532 ORGANS OF RESPIRATION. 



it to the internal tracheae, and some coleopterous insects and 

 dipterous larvae inhabiting the water, come, like cetacea, to 

 the surface to inhale atmospheric air. The branchiae, indeed, 

 are mere external prolongations of the commencements of 

 the internal air-tubes, and assume the form of capillary 

 filaments, or ramose tufts, or lamellae, through which the 

 tracheae pass ; at their metamorphosis, they often lose these 

 branchial terminations of the tracheae, and acquire spiracula 

 for aerial respiration. The aquatic larvae of the common 

 gnat, and of the hydrophilus, chironomus, and dytiscus, 

 inhale air by stigmata near the anus, and the last segment of 

 the trunk is prolonged into an air-tube in the stratiomys and 

 in the larvae of eristalis, which they extend to the surface of 

 the water. The deciduous external branchiae prolonged from 

 the internal tracheae of the larvae of many insects, thus 

 resemble the external deciduous branchiae prolonged in 

 filiform tufts from the permanent organs in many of the 

 higher fishes. 



The tracheal openings of perfect insects are provided with 

 muscles to open and close them, as shown by Lyonet ; they 

 are often extremely minute or entirely wanting on the pos- 

 terior segments of the abdomen and of the thorax, and they 

 are generally largest on the anterior rings of these two divi- 

 sions of the trunk, even where the posterior have been largest 

 in the larva. The tracheae are lined internally with a soft, 

 white, mucous tunic ; externally they are covered with a more 

 dense, shining, serous coat, and between these is an elas- 

 tic fibrous tunic, composed of continuous spiral filaments, 

 twisted closely, like those of a plant, around all the ramifi- 

 cation of the tracheae, from the spiracula to their minutest 

 divisions, thus giving the necessary elasticity for the free 

 passage of air through all parts, as the cartilaginous rings of 

 the trachea and bronchi of vertebrata. These air-tubes ra- 

 mify, like blood-vessels, through all parts of the head, the 

 antennae, the palpi, the legs, the tarsi, the wings, the muscular 

 system of the trunk, the interior of the blood-canals of the 

 wings, the surface of the alimentary canal, the ovary, and on 

 almost all the internal organs, as seen in the annexed figure 

 (Fig. 144,) of the tracheae of melolontha vulgaris. From the 

 stigmatic origin of each tracheae, there generally extend forwards 

 and backwards several large longitudinal trunks (144. b. c.) 



