976 



INSECTA. 



like the omentum of the higher animals, and 

 they are extended also among tlio muscles, 

 between which they occupy the interstices, both 

 between the different layers and the tegumen- 

 tary skeleton. Tliey do not form in the abdo- 

 men one continuous surface like the mesentery, 

 but are simply attached to each other, and to 

 the surrounding structures by constricted por- 

 tions, which allow of the freest communication. 

 They are most abundant in the abdomen, but 

 are extended into the thorax and cover more 

 particularly the nervous cord. There are but 

 very few in the region of the head or in the 

 extremities. We have never yet seen them 

 in actual communication with bloodvessels, 

 although we have observed them attached by 

 minute points along the whole course of the 

 dorsal vessel, in the abdomen, as if they were 

 in some way connected with the return of the 

 blood to the auricular space that appears to 

 surround that organ. It is amongst these 

 vesicles in particular that the Malpighian or 

 so-called biliary vessels extend around the 

 alimentary canal, and the tracheae ramify among 

 them in the greatest abundance, but we have 

 not observed them distributed over the sides of 

 individual vesicles as over some other structures. 

 These circumstances lead us to suspect that 

 the vesicular structures are in some way con- 

 nected with the circulatory system, although 

 they cannot be regarded either as arteries or 

 veins. May they not serve the purpose of 

 lymphatics, while they become at the same time 

 depositaries of the nutrient matter? Oken and 

 Treviranus appear to have considered them as 

 analogous to the liver, and the latter author 

 has supported his opinion by the existence of a 

 somewhat analogous structure in the scorpion, 

 which is believed to be the liver of that animal. 

 That they are most intimately connected with 

 the function of nutrition is proved by the cir- 

 cumstance that they exist in the greatest 

 abundance at the period when the larva ceases 

 to feed, just before it enters the pupa state; 

 that their contents are gradually diminished 

 during that condition ; and that tliey disappear 

 most rapidly towards the latter end of the 

 pupa state, when the organs of generation are 

 in the most rapid progress of development. 

 After the insect has entered the perfect state 

 their contents have nearly disappeared. Added 

 to these circumstances we have observed that, 

 during the earliest periods of the larva state, 

 the quantity of adipose substance contained in 

 the vesicles is very small, and also that in all 

 perfect insects that pass the winter in a state of 

 hybernation the quantity of adipose matter is 

 much greater than in those which do not live 

 through the summer, while it has nearly all 

 disappeared in these insects after they have left 

 their hybernacula in the spring. We have 

 remarked these circumstances particularly in 

 tlie later broods of butteiflies, which being 

 hatched at the end of autumn pass the winter 

 as hybernants and appear again in the spring, 

 and we have constantly noticed the same thing 

 in the large females of Hombus terrestris, which 

 live through the winter. From these circum- 

 stances there can be no doubt but that the 

 adipose matter is intimately connected with (he 



function of nutrition and the circulatory system, 

 while the free communication which we have 

 constantly observed to exist between the vesicles 

 seems to favour our opinion that they may 

 serve the office of lymphatic vessels. That 

 they cannot be supposed to answer the purpose 

 of a liver seems evident from the increase and 

 diminution of their contents at certain periods, 

 while their apparent connexion with the Mal- 

 pighian vessels seems to support the opinion we 

 have advanced, more especially if these be regard- 

 ed as uriniferous rather than as biliary organs. 



Circulatory system. It was formerly sup- 

 posed that there was a total absence of a circu- 

 latory motion of the fluids in insects, and that 

 the whole body was nourished by a simple 

 imbibition of fluids that occupied the cavities 

 of its different regions. This opinion was 

 strengthened by the circumstance of the air- 

 vessels being distributed to every separate 

 structure and ramifying extensively even upon 

 the most delicate organs, a fact so remarkable 

 that it appeared entirely to obviate the necessity 

 for a motion of the fluids, and led to the pro- 

 mulgation of Cuvier's beautifully ingenious 

 theory, that as the blood could not be carried to 

 be aerated in a separate organ or lung, the air 

 was in consequence brought into contact with 

 it throughout the whole body. But the dis- 

 covery of Carus in 1827 of an actual motion 

 of the fluids, and subsequently the discovery 

 by Straus Durckheim of a structure in the 

 dorsal vessel, which clearly indicates the true 

 use of this organ as a centre of circulation, 

 have sufficiently shown that insects do not 

 differ from other animals in the absence of a 

 circulation of their fluids, whatever modifica- 

 tions may exist in the form and situation of 

 the organs by which it is accomplished. 



The heart or great dorsal vessel (fig. 433, A) 

 is an elongated tapering organ, which, in every 

 insect, occupies the middle line of the dorsal 

 surface of the body, and extends from the 

 posterior part of the penultimate segment of 

 the abdomen, through the thorax, into the first 

 segment or head of the animal. That portion 

 of it which is situated in the abdominal region 

 is the proper analogue of the heart of other 

 other animals, and is composed of a certain 

 number of separate compartments or chambers 

 (a). It is distinctly muscular, and is of con- 

 siderable diameter, and is that part which is 

 actively employed in circulating the blood. 

 The other part which extends through the thorax 

 is much narrower than the preceding, and is 

 not divided into chambers, but is one conti- 

 nuous vessel that becomes gradually narrower 

 as it passes through the thorax to the head, 

 where it is divided into separate branches (B). 

 This part is less actively employed than the 

 abdominal, being only the great vessel through 

 which the blood is sent from the muscular 

 heart to the system, and, consequently, repre- 

 sents the aorta. In the structure of the abdo- 

 minal portion or true heart we recognize three 

 separate coats, two of which are most distinctly 

 marked, and form the substance of the organ; 

 but the third or external one is very delicate 

 and not easily observed. Straus Durckheim 

 recognises but two distinct structures, the 



