40. INTERNAL ANATOMY OF INSECTS.- 
water-beetle (Dytiscus maiginalis), the spiracles are closed 
by a semifluid substance, which however, aeons 
Sprengel, is permeable to the air*. 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 rather by contracting and relaxing them. 
Sorg counted in one case (Oryctes nasicornis) twenty, and 
in another (Acrida viridissima) fifty, of these motions to 
take place in little more than ¢wo minutes’: but the 
quickness and force of this motion is not always uniform ; 
for the same physiologist observed, that in Carabus au- 
ratus, when feeding or moving its body rapidly, the con- 
traction of the spiracles took place at very short intervals ; 
but when it was fasting, 
intervals were longer‘: it is probable also, that the tem- 
and its motions were slow, the 
perature may accelerate or retard the motion. In the 
summer I examined a specimen of Melolontha hirticola, 
that had indeed been somewhat injured, with this view : 
the pulses of the abdomen, which alternately 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 walking, I took a spe- 
cimen of some grasshopper (Locusta Leach). Upon 
* Sprengel, Commentar. 7—. 
» Sprengel, from whom I have borrowed this quotation, expresses 
the time by “ scripulo hore’? 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. 
* Sorg, Disquisit. circa resptrat. insect. 27, 46, 66. Sprengel ui 
supr. 1I—, 
