MOTIONS OF INSECTS. 363 



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 spe- 

 cies seems fitted for longer or more frequent excur- 

 sions. As these animals have no circulating fluid ex- 

 cept the air in their tracheas and bronchiae, their loco- 

 motive powers, with few exceptions, must depend alto- 

 gether upon the state of that element. When the ther- 

 mometer descends below a certain point they become 

 torpid, and when it reaches a certain height they re- 

 vive ; 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 quickly 

 in them, invigorates all their motions, enters into the 

 muscles and nervures of their wings, maintaining their 

 tension, and by the greater or less rapidity of its pulsa- 

 tions accelerating or diminishing their action. 



Having given you all the information that I can col- 

 lect with respect to the motions of perfect insects in the 

 air, I must next say something concerning their modes 

 of locomotion in or upon the water. These are of two 

 kinds, swimming and walking. Observe — I call that 

 movement swimming, in which the animal pushes itself 

 along by strokes — while in walking, the motion of the 

 legs is not different from what it would be if they were 

 on land. Most insects that swim have their posterior 

 legs peculiarly fitted for it, either by a dense fringe of 

 hairs on the shank and foot, as in the water-beetles 

 (Di/tiscusy, or the water-boatmen (Notonecta); or by 

 having their terminal joints very much dilated — as in 



• Platf. XIV. riG. 6. 



