548 
MR. NEWPORT ON THE RESPIRATION OF INSECTS. 
through the ramifying and anastomosing tracheae, sufficient for the purposes of re- 
spiration, which is probably the result of several combined actions. The experi- 
mental fact observed by Reaumur, that the anterior pair of spiracles is the most im- 
portant to the insect, connected with that of the gradual obliteration of the last two 
pairs during the changes to the perfect state, and the great extent to which the an- 
terior spiracles are developed as the insect approaches that condition, lead us to con- 
clude at once that it is chiefly through the anterior spiracles that respiration is per- 
formed. The action of the retractores spiracula muscles necessarily tends to contract 
the segments and close the spiracles, and thus, as it were, pump on the air through 
the longitudinal tracheae towards the anterior or thoracic ones ; while the relaxation 
of these muscles, and of the other oblique and longitudinal ones in each abdominal 
segment during the time the muscles of the thorax are in action, must naturally tend 
to enlarge the capacity of the body and induce an act of inspiration. Indeed we have 
full proof that respiration is performed by the alternate contraction and relaxation of 
the abdominal muscles in what takes place in Orthopterous, Hemipterous, and many 
Coleopterous insects. Carus has well remarked, that the abdominal segments, par- 
ticularly in Locusta , are alternately elevated and depressed, like the ribs of Vertebrata; 
and every one must have observed the same thing in the larger Bombi, when fatigued, 
upon alighting after flight, and when excited. The contractions and relaxations of 
the muscles, and consequently the acts of inspiration and expiration, are then short 
and quick in proportion to the degree of excitement, which is sometimes so great that 
the whole abdomen is alternately extended and retracted just as the flanks and ribs 
of the racehorse alternate with each inspiration after a long and severely contested 
struggle on the course. In the Gryllus viridissimns , Linn., when excited I have counted 
thirty-seven contractions, corresponding to as many respirations, per minute, and 
these were precisely analogous to similar acts in Vertebrata. Thus several short 
contractions take place in regular succession at stated intervals, and these are fol- 
lowed by one more long and powerful than the rest. A slight pause then ensues, 
and the short contractions again commence, until they are followed at a certain in- 
terval by another long one, which is evidently a full inspiration, and takes place at 
no stated interval. When the insect is very much excited the interval between the 
long inspirations is greater, and the inspirations when made are more full and power- 
ful. This view of the manner in which respiration is performed in insects is supported 
by the test of experiment. 
If one of the larger moths be submerged in water, a few bubbles of air will be seen 
to arise from all the spiracles on each side of the body, but chiefly from the first, 
second, and third pairs. These bubbles diminish in size as we proceed towards the 
posterior extremity of the insect, the first and second pairs being the largest. During 
the changes of the insect these spiracles are greatly increased in size, and often con- 
siderably altered in form, while the spiracles and tracheae in the posterior portion of 
the body are greatly diminished, and the anal pair which existed in the larva is 
