320 INSECTA. 



considering. There is, however, another reason for rejecting the opinion 

 that these accessory vessels secrete urine ; and that is, that they are only 

 met with in a few heetles and some species of Orthoptera a circum- 

 stance that alone would be sufficient to disprove such a supposition. 



(842.) In the vertebrate animals, as the reader is well aware, the 

 nutritious products of digestion are taken up by a system of absorbing 

 vessels that ramify extensively over the coats of the intestine ; and the 

 nutriment is thus conveyed into the mass of the circulating 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 ab- 

 sorbents nor veins, a different arrangement 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 immediately mixed up with the blood. This transudation has 

 indeed been actually witnessed by Ramdohr and Bengger*, and even 

 analysed by the last-mentioned physiologist, who found it to consist 

 almost entirely of albumen. 



(843.) The respiratory organs of the Insecta, as well as their circu- 

 latory apparatus, are constructed upon peculiar principles, and are evi- 

 dently in relation with the capability of flying which distinguishes 

 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 apparatus of 

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

 purifying it by securing its exposure to the influence of air. Ey 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 state ; for in the larva and pupa conditions, where 

 flight is not possible, various additional organs, frequently of consider- 

 able bulk, are provided, that we shall speak of in another place. If we 

 examine the external skeleton of any large insect (a beetle, for example), 

 we shall find, between the individual segments of the body, minute 

 apertures or pores (spiracles} through which the air is freely admitted : 



* Physiologische Untersuchungen iiber die thierische Haushaltung der Insekten. 

 8vo. 1817. 



