870 



INSECT. 



is most distinctly teen in the aquatic larvae, and in 

 the wings ot'insecu, owing to their transparency. 



D. The Respiratory System. 



The systems of circulation and respiration are ne- 

 cessarily dependent upon each other. In the higher 

 animals the lungs are the receptacles of the air, and 

 the blood rushes into these organs to be oxygenated. 

 In insects we hare, however, seen that the circulatory 

 system extends throughout the body, the great dorsal 

 vessel equally extending from one end of the body to 

 the other ; hence it is requisite that the respiratory 

 apparatus should be developed to an equivalent de- 

 gree, and we accordingly find that respiration in 

 insects is effected by means of two great longitudinal 

 vessels or canals called tracheae, running along the 

 sides of the body beneath the outer integuments and 

 muscles, and which open to the outer atmosphere by 

 means of short tubes, terminating in breathing pores 

 (spiracles or stigmata). The number of these spiracles 



*ig. ISO, Respiratory apparatus of Nepu cinerea, in which (he 

 :>nly pair of spiracles are placed at the sides of the anus, the 

 lateral spiracles being obsolete or rudimeutal. 



is various ; those of the larva, in which the body is 

 composed of a more regular and uniform series of 

 segments, being more numerous than the spiracles of 

 the imago. In the former they amount to eighteen, 

 that is, nine pairs; whereas, in the generality of perfect 

 insects, the number is, or appears to be, diminished ; 

 moreover, in the latter, the spiracles, owing to the 

 greater development of external covering, are gene- 

 rally more or less concealed. Spiracles are of two 

 kinds, simple or composite. In the former the aper- 

 ture is a simple orifice guarded only by hairs, which 

 prevent the passage of any foreign substance which 

 might be injurious ; but in the composite spiracle the 

 aperture is closed by two horny valves, which move 

 backwards and forwards like a pair of folding doors 

 at each inspiration. The former sort of spiracles are 

 generally found at the sides of the abdomen, where 

 e upper and lower arcs of each segment are united 



together. The composite spiracles are, on the con- 

 trary, appropriated to the thorax. 



The tracheae, which originate in the spiracles, are 

 connected with the two great longitudinal air tubes 

 above mentioned, which likewise emit an infinite 

 number of ramifications extending to all parts of the 

 body, like the branches of a tree. These tracheae are 

 of two sorts, tubular or vesicular : the former are 

 tubes composed of three distinct membranes, the ex- 

 ternal and internal layers being of a cellular texture, 

 and the central being composed of a cartilaginous 

 thread, rolled in a spiral direction, and very elastic. 

 The vesicular tracheae are destitute of this spiral 

 cartilage, and are small bags of a cellular texture, 

 which, when not distended with air, naturally become 

 flaccid. The tracheae often communicate with each 

 other, and penetrate all the organs, such as the legs, 

 wings, &c. ; they are also distributed throughout the 

 intestinal parts of the body, so that the air circulates 

 freely in the most obscure parts. 



We have already noticed in our articles DYTICID^:, 

 CULICID.*, CHIRONOMUS, EPHEMERA, and in the 

 present article, various modifications exhibited by the 

 respiratory apparatus of the insects belonging to these 

 groups, which are chiefly aquatic, and in which we 

 perceive two principal modifications: 1st, A series of 

 large lateral organs which act upon the water, and 

 abstract from it the oxygen ; and, 2ndly, a concentra- 

 tion of the respiratory functions, by means of an 

 apparatus placed at the extremity of the body, 

 whereby the insect is enabled to obtain a supply of 

 air by protruding this part of the body above the 

 surface of the water, whilst it still remains with its 

 greater part submerged. M. Audouin has also 

 recently noticed another remarkable modification of 

 the respiratory apparatus in insects, which, although 

 terrestrial, are occasionally covered for a great length 

 of time with water. (See his Memoir upon Nepiis 

 fulvesccns in the Nouvelles Annales du Museum). 



Fig. 151, Dorsal muscles of the anterior segments of the caterpil- 

 lar of Costut. 



E. The Motive, or Muscular System. The varied 

 structures which we have observed in the preceding 

 parts of this article, upon the external anatomy of 

 insects, are but the visible expression of certain prin- 

 ciples which constitute the economy of the various 

 species, and which are regulated in the performance 

 of their various functions by a series of internal 

 muscles, attached to the internal surface of the ex- 

 ternal covering of the animal. Of the extent of this 



