ANAT< 'MM,-, 



117 





tubes, or tr,i< lii-ir. which receive ;iir 1 lirmigli paired >e^mentally arranged 

 spiracles. Kach spiracle is commonly the mouth oi a diort In 

 opens into a ;;/.//;/ t rat heal trunk ' I''i,<^. 170) cxten< II'ML' along tin- >ide of 

 the body, l-'rom the two main trunks bram 1 -nt which divide 



and subdivide and terminate in extremely deli<atr tubes. whi< h pene- 

 trate even between muscle libers; between the omma t idia of the com- 

 pound eyes and pnssibly enter cells. In 

 moM CftSCS each main longitudinal trunk 

 gives off in each segment (Fig. i 7 i ) t hree 

 branches: (i) an upper, or dorsal, 

 branch which goes to the dorsal muscles; 

 (2) a middle, or visceral, branch, which 

 supplies the alimentary tract and the 

 reproductive organs; (3) a lower, or 

 ventral, branch, which pertains to the 

 ventral ganglia and muscles. 



In many swiftly flying insects (dragon 

 flies, beetles, moths, flies and bees) there 

 occur tracheal pockets, or air-sacs, which 

 were formerly and erroneously supposed 

 to diminish the weight of the insect, but 

 are now regarded as simply air-reser- 

 voirs. Sacs filled with air lessen the 

 specific gravity of an insect in a fluid 

 medium; but do not increase the buoy- 

 ancy of an insect in the air, unless the 

 contained air is warmer than the sur- 

 rounding air; and in the case of birds, 

 it has been found that the air contained 

 in the bones, though warmer than the 



.. .. . ... FIG. 170. Tracheal system of an 



Surrounding medium, has no appreciable insect, a, antenna; b, brain; I, leg; 



effect on flight. " ne . rve ( i ord; *- P alpus; , * spi"fie; 



st, spiracular, or stigmatal, branch; /, 



Types Of Tracheation. TWO types main tracheal trunk; v, ventral branch; 

 c . 11 ! , i i r vs, visceral branch. After KOLBE. 



of tracheal system are distinguished for 



convenience: (i) the primary, open, or holopneustic type described above, 

 in which the spiracles are functional; (2) the secondary, closed, or apneus- 

 tic type, in which the spiracles are either functionless or absent. This 

 type is illustrated in Collembola and such aquatic nymphs and larvae as 

 breathe either directly through the skin or else by means of gills. The 

 two types are connected by all sorts of intermediate stages. 



