NATURAL HISTORY OF HARTING. 325 



perfect insect with all its parts complete, the gorgeous 

 butterfly, or the brilliant beetle. There is no insect so 

 small as not to afford ample space within its external 

 crust for the unobstructed performance of all the vital 

 functions. The most minute of beetles possess the 

 same number of segments and jointed organs as the 

 most gigantic ; and perfect systems of digestion, cir- 

 culation, and respiration are common to them all. 



The function of respiration in true insects is carried 

 on by means of a remarkably complex apparatus ; 

 instead of lungs they are supplied with an incredible 

 number of wonderfully-constructed air tubes called 

 tracheae, running through every part of the body like 

 a fine network. Each of these tubes, however minute, 

 is kept in a state of distension by a fine elastic thread 

 wound round and round from end to end like a 

 spiral spring, and the air is drawn into them through 

 several openings in different parts of the body called 

 spiracles, or breathing pores, which some insects have 

 the power of opening and closing fifty or sixty times 

 in a minute. These breathing pores, where they are 

 not protected by wings or wing-cases, have most of 

 them a provision for filtering the air that passes into 

 them, so as to exclude the finest floating particle of 

 dust which might otherwise be drawn into the air 

 tubes and cause an obstruction. The construction of 

 this filter is not limited to one plan ; in some insects 

 it consists of an elastic fringe attached to the inner 

 border of the spiracle, the free ends of which meet in 

 the centre, so as to leave no part of the opening un- 

 defended. In others it is a delicate membrane stretched 

 over a framework fitted across the aperture and pierced 

 with minute holes like perforated zinc. Other forms 

 might be described, but these are sufficient to show 

 that the members of this class have been in the habit 

 of wearing respirators from time immemorial. 



In the typical forms of the most perfect insects we 

 find six legs fitted for various modes of progression . 



