“There are: (1) a transparent, structureless intima, only visible when thrown into folds; (2) a partial endocardium, of scattered, nucleated cells, which passes into the interventricular valves; (3) a muscular layer, consisting of close-set, annular, and distant, longitudinal fibres. The annular muscles are slightly interrupted at regular and frequent intervals, and are imperfectly joined along the middle line above and below, so as to indicate (what has been independently proved) that the heart arises as two half-tubes, which afterwards join along the middle. Elongate nuclei are to be seen here and there among the muscles. The adventitia (4), or connective tissue layer, is but slightly developed in the adult cockroach.”
Fig. 370.—Part of the heart of Lucanus cervus: a, the posterior chambers (the anterior ones are covered by a part of the ligaments which hold the heart in place); i, auriculo-ventricular openings; g, g, the lateral muscles fixed by the prolongations h, h, to the upper side of the abdomen.—After Straus-Dürckheim.
Graber says that the heart of insects may be regarded not as an organ de novo, but only as the somewhat modified contractile dorsal vessel of the annelids, in which, however, the transverse arteries arising on each side became, with the gradual development of the tracheæ, superfluous and finally abortive. He describes it as a muscular tube composed of very delicate annular fibres, which within and without is covered by a relatively homogeneous, strong, elastic membrane.
The division into separate chambers is effected by means of a folding inwards and forwards of the entire muscular wall. “A portion of each side of the heart is first extended inwards so as very nearly to meet a corresponding portion from the opposite side, and then, being reflected backwards, forms, according to Straus (Consid., etc., p. 356), the interventricular valve which separates each chamber from that which follows it. Posteriorly to this valve, at the anterior part of each chamber, is a transverse opening or slit (Fig. 371, b), the auriculo-ventricular orifice, through which the blood passes into each chamber, and immediately behind it is a second, but much smaller, semilunar valve (c), which, like the first, is directed forwards into the chamber. It is between these two valves on each side that the blood passes into the heart, and is prevented from returning by the closing of the semilunar valve. When the blood is passing into the chamber, the interventricular valve is thrown back against the side of the cavity, but is closed when, by the contraction of the transverse fibres, the diameter of each chamber is narrowed, and the blood is forced along into the next chamber.” (Newport.)
Fig. 371.—A, heart of Lucanus cervus: a, valves or chambers; bb, alary muscles; c, supposed auricular space around the heart. B, division into arteries of the end of the aorta in larva of Vanessa urticæ. C, interior of the chamber, showing the transverse fibres; b, auriculo-ventricular opening and valve into the chambers; c, semilunar valve; d, interventricular valve.—After Straus-Dürckheim, from Newport.
Fig. 372.—Heart of Belostoma.—After Locy.
According to Müller, there is but a single pair of ostia in Phasma, and, in the larva of Corethra, the heart is a simple, unjointed tube, not divided into chambers, and Viallanes states that, in the very young larva of Musca, there are no ostia (Kolbe). In the larva of Ptychoptera, Grobben found a short oval heart, with one pair of ostia situated in the 6th abdominal segment; a long aorta proceeds from it, the thoracic portion of which pulsates; from behind the heart arises a pulsating pouch, which connects with the hinder aorta, which does not pulsate, and ends at the base of two tracheal gills. Burmeister was able to find only four pairs of openings in the larva of Calosoma. Newport states that, while Straus figures nine chambers in Melolontha, and, consequently, eight pairs of openings, he has not been able to observe more than seven pairs of openings in Lucanus cervus. He has invariably found eight pairs of openings both in the larva and imago of Sphinx ligustri, as well as in other Lepidoptera. According to Béla-Dezso, the number of pairs of ostia corresponds to that of the pairs of stigmata.