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 oi'gans 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 (Otyctes nasicomis) 
twenty, and in another (Acridaviridissima) fifty, 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 : it is probable also, 
that the temperature may accelerate or retard the mo- 
tion. In the summer I examined a specimen of Phyllo- 
pertha horticola, 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 (Locusta). 
a Sprengel, Commcntar. 7 — . 
b Sprengel, from whom I have borrowed this quotation, expresses 
the time by " scripulo hone." 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 respirat. insect. 27, 46, 66. Sprengel ubi 
supr. 1 1 — . 
