The Circulatory System

rhythmically. These observations suggest that the heartbeat is normally coordinated by a 521

pacemaker located posteriorly. In adult Manduca sexta in which heartbeat reversal occurs, distinct pacemakers exist for the anteriorly and posteriorly directed contractions (Dulcis etal., 2001).

Whether or not an insect heart is innervated, its beat is myogenic, that is, the beat originates in the heart muscle itself (Jones, 1977; Miller, 1985, 1997). This contrasts with the situation in Crustacea and Arachnida, which have neurogenic hearts. For innervated insect hearts, it is generally assumed that, as in vertebrates, control of the rate and amplitude of the heartbeat resides in the cardiac neurons. However, as Miller (1985) pointed out, such regulation has been demonstrated in only a few cases.

The rate at which the heart beats varies widely both among species and even within an individual under different conditions. In the pupa of Anagasta kuhniella, for example, the heart beats 6-11 times per minute. In larval Blattella germanica rates of 180-310 beats/min have been recorded (Jones, 1974). Many factors affect the rate ofheartbeat. Generally, there is a decline in heartbeat rate in successive juvenile stages, and in the pupal stage the heart beats slowly or even ceases to beat for long periods. In adults the heart beats at about the rate observed in the final larval stage. Heartbeat rate increases with activity, during feeding, with increase in temperature or in the presence of carbon dioxide in low concentration, but is depressed in starved or asphyxiated insects. Hormones, too, may affect heartbeat rate. Authors have reported a wide range of cardioaccelerating and cardioinhibiting factors, in-cludingjuvenile hormone, neurosecretory peptides, octopamine, and 5-hydroxytryptamine. However, in many instances, an effect of these substances on the metabolism of the insect may cause the change in heartbeat rate observed.

As noted, the ventral diaphragm and accessory pulsatile organs may or may not be innervated. Thus, it may be anticipated that, as with the dorsal vessel, contraction of these structures may be controlled neurally and hormonally or by hormones alone. Pharmacological studies have shown that a range of amines and small peptides can modulate contraction of these structures. However, immunohistochemistry has identified amine- and peptide-releasing neurons terminating at these structures, tending to cloud the picture with respect to which system is regulating their activity (Hertel and Pass, 2002; Koladich et al., 2002).

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

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