358 MOTIONS or INSECTS. 



carry it five feet, every second. But if alarmed, be states 

 their velocity can be increased six- or seven-fold, or to 

 thirty or thirty-five feet, in the same period. In this 

 space of time a race-horse could clear only ninety feet, 

 which is at the rate of mo*re than a mile in a minute. Our 

 little fly, in her swiftest flight, will in the same space of 

 time go more than the third of a mile. Now compare 

 the infinite difference of the size of the two animals (ten 

 millions of the fly would hardly counterpoise one racer), 

 and how wonderful will the velocity of this minute crea- 

 ture appear ! Did the fly equal the race-horse in size, 

 and retain its present powers in the ratio of its magni- 

 tude, it would traverse the globe with the rapidity of 

 lightning. 



It seems to me, that it is not by muscular strength 

 alone that many insects are enabled to keep so long 

 upon the wing. Every one who attends to them must 

 Lave noticed, that the velocity and duration of their 

 flights depend much upon the heat or coolness of the 

 atmosphere : especially the appearance of the sun. The 

 warmer and more unclouded his beam, the more insects 

 are there upon the wing, and every diurnal species seems 

 fitted for longer or more frequent excursions. As these 

 animals have no circulating fluid except the air in their 

 tracheae and bronchiae, their locomotive powers, with 

 few exceptions, must depend altogether upon the state of 

 that element. When the thermometer descends below 

 a certain point they become torpid, and when it reaches 

 a certain height they revive ; so that the air must be re- 

 garded, in some sense, as their blood, or rather the ca- 

 loric that it contains ; which when conveyed by the air, 

 it circulates quickly in them, invigorates all their mo- 



