40 INTERNAL ANATOMY OF INSECTS. 



water-beetle (Dytiscusmarginalis), the spiracles are closed 

 by a semifluid substance, which however, according to 

 Sprengel, is permeable to the air a . The animal, where 

 these organs are furnished with lips, has doubtless, by 

 means of a muscular apparatus, the power of opening 

 and shutting them : this is done, we are told, by elevating 

 and depressing, or ratker by contracting and relaxing 

 them. Sorg counted in one case (Oryctes nasicornis) 

 twenty, and in another (Acridavtridissima) Jiffy, of these 

 motions to take place in little more than two minutes b : 

 but the quickness and force of this motion is not always 

 uniform ; for the same physiologist observed, that in 

 Carabus auratus, when feeding or moving its body ra- 

 pidly, the contraction of the spiracles took place at very 

 short intervals ; but when it was fasting, and its motions 

 were slow, the intervals were longer c : it is probable also, 

 that the temperature may accelerate or retard the mo- 

 tion. In the summer I examined a specimen of Phyllo- 

 pertJia hortzcola, that had indeed been somewhat injured, 

 with this view : the pulses of the abdomen, which alter- 

 nately rose and fell, were at about the rate of the pulse 

 of a man in health, sixty in a minute, and the spiracles 

 appeared to me to keep pace with this motion : later in 

 the year, when the temperature was lower, as I was walk- 

 ing, I took a specimen of some grasshopper (Locust a). 



* Sprengel, Commentar. 7 . 



b Sprengel, from whom I have borrowed this quotation, expresses 

 the time by " scripulo hora:" This word is of uncertain meaning, 

 being scarcely ever applied to time ; but as it means the twenty-fourth 

 part of an ounce, Faber conjectures it may mean the same portion 

 of an hour. 



c Sorg, Disquisit. circa rcspirat. insect. 27, 46, 66. Sprengel ubi 

 supr, 11. 



