116 



PRINCIPLES OF ANIMAL BIOLOGY 



do not collapse. The bronchioles end in expanded chambers, the 

 alveoli, which are in close contact with blood capillaries. The aggregate 

 interior surface of the alveoli in man (Fig. 94d) is more than 1000 square 

 feet or about fifty times as great as the general surface of the body. 



In most insects, air is taken in by tracheae. These are tubes opening 

 at the surface of the body at various points. The tracheae branch, tree- 

 fashion, in such a way as to reach all parts of the body (Fig. 95). No 



part of any insect tissue is more than a 

 few cells away from the nearest tracheal 

 branch. Formerly it was thought that 

 air pulsed back and forth, into and out 

 of these tracheae. It is nOw known for 

 some insects, however, that air goes in at 

 certain tracheae, out at others, thus 

 implying a circulation of the air. The 

 tracheae are connected with one another 

 by branches, so that such a circulation is 

 possible. 



The young stages of May flies, dragon 

 flies, and some beetles live in the water, 

 yet respiration is carried on by tracheae. 

 Instead of opening at the surface of the 

 body, such tracheae begin in fine closed 

 branches which spread out in external 

 gills (flat plates or tubes), from which 

 they receive their oxj^gen by absorption. 

 Such gills richly supplied with tracheae 

 are known as tracheal gills. 



Breathing Movements. — Whatever 

 mechanism an animal possesses for the 

 absorption of oxygen, it is necessary that 

 there be a continuous supply of oxygen 

 to absorb. An animal that lives fully 

 exposed but attached to some object in 

 swiftly flowing water usually requires no 

 special device to ensure that supply. But one that lives in still water 

 and i-emains motionless soon absorbs all the neighboring oxygen; and 

 since oxygen diffuses only very slowly through water, the supply is not 

 ([uickly renewed. Fishes swim about; but since the gills are under a 

 protective plate (the operculum) at each side just behind the head, mere 

 moving about does not suffice. Renewal of the oxygen supply next to 

 the gills is effected by taking water into the mouth and then pumping it 

 out through clefts among the gills. The opercula are raised to allow 



Fig. 95. — Tracheal system of an 

 insect, a, antenna; b, brain; I, leg; 

 n, nerve cord; p, palpus; s, spiracle; 

 st, spiracular branch; t, chief 

 tracheal trunk; v, ventral branch; 

 vs, visceral branch. (From Folsom, 

 "Entomology," after Kolbe.) 



