$62. MOTIONS OF INSECTS. 
its ordinary flight, the common house-fly (Musca domes- 
tica, L.) makes with its wings about 600 strokes, which 
carry it five feet, every second. But if alarmed, he states 
their velocity can be increased six- or seven-fold, or tothirty 
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 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 dif- 
ference of the size of the two animals (ten millions of the 
fly would hardly counterpoise one racer), and how won- 
derful will the velocity of this minute creature appear ! 
Did the fly equal the race-horse in size, and retain its 
present powers in the ratio of its magnitude, 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 enabied 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 tracheze 
and bronchi, their locomotive powers, with few excep- 
tions, must depend altogether upon the state of that ele- 
ment. 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 regarded, in 
some sense, as their blood, or rather the caloric that it 
contains; which when conveyed by the air, it circulates 
