Studies of cardio-regulation in the cockroach, Periplaneta americana.

The circulation of haemolymph in insects is maintained by a system of muscular pumps and fibromuscular septa. The most important muscular pump is the dorsal vessel which collects blood from the abdominal cavity and discharges it into the head. In cockroaches the dorsal vessel is a straight tube comprising the 'heart' in the abdominal and thoracic segments and narrowing to form the aorta in the head. In the American cockroach, Periplaneta americana, there are three heart chambers in the thorax and nine chambers in the abdomen. A heart chamber is denned as the region of the dorsal vessel between two pairs of ostial valves. Four of the chambers in the middle of the abdomen give off paired segmental vessels which are innervated and therefore possibly muscular. By means of the segmental vessels some of the intracardial haemolymph is distributed laterally in the abdomen. The walls of the heart consist of a single layer of multinucleate striated muscle cells. The muscle fibres are about 3 jim x 40 /tm x 200 /tin, and are arranged in a semicircular fashion. The fibres are often connected by intercalated disks similar to those found in vertebrate heart muscle (Edwards & Challice, i960) and in moth heart muscle (Sanger & McCann, 1968). References to the detailed anatomy of Periplaneta heart can be found in the comprehensive reviews by Jones (1964) and McCann (1970). Especially pertinent works cited in these reviews are those of Alexandrowicz (1926), Mclndoo (1939, 1945) and Johnson (1966). The cardiac nervous system of P. americana consists of paired lateral cardiac nerve cords running along the entire length of the dorsal vessel, one on either side, and the processes from paired segmental nerves which originate lateroposteriorly at each central ganglion. The paired segmental nerves join the respective right and left lateral cardiac nerve cords opposite the ostial valves in each heart chamber. Each lateral cardiac nerve cord contains nerve cells which are usually bipolar and are of two types. Ganglion cells, which are probably motor in function, send axons to the myocardium and also send processes anteriorly and posteriorly along the lateral cardiac nerve cord. The second type of nerve cell is probably neurosecretory. It also sends processes along the lateral cardiac nerve cords, but as yet there is no evidence that these cells innervate the myocardium (Miller, 1968). Mclndoo (1945) reported that the lateral cardiac nerve cords in the American cockroach contain about 32 nerve cells. It is not clear as yet how many of these neurones