976 



! .ng backwards from the head and thorax 



and laterally from the sides of the abdomen. 



We have observed a similar space in many 



insects, particularly in Asilus crabroniformis 



(fig. 434, D), and also in Bornbus terrestris. 



Fig. 434. 



One valve of the heart of Asiltts crabroniformis. 

 a, the chamber; b, the lateral muscles; c, the 

 auricular space ; the arrows denote the course of 

 the blood. 



In this insect we have observed the fibres of 

 the heart crossing each other in an oblique di- 

 rection, forming as it were a series of festoons 

 around the posterior part of each chamber. 

 These, like the transverse fibres observed in 

 Melolontha by Straus, contract the diameter 

 of each chamber, and extend the vessel. Be- 

 sides the proper muscular structure of the heart 

 itself, there are attached on each side of the 

 organ several sets of muscular fibres, arranged 

 in pairs along the upper and under surface of 

 each chamber. Each set of these fibres, con- 

 verging to a tendon, and passing outwards, 

 forms a triangular muscle (A, b, 6), which 

 is attached to the lateral surface of each seg- 

 ment. These, which have been called the 

 wings of the heart, assist by their contraction 

 to shorten and expand the chambers at the 

 auricular, or receiving period of the heart's mo- 

 tions, while, as just explained, the transverse 

 and diagonal muscles occasion the ventricular, 

 by their contracting and narrowing the diameter 

 of each chamber. It is between the upper and 

 under set of the lateral muscles that we believe 

 'the auricular space to exist, bounded by a de- 

 licate membrane. The thoracic or aortal por- 

 tion of the heart commences at the anterior 

 part of the first abdominal segment, where the 

 organ bends downwards to pass under the 

 metaphragma, and enter the thorax. When it 

 has entered that region it immediately ascends 

 again between the great longitudinal dorsal 

 muscles of the wings, and passes onwards 

 until it arrives at the posterior margin of the 

 pronotum; it then again descends and con- 

 tinues its course along the upper surface of the 

 oesophagus, with which it passes beneath the 

 cerebrum, anterior to which, and immediately 

 above the pharynx, it is bifurcated and divided 

 into several branches, as formerly noticed by us 

 in the sphinx.* Previously to our notice, how- 



Phil. Trans. 1832, p. 



385. 



INSECTA. 



ever, Carus had seen the course of the blood 

 in the head of insects following directions cor- 

 responding to the situations in which we have 

 been able to trace a distinct division of the aorta 

 into vessels. We have found a similar division 

 of the aorta into branches in several species of 

 Coleopterous insects, as in Melue, Blctps, and 

 Timarc/ia, although we have omitted to trace 

 it in Lucanus, In the Sphinx and Vanessa 

 urtictf, immediately after the aorta has passed 

 beneath the cerebrum it gives off laterally two 

 large trunks, which are each equal in capacity to 

 about one-third of the main vessel. These pass 

 one on each side of the head, and are divided 

 into three branches, which are directed back- 

 wards, but have not been traced farther in conse- 

 quence of their extreme delicacy. Anterior to 

 these trunks are two smaller ones, which appear 

 to be given to the parts of the mouth and an- 

 tennae, and nearer the median line are two others, 

 which are the continuations of the aorta. These 

 pass upwards and are lost in the integuments. 

 The whole of these parts are so exceedingly 

 delicate that we have not as yet been able to 

 follow them beyond their origin at the termina- 

 tion of the aorta, but believe them to be con- 

 tinuous with very delicate circulatory passages 

 along the course of the tracheal vessels. It is 

 in the head alone that the aorta is divided into 

 branches, since throughout its whole course 

 from the abdomen it is one continuous vessel, 

 neither giving off branches nor possessing la- 

 teral muscles, auricular orifices, or separate 

 chambers. In the larva state it is far more 

 difficult to recognise the true structure of the 

 vessel by actual dissection than in the perfect, 

 because the valves are only in a rudimentary 

 condition. But it is easy to observe it in the 

 bodies of living transparent specimens, as done 

 by Carus, Wagner, Bowerbank, and others in 

 the Ephemeridtf and Agrionida, in which not 

 only the form of the valves and motions of the 

 vessel are distinct, but also the abundance of 

 globules that circulate in every direction. Even 

 in some of the opaque-bodied maggots of Dip- 

 tera we have seen the form of the valves very 

 distinctly through the tegument in the eight 

 posterior segments. When viewed in that state 

 each chamber appears to be much narrower at 

 its anterior and posterior extremity than in its 

 middle (Jig. 358, D), and the valves formed 

 by the reflexion of its parietes inwards, although 

 distinct, are very small. Near the middle of 

 each chamber there is attached on each side a 

 narrow muscle, which passing backwards is 

 attached to the anterior margin of each seg- 

 ment. Between the muscle and the heart in 

 each segment, a large tracheal vessel crosses to 

 anastomose with its fellow on the opposite side, 

 and on each side of the dorsal vessel, nearly 

 in the course of the lateral muscles, there is u 

 faint indication of a line which seems to form 

 the boundary of what we regard as the auricular 

 space in which the blood is collected before 

 passing into the chambers, of which there are 

 eight very distinct ones in these larvae. 



The motion and course of the blood, as will 

 be seen from the above account of the structure 

 of the chief organ of the circulation, is first 



