140 Marvels of Insect Life, 



deflation of the lungs ; but it is sufficient, as Insects are not such large con- 

 sumers of oxygen as the backboned animals. Insects are often enclosed in 

 tight-fitting tin boxes for many hours on collecting expeditions without any 

 injury to their health. IMiall and Denny state that though " cockroaches in 

 carbonic acid speedily become insensible," yet " after twelve hours' exposure tO' 

 the pure gas thev survive and appear none the worse." A caterpillar that has 

 fallen into water soon becomes flaccid and apparently dead through the entry of 

 water into the air-tubes, but if rescued and placed where the water can drain out, 

 will revive, and resume its former activity. Insects such as bees and wasps, which 

 exhibit an almost ceaseless activity, are affected by close confinement in restricted 

 air-space much more speedily than the leisurely flying though larger butterfly or moth. 

 A main air-trunk extends along each side of the body, with short branches- 

 connecting wdth each spiracle, and three ramifying branches to each segment of the 

 body : one to the muscles of the upper side, one to the digestive canal and other 





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Photo bv] [«• '*"*""• 



How AN Insect Breathes. 



In this photograph a silkworm has been made transparent to show the ramifications of the system of air-tubes, which are here seen 

 as dark, branching threads. 



central parts, and the third to the nerves and muscles of the lower side of the body.. 



The spiracles are sht-Hke openings along the sides, but they are not as simple- 

 as they appear to be from the exterior. In different families there are a variety of 

 means adopted for excluding dust, and valves for closing them when necessary. 

 The simplest forms are seen in some bugs, flies, and beetles. These have round or 

 eUiptical openings surrounded by a ring of chitin, whilst within is a funnel-like 

 contraction. In the cockroach the spiracles are permanently open, but there is an 

 internal valve which can close access to the air-tubes. In the grubs and jiupc-e of 

 some flies each spiracle consists of a series of small openings, each with a separate 

 short tube which unites within before joining the trunk tube. None of the foregoing 

 have lips to the spiracles. 



Others are more specialized in character, with hps, furnished with bristles or 

 hairs, sometimes branching so that they form a kind of sieve. In some cases these- 

 hairs are external, in others internal, according to the luibits of the species, the 



