

222 DR. PETTIGREW ON THE MECHANISM OF FLIGHT. 



The inference to be deduced from the foregoing observations and experiments is plainly 

 this, that even in large-bodied, small-winged insects and birds, the wing-surface is greatly 

 in excess, the surplus wing-area supplying that degree of sustaining- or elevating-power 

 which is necessary to prevent undue exertion on the part of the animal. In this we 

 have a partial explanation of the buoyancy of insects, and the great lifting-power pos- 

 sessed by bats and birds — the bats carrying their young without inconvenience, the 

 birds elevating surprising quantities of fish, game, carrion, &c. 



The movements of the wings in insects, bats, and birds are essentially the same. It is 

 otherwise with the structure of the wings, the wing of the insect being in some respects 

 rudimentary as compared with that of the bat and bird. The pinion in either case is to 

 be regarded as a living, delicately- organized structure, whose parts and proportions are 

 adjusted with mathematical accuracy to the strength of the animal and the rapidity of 

 the vibrations necessary to raise its weight from the ground. The wing, as a rule, is more 

 or less triangular in shape (Plate XV. figs. 63, 65, and 66), the base of the triangle being 

 directed towards the body, the sides anteriorly and posteriorly. It is also conical in sec- 

 tion from within outwards and from before backwards, this shape converting the pinion into 

 a delicately graduated instrument, balanced with the utmost nicety, to satisfy the require- 

 ments of the muscular system on the one hand and the resistance and resiliency of the air on 

 the other. Those conditions are necessary to correct the unequal strain to which the dif- 

 ferent portions of the wing are exposed in action, as well as to prevent shock to the 

 system. "While all wings are graduated as explained, innumerable varieties occur as to 

 their general contour, some being falcated or scythe-like, others oblong, others rounded 

 or circular, some lanceolate, and some linear. Thus far a parallelism may be established 

 between the wing of the insect, bat, and bird ; but when we come to speak of the margins 

 of the pinion, we find the wings of bats and birds present little variety, whereas those of 

 insects may be crenated, dentated, ciliated, fimbriated, digitated, or caudated. Still 

 greater differences occur as to surfaces, the wings of bats and birds presenting a uniform 

 appearance, those of insects being hairy, naked, farinose, scaly, veined, reticulated, and 

 striated, as happens. 



THE WIXGS OF INSECTS. 



Elytra or wing-cases—their shape and tises.— The wings of insects may consist either 

 of one (Plate XIII. fig. 20) or two (Plate XIII. fig. 24) pairs,— the anterior or upper 

 pair, when two are present, being in some instances greatly modified and present- 

 ing a corneous condition (Plate XIII. fig. 16 r). When so modified, they cover the 

 under wings (e) when the insect is reposing, and have from this circumstance been 

 named elytra, from the Greek ?X UT/ >ov, a sheath. The elytra, or wing-cases, as they are 

 sometimes called, are dense, rigid, and opaque in the beetles (Plate XIII. fig. 16 r), 

 solid in one part and membranaceous in another in the cockroaches (Plate XIII. fi 

 21 r), more or less membranous throughout in the grasshoppers, and completely mem- 

 branous in the dragonflies (Plate XIII. fig. 24 *} The elytra, otherwise known as the 

 superior or upper wings, are inclined at a certain angle when extended, and are indirectly 

 connected with flight in the beetles, cockroaches, and grasshoppers. They are actively 

 engaged in this function in the dragonflies and butterflies. The elytra or antnior wings 



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