152 THE VITAL FUNCTIONS. 



The distinction into small and great intestine is more or 

 less marked, in different insects, in proportion to the quan- 

 tities of food consumed, and to its vegetable nature; and in 

 herbivorous tribes, more especially, the dilatations in the 

 lower part of the canal are most conspicuous, as well as the 

 duplicatures of the inner membrane, which constitute im- 

 perfect valves for retarding the progress of the aliment. 'It 

 is generally at the point where this dilatation of the canal 

 commences, that a second set of hepatic vessels is inserted, 

 having a structure essentially the same as those of the first 

 set, but generally more slender, and uniting into a small 

 number of ducts before they terminate. The number and 

 complication of both these sets of hepatic vessels, appear to 

 have some relation to the existence and development of the 

 gizzard, and consequently, also, to the nature and bulk of 

 the food. Vessels of this description are, indeed, constantly 

 found in insects; but it is only where a gizzard exists, that 

 two sets of these secreting organs are provided; and in some 

 larvas, remarkable for their excessive voracity, even three 

 orders of hepatic vessels are met with.* 



A muscular power has also been provided, not only for 

 the strong actions exerted by the gizzard, but, also, for the 

 necessary propulsion, in different directions, of the contents 

 both of the stomach and intestinal tubes. The muscular 

 fibres of the latter are distinctly seen to consist of two sets, 

 the one passing in a transverse or circular, and the other in 

 a longitudinal direction. Glandular structures, analogous to 

 the mucous follicles of the higher animals, are also plainly 

 distinguishable in the internal coat of the canal, more espe- 

 cially of herbivorous insects.t The whole tract of the ali- 

 mentary canal is attached to the sides of the containing ca- 

 vity by a fine membrane, or peritoneum, containing nume- 

 rous air-vessels, or trachese.^ 



* See the Memoirs of Marcel des Serres, in the Annales du Museum, 

 xx. 48. 

 f Lyonet. 

 4 It has been stated by Malpighi and by Swammerdam, and the statement 



