1 3 6 FAT-BODY CHAP. 



As the dorsal vessel consists of a number of distinct chambers, 

 it has been suggested that there is normally one of these for 

 each segment of the body ; and it appears that the total number 

 is sometimes thirteen, which is frequently that of the segments 

 of the body without the head. The number of chambers differs, 

 however, greatly, as we have previously stated, and cannot be 

 considered to support the idea of an original segmental arrange- 

 ment of the chambers. The dorsal vessel, though in the adult a 

 single organ, arises in the embryo from two lateral, widely 

 separated parts which only in a subsequent stage of the embry- 

 onic development coalesce in the median line. 



Fat-Body. 



In discussing the tracheae we remarked on the importance of 

 their function and on their abundant presence in the body. 

 Equally conspicuous, and perhaps scarcely less important in func- 

 tion, is the fat-body, which on opening some Insects, especially 

 such as are in the larval stage, at once attracts attention. It 

 consists of masses of various size and indefinite form distributed 

 throughout the body, loosely connected together, and more or 

 less surrounding and concealing the different organs. The colour 

 varies according to the species of Insect. This fat-body is much 

 connected with fine tracheal twigs, so that an organisation extend- 

 ing throughout the body is thus formed. It may be looked on as 

 a store of nutritious matter which may be added to or drawn 

 on with great rapidity ; and it is no doubt on this that many of 

 the internal parasites, so common in the earlier stages of Insects' 

 lives, subsist before attacking the more permanent tissues of their 

 hosts. There is some reason to suppose that the fat-body may 

 have some potency in determining the hunger of the Insect, for 

 some parasitised larvae eat incessantly. 



The matter extracted from the food taken into the stomach 

 of the Insect, after undergoing some elaboration on which point 

 very little is known finds its way into the body-cavity of the 

 creature, and as it is not confined in any special vessels the fat- 

 body has as unlimited a supply of the nutritive fluid as the 

 other organs : if nutriment be present in much greater quantity 

 than is required for the purposes of immediate activity, meta- 

 morphosis or reproduction, it is no doubt taken up by the fat- 



