RESPIRATION IN INSECTS. 867 



most of which are organized for flight, the respiratory apparatus consists, 

 first of tubuli. which commence at the spiracles or stigmata found on each side 

 of certain segments of the body, and, after a short course, lead into two lon- 

 gitudinal lateral tubes, extending from one end of the animal to the other. 

 From these principal tubes, branches are given off for each segment ; these 

 branch again and again, until the finest ramifications penetrate the substance 

 of every organ, especially the muscles, and even the ganglionic nervous cen- 

 tres, and the complex eyes. These tubuli, large and small, constitute the 

 well-known tracheae of Insects. They are recognized under the microscope, 

 by the beautiful structure of their walls, which are composed of coherent spi- 

 ral fibres arranged in the most regular manner, and maintaining by their elas- 

 ticity, the whole tracheal system in a patent state, resisting the pressure to 

 which they are subjected during muscular effort. By these open canals, air is 

 freely introduced into every portion of the Insect. In many perfect Insects, 

 especially in those of powerful flight, the body or abdomen is made to perform 

 active movements, by which air is drawn in and out through the spiracles. 

 Closure of the spiracles, or the filling of the tracheae with oil, speedily as- 

 phyxiates these animals. Although, therefore, the circulatory apparatus is 

 limited to a dorsal vessel, with large venous sinuses and interstitial lacunae, 

 without capillary vessels, yet the respiratory apparatus is diffused through 

 every organ and portion of the body, and the aeration of the blood is most 

 complete. The respiratory function of these, the most perfectly developed 

 examples of the Annulose animals, is, in the majority of species, extremely 

 active, in harmony with their general energy and comparatively high tem- 

 perature. In the largest Insects, and in those which, though of smaller size, 

 possess remarkable powers of sustained flight, as the Bee tribe, the longitu- 

 dinal tracheal trunks are dilated in certain segments, so as to form air-sacs 

 with rigid walls. The size of these presents considerable variation, being 

 largest in species possessing the greatest powers of flight. The wings of the 

 perfect Insect have no homology with the true limbs, for they spring from the 

 dorsal, and not from the abdominal surface ; in their structure and mode of 

 evolution, they are more like respiratory organs, being filled with ramified 

 tracheae. 



The tracheae and their dilatations in the flying Insects, sometimes desig- 

 nated the Birds of the Non-vertebrate Creation, have been supposed, like the 

 membranous and osseous air-cavities in Birds, to diminish the specific gravity 

 of the body during flight, by the warmer air in them ; but any such effect 

 must be immeasurable. Their hollow, stiff' structure, however, utilizes the 

 material employed in the most admirable manner, and so relatively diminishes 

 weight ; whilst the large dilatations or sacs met with, especially in Insects of 

 flight, may serve as store-chambers for air, the spiracles being, perhaps, more 

 or less closed during that act. 



Insects absorb, and convert into carbonic acid, a relatively large quantity of 

 oxygen, even during rest ; but the respiratory interchanges are much more 

 active during locomotion. A bee will perform during the excitement immedi- 

 ately following its capture, as many as 125 respiratory movements of its body 

 per minute ; but after an hour and a half, these may decline to 46 per minute. 

 In the first hour of an excited respiration, one-third of a cubic inch of carbonic 

 acid has been found to be generated, a larger quantity than was produced 

 in twenty-four hours by a bee in its quiescent state, and far more than is 

 given off' by the lungs of a Man, in proportion to his weight. (Newport. ) In 

 the larval condition, as exhibited in caterpillars and grubs, Insects are also 

 provided with stigmata, lateral tubes, and tracheae ; and so is the pupa or 

 chrysalis of those insects, which pass through the perfect stages of metamor- 

 phosis. But the respiratory process in the pupa is less active than in the 

 perfect Insect, and in the chrysalis, less active than in the larva. It is curious 

 that the larvae of certain insects, as of the gnats and Ephemerae or day-flies, 

 and dragon-flies, are purely aquatic in their habits ; but still, for the most 

 part , they also breathe by tracheae, the spiracles of which exist only in the 

 hinder portion of the animal ; these they protrude above the surface of the 

 water, for respiratory purposes. In the larva of the gnat, one of the spiracles 

 of the tail segment is provided with a tubular prolongation, the mouth of 



