26 ROMANCE OF THE INSECT WORLD CHAP. 



termination of that state. Then the cerebral mass 

 acquires a compact form, it is more uniformly opaque 

 than the other ganglia, and fills up most of the cavity 

 that incloses it. 



If the metamorphoses of the nervous structures are 

 wonderful, calling fresh senses and faculties into play, 

 how shall be characterised the changes that take 

 place in the organs of respiration ? There can be no 

 comparison between the feeble respiratory * efforts of 

 a sluggish caterpillar or grub, and the amount of 

 breathing energy of a vigorous and nimble insect. 

 The mode whereby an insect breathes is very dif- 

 ferent from ours. It does not draw in air through 

 one mouth, neither has it lungs, nor is the fluid 

 answering to blood, which the air has to purify, 

 contained in veins, it bathes the internal system. 



When the interior of an insect is carefully investi- 

 gated, a multitude of the tiniest and most delicate 

 tubes are seen branching to right and left, on one side 

 towards the skin ; on the other they are distributed 

 to the middle of the body, they cover the digestive 

 organs, and enter amongst the tissues.* These are 

 air-tubes, known as tracheae, their duty being to 

 convey the air through every part of the insect. A 

 trachea is a tube formed of two layers of membrane, 

 inclosing between them a spiral convoluted thread. 

 It is kept open by means of the spiral, which is very 

 compressible, so that the amount of air any trachea 

 may contain varies according to circumstances. The 

 outside air gains admission to the tubes through 

 small apertures, or mouths, called spiracles, which 

 are situated generally at regular distances, their 



