and adults of Lepidoptera and Diptera.

In some actively flying insects, for example, locusts, butterflies, saturniid moths, and possibly some Hymenoptera, as well as in diapausing lepidopteran pupae, hemolymph movements are closely coordinated with the ventilation movements for gas exchange (Chapter 15, Sections 3.2 and 3.3). Abdominal pumping not only improves gas exchange within the tracheal system but also brings about tidal flow (oscillating circulation) of hemolymph. In other words, hemolymph flows back and forth between the abdomen and the anterior part of the body. Tidal flow of hemolymph into the abdomen is aided by reverse peristalsis of the dorsal vessel (Miller, 1997).

Control of circulation is especially important in large flying insects such as bumble bees, dragonflies, and night-flying moths that thermoregulate. Thermoregulation allows these insects to warm up their wing musculature at low ambient temperatures and to dissipate heat produced during flight at high temperatures. At low ambient temperatures, the heartbeat is weak and contraction of the ventral diaphragm infrequent, so that heat produced by pre-flight contraction of wing muscles is retained within the thorax. As the thoracic temperature becomes suitable for flight, heartbeat rate and amplitude increase, as do the frequency and strength of contractions of the ventral diaphragm, taking heat away from the thorax to prevent overheating (Miller, 1997).

3.2. Heartbeat

Contraction of the heart (systole) is followed, as in other animals, by a phase of relaxation (diastole) during which muscle cell membranes become repolarized. A third phase, diastasis, may follow diastole, when the diameter of the dorsal vessel suddenly enlarges because of the influx of hemolymph. Diastole in many insects seems to be passive, that is, the result of natural elasticity of the heart muscle. Though alary muscles may be quite well developed in such species, they apparently have no role in the relaxation process. They have been shown to be electrically inexcitable in locusts and cockroaches, and cutting them has no effect on the rate and strength of the heartbeat. In a few species structural integrity of the heart and alary muscles is vital, and cutting the alary muscles terminates the heartbeat.

In most pterygotes, where hemolymph flow is unidirectional, contraction of the dorsal vessel begins at the posterior end and passes forward as a peristaltic wave. Experimentally contraction can be induced at any point along the length of the vessel and individual semiisolated segments (portions of the heart with tergum still attached) continue to beat

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