1NSECTA. 



between the fibres of the muscular coat of the 

 alimentary canal, and ramify extensively be- 

 tween the mucous coat and a structure which we 

 have described* as the adipose coat, which lies 

 between the mucous and muscular, as is well 

 seen in the colon and crecum of the puss-moth, 

 Centra vinula, in the perfect state. It is in 

 this layer that the ramifications of trachea 

 anastomose very freely, but do not enter the 

 mucous or internal coat. Besides these parts 

 all the secretory and generative organs are sup- 

 plied with anastomosing branches in abun- 

 dance, and tracheae are extended even to the 

 very last joints of the tarsi in the limbs. The 

 only parts into which we have not observed 

 tracheae penetrate are the adipose vesicles, 

 upon which we have not often observed rami- 

 fications, although branches of trachese are dis- 

 tributed very extensively among them. In the 

 larva state the trachese are always smaller than 

 in the perfect, compared with the size of the 

 individual, and they are smallest in those 

 apodal larvae of Hymenoptera which reside 

 long in closed cells, as the Anthophora return, 

 in which insect the communications of the 

 trachese across the body are very distinct, as 

 was shewn long ago by Swammerdam in the 

 larva of the hive-bee. 



The structure of the tracheae has been des- 

 cribed by Swammerdam, Sprengel, and others. 

 Sprengel has described the structure as con- 

 sisting of an external serous and an internal 

 mucous membrane, inclosing between them a 

 spirally convoluted fibre (JigA35, a\ which is 

 elastic, and gives to the tracheae the appearance 

 exhibited by the tracheae in other animals. 

 The external or serous membrane (6) very 

 loosely surrounds the spiral fibre (). The 

 mucous or internal lining, as we formerly re- 

 marked, and as noticed by Swammerdam, De 

 Geer, Lyonet, and Bonnet, is continuous with, 

 and is thrown off at the change with the external 

 covering of the larva, and certainly is a distinct 

 membrane, renewed at those periods, although 

 Sprengel believes that it is only a means of 

 connexion between the coils of the spiral fibre, 

 and not a distinct structure. 



The vesicles, or dilated trachese, exist in the 

 greatest abundance in volant insects, although 

 they also exist in a much less developed form 

 in the saltatorial. These vesicles exhibit an 

 appearance which was formerly noticed by 

 Swammerdam in Oryctes nasicomis, and sub- 

 sequently by Sprengel in other insects. It con- 

 sists of an amazing number of punctured spots, 

 discoverable only under a good microscope, but 

 which, when attentively examined, exhibit 

 somewhat the appearance of perforations. The 

 precise nature of these spots is not well under- 

 stood. Burmeister conceives that they are oc- 

 casioned by the rupture of the spiral fibre 

 during development, and that the spots are the 

 spaces between the broken fibres. But Marcel 

 de Serres and Straus Durckheim deny the 

 existence of spiral fibre in the vesicles, while 

 Suckow and Burmeister contend that it cer- 

 tainly does exist, and we also are of this 



* Phil. Tians. 1836. 



opinion. Indeed, when it is remembered that 

 the vesicles exist only in the perfect insect, and 

 are only dilated tracheae, and that the existence 

 of spiral fibre in the tracheae is undoubted, 

 surely its existence can scarcely be questioned 

 in the vesicles, although, probably, it is in an 

 almost atrophied condition. Now the fact that 

 the spots are not observed on the vesicles 

 until the insect has entered the perfect state, 

 was, perhaps, one of the circumstances that 

 led Burmeister to his opinion respecting them ; 

 but that they are not caused by ruptured spiral 

 fibre is proved by the existence of these spots 

 in some of the tracheae that communicate direct- 

 ly with the vesicles, and have not been dilated, 

 and in which the spiral fibre is unbroken. It 

 is also shewn by the circumstance of their not 

 being in a regular series, over the course of the 

 fibres, but distributed thickly and irregularly 

 over the surface of the vesicles, and by their 

 existing in the space between two parallel 

 fibres in the trachese, and even in the substance 

 of the fibre, as we have seen them in the 

 vesicles of the male of Bombus terrestris. Be- 

 sides this, they are sometimes seen to terminate 

 in an abrupt and remarkable manner in the 

 dilatations of the larger tracheae in the same 

 insect. The results of our own observations 

 lead us to conclude that these spots are not 

 ruptures of the spiral fibre, but are partial per- 

 forations of the vesicles, that they do not 

 pass through the internal or lining coat, and 

 probably are little cells in the coats of the vesi- 

 cles, through which the circulatory fluid can be 

 freely submitted to the action of the air in the 

 vesicles, as in the minute terminal cells in the 

 lungs of vertebrata. 



The use of the vesicles, as above remarked, 

 and as formerly suggested by Hunter, appears 

 to be to enable the insect to alter its specific 

 gravity at pleasure, by enlarging its bulk, and 

 thus rendering it better able to support itself on 

 the wing with little muscular effort. That this 

 is the use of the sacs may be inferred from 

 their non-existence in the larva state, or in in- 

 sects that constantly reside on the ground, more 

 particularly in creeping insects ; and it seems 

 further confirmed by the fact that, among volant 

 insects, those have the largest and greatest 

 number of vesicles which sustain the longest 

 and most powerful flight. Thus the vesicles 

 are found most developed in the Hymenoptera, 

 Lepidoptera, Diptera, and some Coleoptera and 

 Hemiptera, in all which, in the larva suite, there 

 is not the slightest trace of them. A still 

 further proof that they are for lightening the 

 body is found in Lucanus cervus. In the 

 male of this insect the large and heavy man- 

 dibles and head, but more especially the man- 

 dibles, are not filled with solid muscle, as in 

 the Hydrous and others in which these parts 

 are more in proportion to the size of other 

 parts of the body, but with an immense 

 number of vesicles, which in the mandibles 

 are developed in the greatest abundance in 

 rows from long tracheae, that are extended 

 from one end of the organ to the other, so that 

 the interior is almost entirely filled with vesi- 

 cles. By this beautiful provision these pro- 



