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* (Spiraculd), or breathing pores, are small 

 orifices in the trunk or abdomen of insects, opening into 

 the trachea,, by which the air enters the>boc!y, 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 opening 

 and shutting. A spiracle therefore, speaking analogi- 



a PLATE XXIII. FIG. 2. arid PLATES VIII. IX. XVI. XXIX. c', h", 

 m", A", JO". 



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. 



