24 CIRCULATION. 



a number of long, delicate tubes, filled with a yellowish liquid, 

 terminating in the digestive tube ; these are the biliary vessels 

 (&), which perform the functions of the liver. 



46. We find salivary organs in a great many insects, and 

 generally they are more developed in the suctorial than in the 

 triturating species. They are simple, floating tubes, which some- 

 times terminate in a kind of utri'culse or little membranous sacs, 

 which communicate with the pharynx by means of intermediate 

 excretory ducts or canals. 



47. Towards the posterior extremity of the intestinal canal, we 

 also find other secreting organs of various forms (I) which serve 

 for the elaboration of those particular liquids which many insects 

 cause to exude from the posterior part of the abdomen when they 

 are disturbed ; the venom of the bee is an instance. 



48. Sometimes the nutritive liquid resulting from the digestion 

 of food is immediately appropriated to assimilation, sometimes, on 

 the contrary, a part of it seems to be held in reserve to be em- 

 ployed on a future occasion. The species of reservoir which is 

 regarded as subserving this curious purpose is the mass of fatty 

 tissue Gurrounding the viscera. 



49. Insects have no circulation properly so called ; the nutri- 

 tive liquid is diffused among all the organs and penetrates them 

 by imbibition. But there exists, nevertheless, on the dorsal sur- 

 face of the animal, immediately beneath the integuments, a sort 

 of longitudinal tube, surrounded by fleshy fibres, which appears 

 to be the rudiment of a heart, for we observe in it alternate con- 

 tractions and dilatations similar to those of the same organ in 

 other animals. But this canal does not appear to give off any 

 branches ; there are no arteries nor veins. 



The blood, become venous by its action on the different tissues 

 of the economy, is not carried to any particular point to come in 

 contact with the oxygen of the air, to regain its vivifying quali- 

 ties. If respiration were carried on in the ordinary way, by 

 means of lungs or the external surface of the body, it would be 

 extremely imperfect ; but the disadvantage which seemingly must 

 result from this great imperfection in so important a function as 

 the circulation does not really exist. Nature has dispensed with 

 the necessity of circulating the blood in insects, by carrying, 

 the air in them, to all parts of the body, by means of a multitude 

 of canals which ramify almost infinitely in the substance of the 

 organs (fig. 13). 



46. What are the characters of salivary glands in insects ? 



47. Where is the venom of the bee formed ? 



48. Is digested food in all cases immediately appropriated to the purposes 

 of assimilation ? 



49. How is the blood circulated in insects ? How is the want of circula- 

 tion compensated for in insects ? 



