324 INSECTA. 



ther, the inference would be that the sole design of the tracheal appa- 

 ratus of the insect consisted in aerating the fluids. Since, however, 

 the blood returns much before the tracheae reach their remote pene- 

 tralia, it is evident that the tracheal system in the insect fulfils some 

 other function. What can be the meaning of those incomparable 

 pneumatic plexuses veritable retia mirabilia which embrace imme- 

 diately the very ultimate elements of the solid organs of the body ? those 

 microscopic air-tubes, which carry oxygen in its gaseous form, unfluidi- 

 fied by any intervening liquid, to the very seats of the fixed solids which 

 constitute the fabric of the organism? The intense electrical and 

 chemical effects developed by the immediate presence of oxygen at the 

 actual scene of all the nutritive operations of the body, fluid and solid, 

 give to the insect its vivid and brilliant life, its matchless nervous 

 activity, its extreme muscularity, its voluntary power to augment the 

 animal heat. Such contrivance, subtle and unexampled, reconciles the 

 paradox of a being, microscopic in corporeal dimensions and remarkable 

 for the relative minuteness of the 'bulk of its blood, sustaining a frame 

 graceful in its littleness, yet capable of prodigious mechanical results. 



(848.) We must now consider the mechanism by which air is per- 

 petually drawn into the body of the insect, and again expelled. If the 

 abdomen of a living insect be carefully watched, it will be found con- 

 tinually performing movements of expansion and contraction that suc- 

 ceed each other at regular intervals, varying in frequency, in different 

 species, from twenty to fifty or sixty in a minute*, but occurring more 

 rapidly when the insect is in a state of activity than when at rest. At 

 each expansion of the abdomen, therefore, air is sucked in through all 

 the spiracles, and rushes to every part of body ; but when the abdomen 

 contracts, it is forcibly expelled through the same openings. Burmeister 

 even supposes that the humming noises produced by many insects during 

 their flight must be referred to the vibration caused by the air stream- 

 ing rapidly in and out of the spiracular orifices. Insects which live in 

 water are obliged, at short intervals, to come to the surface to breathe, 

 at which time they take in a sufficient quantity of air to last them 

 during the period of their immersion ; but if the spiracles are closed by 

 any accident, or by the simple application of any greasy fluid to the 

 exterior of their body, speedy death, produced by suffocation, is the 

 inevitable result. 



(849.) A moment's reflection upon the facts above stated concerning 

 the respiration of insects will suggest other interesting views connected 

 with the physiology of these little creatures. It is evident, in the first 

 place, that their blood is all arterial ; they can have no occasion for 

 veins, as they have no venous blood, the whole of the circulating fluid 

 being continually oxygenized as its principles become deteriorated. The 

 perfection of their muscular power, their great strength and indomitable 

 * Sorg, Disquisitiones Physiologies circa Kespirationem Insectorum et Vermium- 



