358 iMOTIONS OF INSECTS. 



cany it five feet, every second. But it" alarmed, he 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 more than a mile in aminute. 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 mirmte 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 

 lijihtning. 



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 

 have 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 

 traclieu) and bronchia^, their locomotive powers, with 

 few exceptions, must depend altogether upon the state of 

 tliat 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 nnist 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- 



