264 1NSECTA. 



ever, another reason for rejecting the opinion that these accessory 

 vessels secrete urine, and that is, that they are only met with in a 

 few beetles and some species of Orthoptera ; a circumstance that 

 alone would be sufficient to disprove such supposition. 



In the vertebrate animals, as the reader is well aware, the nu- 

 tritious products of digestion are taken up by a system of absorb- 

 ing vessels, that ramify extensively over the coats of the intestine, 

 and the nutriment is thus conveyed into the mass of the circu- 

 lating fluid by ducts appropriated specially to this office ; in 

 animals of less perfect structure than these, such as the Mollusca, 

 the veins themselves absorb the nutritive materials. But in insects, 

 in which we find neither absorbents nor veins, a different arrange- 

 ment is necessary ; and, in the little creatures before us, nutrition 

 appears to be carried on by the simple transudation of the chyle 

 through the coats of the intestine, so that it escapes into the 

 general cavity of the abdomen, where, as we shall see when we 

 examine the arrangement of their circulating organs, it is im- 

 mediately mixed up with the blood. This transudation has in- 

 deed been actually witnessed by Ramdohr and Rengger,* and 

 even analyzed by the last-mentioned physiologist, who found it to 

 consist almost entirely of albumen. 



(306.) The respiratory organs of the INSECT A, as well as their 

 circulatory apparatus, are constructed upon peculiar principles, and 

 are evidently in relation with the capability of flying, which distin- 

 guishes these minute yet exquisitely constructed articulated animals. 

 Any localized instruments for breathing, whether assuming the shape 

 of branchiae or lungs, would materially have added to the weight of 

 the body, and moreover have rendered necessary an elaborate ap- 

 paratus of arteries and veins for conveying the blood to and fro 

 for the purpose of purifying it by securing its exposure to the in- 

 fluence of air. By the plan adopted, however, all these organs are 

 dispensed with ; and the organs of respiration, so far from increasing 

 the weight of the animal, actually diminish its specific gravity to 

 the greatest possible extent. The blood, in fact, in insects is not 

 brought to any given spot to be exposed to oxygen, but the air is 

 conveyed through every part of the system by innumerable tubes 

 provided for that purpose, and thus all the complicated parts 

 usually required to form a vascular system are rendered unnecessary. 

 These observations, however, only apply to the insect in its perfect 



* Physiologische Untersuchungen iiber den thierischen Haushalt der Insekten- 

 8vo. 1817. 



