HARBWICKE'S SCIENCE-GOSSIP. 



;o9 



tracheal gills, either with respect to their number or 

 position ; both may occur in the same segment ; and 

 the tracheal gills themselves are not everywhere 

 homodynamous or equivalent, ' but have merely a 

 physiological correspondence. 



A very long chapter might be written upon the 

 views advanced by different writers as to the 

 circulation of insects. Malpighi first discovered the 

 heart or dorsal vessel in the young silkworm, and 

 observed its progressive contraction ; he regarded it 

 as a large pulsating vein.* Swammerdam thought 

 that his injections ascertained the existence of lateral 

 vessels, but this proved to be a mistake. Lyonnet 

 added many details of interest to what was previously 

 known, but came to the conclusion that there was 

 no system of vessels connected with the heart, and 

 even doubted whether the organ so named was in 

 effect a heart at all. Marcel de Serres maintained 

 that it was merely the secreting organ of the fat- 

 body. Cuvier and Dufour doubted whether any 

 circulation, except of air, existed in insects. This 

 was the extreme point of scepticism, and naturalists 

 were drawn back from it by Herold,t who repeated 

 and confirmed the views held by the seventeenth 

 century anatomists, and insisted upon the demon- 

 strable fact that the dorsal vessel of an insect does 

 actually pulsate and impel a current of fluid. Carus, 

 in 1S26, saw the blood flowing in definite channels in 

 the wings, antennre, and legs. Strauss-Durckheim 

 followed up this discovery by demonstrating the 

 contractile and valvular structures of the dorsal 

 vessel. Blanchard affirmed that a complex system 

 of vessels accompanied the air-tubes throughout the 

 body, occupying peritracheal spaces supposed to 

 exist between the inner and outer walls of the 

 trachete. This peritracheal circulation has not 

 withstood critical inquiry, and it might be pro- 

 nounced wholly imaginary, except for the fact that 

 air-tubes and nerves are found here and there within 

 the veins of the wings of insects. 



The insect heart is a chambered tube occupying 

 the middle line of the dorsal surface of the abdomen. 

 The chambers may correspond, as in the cockroach, 

 to the eight foremost abdominal segments, or they 

 may be fewer ; they increase regularly in length and 

 width as they approach the thorax. From the 

 anterior chamber a simple tube, the aorta, passes 

 forwards to the head. If the abdomen is marked off 

 by a decided constriction, the aorta is bent down at 

 its commencement. In the thorax it lies above the 

 oesophagus and crop, sunk among the muscles of the 

 dorsal wall. The fore end of the aorta is usually a 

 trumpet-shaped orifice, from which distinct vessels 

 have only in few and doubtful cases been seen to 

 proceed ; it is possible that the blood escapes freely 

 from the aorta, but the course of the circulation has 



* Dissert, de Bombyce, i66g. 



t Schrift. d. Marburg. Naturf. Gesellschaft, 1823. 



not been distinctly and continuously traced beyond 

 this point. A sinus resembling a pericardium, but 

 containing blood, can be plainly seen to surround 

 the heart in the cockroach and other large insects. 

 Other sinuses or lacunar occupy the interspaces 

 of the viscera, and one large ventral sinus 

 envelopes the nerve-cord in some insects. The wall 

 of the heart is muscular, and the fibres are wound 

 about the tube in opposite spirals. A pair of lateral 

 valvular inlets lead from the pericardial sinus into the 

 hinder end of each cardiac chamber, and every 

 chamber communicates with the next in front by 

 another valvular opening. In the living insect a 

 wave of contraction passes rapidly along the heart 

 from behind forwards ; and the blood may under 

 favourable circumstances be seen to flow in a steady, 

 backward stream along the pericardial sinus, to enter 

 the lateral aperture of the heart. The peristaltic 

 movement of the dorsal vessel may often be observed 

 to set in at the hinder end of the tube before the 

 preceding wave has reached the aorta. In white 

 cockroaches which had just moulted the pulsations 

 were counted by Cornelius to eighty per minute. 



Lyonnet, in his famous memoir on the larva of the 

 goat moth, describes what he terms the wings of the 

 heart — paired lateral muscles, which radiate from 

 opposite points of the dorsal integument, and unite by 

 their broad bases beneath the heart, so as to form 

 transversely elongate lozenges. These alary muscles 

 form thin sheets of loosely connected fibres, the 

 interspaces being occupied by granular matter. It 

 has often been explained that the muscles by their 

 contraction dilate the heart, but this cannot be true. 

 A pull from opposite sides upon a flexible, cylindrical 

 tube would narrow, and not expand its cavity; 

 moreover, the muscles are not inserted directly into 

 the wall of the heart, though a muscle passes upwards 

 from their junction to the ventral surface of that 

 organ. They form a transverse diaphragm which, 

 whenever it contracts, depresses the heart and 

 enlarges the pericardial space. The same action 

 compresses the abdominal viscera, and* may be 

 supposed to force out the blood from the surrounding 

 lacuna;, impelling it towards the pericardial sinus. 

 It should be explained that direct observation of this 

 and similar points is nearly impossible in large and 

 opaque insects. We are therefore largely dependent 

 upon inference from minute structure, and upon the 

 microscopic examination of transparent aquatic larva;. 



No satisfactory injections of the vessels of insects 

 have been made ; and the large lacunse, or cavities 

 without proper wall, which lie in the course of the 

 circulation, render complete injections impracticable. 

 Reticulated blood-vessels can however be traced in 

 the wings and other transparent organs. The 

 course of the blood is in general forwards along the 

 anterior, backwards along the posterior, side of the 

 appendage. The direction of the current is not 

 absolutely constant in all the branches, but the same 



