Ventral Diaphragm

The ventral diaphragm plays a prominent role in perfusing the ventral nerve cord of insects (Fig. 1). Nearly 40 years ago Glenn Richards surveyed the ventral diaphragms in insects and found that insects with a well-defined ventral nerve cord in the abdomen also had a well-developed ventral diaphragm. In contrast, insects with the ventral nerve cord condensed into a complex ganglion structure in the thorax invariably lacked a ventral diaphragm. This correlation suggests that the role of the ventral diaphragm is inexorably tied to perfusion of the ventral nerve cord in the abdomen.

The thorax of most insects is so packed with muscles involved in locomotion that other tissues are greatly reduced. Thus, the foregut is a simple tube passing through a small opening in the middle of the thorax and a well-defined ventral diaphragm (if present) is reduced. When present, the ventral diaphragm loosely defines a perineural sinus below and the perivisceral sinus above containing the gut.

In some insects, the ventral diaphragm is a strong muscular structure with a great deal of contractile activity. The activity of the ventral diaphragm is dictated by innervation from the central nervous system. In some large flying insects, the ventral diaphragm assists in hemolymph flow during thermoregulation by facilitating the removal of warm hemolymph from the hot thoracic muscles to the abdomen for cooling.

The intimate association between the ventral diaphragm in insects and perfusion of the ventral nerve cord is strengthened by considering the structure in the American and Madeira (Leucophaea maderae) cockroaches that takes the place of a proper diaphragm. In these two insects, four strips of muscles are attached at the back of the thorax and inserted on the ninth sternite. This structure has been called the hyperneural muscle because it does not form a true diaphragm above the ventral nerve cord and therefore is given a distinctive name. The hyperneural muscle is attached near the back of each of the abdominal ganglia, and the muscles contract slowly but not in a rhythmic order.

The hyperneural muscles are electrically inexcitable, which means that they do not contract myogenically, as the myocardium does, but instead are neurally driven by motor neurons located in the ventral ganglia. Thus each of the ventral nerve cords in these two cockroach species (P. americana and L. maderae) has its own muscle supply that pulls the ganglia back and forth along the midline of the abdomen upon demand. This entire structure is designed to increase the mixing and contact between the ganglia and the hemolymph.

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