MICROSCOPIC OBJECTS. 53 



these animals are interesting' microscopic objects. " In 

 the marine tribes there is a pair of these instruments 

 acting horizontally ; but they differ so much in size, form, 

 and even consistence, in the different genera, that no 

 general description could be made applicable." — Mag. 

 Nat. Hist. vol. viii. p. 74. 



The Respiratory System of Insects. — ^There are few sub- 

 jects which offer more interesting materials for microsco- 

 pic examination than the beautiful and varied organism 

 by which the vital functions are performed in insects. 

 In the larva or early stage of their being, this apparatus 

 has several modifications, according as the creature is an 

 inhabitant of the earth or the water. 



These creatures do not breathe by lungs, nor do they 

 inhale the air by the mouth. In the terrestrial kinds the 

 example best suited for examination is the larva or cater- 

 pillar of the WiUow Moth fCossus ligniperdaj , on account 

 of its large size. This larva often attains the length of 

 nearly tliree inches, as shown in fig. 2 (p. 54). Below the 

 head are seen the six feet, and towards the middle com- 

 mence the pairs of false feet. Along each side of the 

 body is a row of oval apertures called spiraculse ; by 

 means of these the air is admitted and passes along the 

 tubes on each side. The arrangement of these tubes, 

 called tracheae, with their numerous branches, is shown 

 in fig. 1 . 



In the construction oi the tracheae, the chief aim is to 

 obtain a delicate, light and flexible tube which shall not 

 coUapse. This is effected by means of a strong spiral 

 fibre wound inside the membrane composing the tube, as 



