INTERNAL ANATOMY OF INSECTS. 37 
action of the inspired air. They do not breathe through 
the mouth, but through numerous orifices called spiracles, 
and the respiratory vessels connected with these are con- 
ducted to every part of the body. In some indeed, that 
we have included under the denomination of insects, as 
the Arachnida, an approach is made to the branchial 
respiration of fishes. 
The respiratory apparatus of insects may be consi- 
dered under two principal heads : — viz. the orifices or 
spiracles, and other external organs by which the air is 
alternately received and expelled ; and the internal ones, 
by which it is distributed. Each of these is well worthy 
of your attention. 
I. The external respiratory organs of insects may be 
divided into three kinds. Spiracles; Respiratory plates ; 
and branchiform and other pneumatic appendages. 
i. Spiracles 71 (Spiracula), or breathing pores, are small 
orifices in the trunk or abdomen of insects, opening into 
the trachea, by which the air enters the body, or is ex- 
pelled from it b . They may be considered principally as 
to their composition and substance ,• shape ,- colour ; mag- 
nitude ; situation ; and number. ' 
1. Composition and substance. Perhaps you may not 
be aware that the structure of these minute apertures is 
not so simple as at the first view it may seem ; but when 
you recollect that by them the insect breathes, you will 
suspect that provision may be made for their openino- 
and shutting. A spiracle therefore, speaking analogi- 
3 Plate XXIII. Fig. 2. and Plates VIII. IX. XVI. XXIX. c' } k", 
vi", A", D". 
b Moldenhawers {Anat. der Pflanz. 314 — .) affirms that the spira- 
cles of most insects are quite closed : but Sprengel (Commentar. § 8.) 
has satisfactorily refuted that opinion. 
