June 1, 1891.] 



KNOWLEDGE 



113 



kingdom, iu which are also included spiders and crabs ; and 

 the Vertebrates, which form a sub-kingdom to themselves. 

 Whereas, however, by far the great majority of Insects 

 are endued with this faculty, among the Vertebrates it is 

 only in the class of Birds that we meet with a similar 

 preponderance of species which enjoy this kind of locomo- 

 tion, although all the members of certain orders — the 

 Pterodactyles and Bats — are similarly endowed. More- 

 over, we have to draw a distinction between true flight, 

 as exemplified by Tiirds. Biits, and Insects, and what wo 

 may call spurious Hight, of which we have examples in 

 Flying Phalaugers, Flying Squirrels, and Flying Fish. 

 True flight is performed by an alternate upward and down- 

 ward motion of the wings, or special organs of flight, and 

 can be indefinitely prolonged until the muscular powers of 

 the flyer are exhausted. On the other hand, spurious 

 Hight is merely a prolongation of a downward or upward 

 leap by means of parachute-like expansions developed 

 on the sides of the body, or, as in the Flying Fish, 

 by passive extension of wing-like organs, and it can 

 never be extended beyond the limits of the initial velocity 

 of the original leap. This distinction between true and 

 spurious flight is a very important one, since it shows us 

 that the animals endowed with the former power are limited 

 to four groups, namely. Insects, the extinct Pterodactyles 

 or Flying Dragons, Birds, and Bats. Spurious flight, on 

 the other hand, is found in Flying Fish, Flying Lizards, 

 Flying Phalangers, Flying Squirrels, and Flying Lemurs. 

 Among those animals capable of true flight a broad line 

 of distinction separates the Insects from the Vertebrates 

 in regard to the organs set apart for this particular pur- 

 pose. Thus, whereas in Insects, all of which are provided 

 with six pairs of legs, the wings, or special organs of Hight, 

 are frcijuently four in number, and are in all cases dcvelojied 

 from the back of the body, entirely independent of the legs ; 

 in Vertebrates, where the number of legs never exceeds 

 four, the two wings are always formed by special modifi- 

 cation of iho first pair of legs. It is therefore evident 

 that although the wings of Insects, as performing similar 

 functions, are analogous with those of Vertebrates, yet, 

 as being structurally quit^ difl'erent, they are in no sense 

 homologous with the same. 



The special modification of the first pair of legs to sub- 

 serve the purpose of flight in those Vertebrates which 

 possess this power iu its true form, may be taken as an 

 indication that such Vertebrates have originally descended 

 from others in which that power was not developed. 

 Although we have no such guide in the case of Insects, 

 yet the circumstance that in all those kinds which undergo 

 a complete metamorphosis no traces of wings are obser- 

 vable in their larvse, points with equal clearness to the 

 conclusion that those creatures have been likewise derived 

 from crawling ancestors, and that their power of flight is 

 an acquired one. Those Insects which are unable to fly 

 must not, however, be regarded as ancestral forms, since 

 there is clear evidence that their wings have been lost or 

 have became rudimentary. It has been already mentioned 

 that while all flying Vertebrates have only a single pair 

 of wings, many Insects are provided with two pairs of 

 these organs ; and from the tendency among Insects for 

 one or other of these pairs of wings either to disappear or 

 to be modified for other purposes, it would appear that a 

 single pair is decidedly the best suited for flight. 



We .shall now proceed to trace some of tlie chief modi- 

 fications in the organs of flight in the difl'ercnt groups of 

 animals, commencing witli Insi'cis, in which, as already 

 observed, we nevi'r meet with spurious flight. 



hi all the iSeetles, or C'oleoptera, whicli form the first 

 order of Insects, the front pair of wings are nuKHfiid into 



the well-known horn-like wing-covers, or I'hjtra, beneath 

 which the membranous second pair are neatly folded 

 during such times as the creatm'es are not engaged in 

 flight. In some kinds, such as the Stag-Beetle and the 

 Water-Beetles, these wing-cases are long, and extend 

 backwards to the hinder extremity of the body ; but in 

 others, like the well-known " Devil's Coach-Hor.se," they 

 are extremely short. The modification of the front wings 

 into wing-covers clearly indicates that Beetles are a 

 highly specialised group ; the extreme development of 

 this specialisation occurring iu certain species like the 

 Oil Beetle, where the second pair of wings have also 

 become rudimentary, so as to render their owner incapable 

 of flight. In some degree a confirmation of this spe- 

 cialisation is aflbrded by the circumstance that Beetles are 

 not known in the fossil condition so far down in the 

 geological scale as are some of the more generalised 

 groups of insects. 



In the Bees, Wasps, Ants, and other members of the 

 second order Hymenoirtera, both pairs of wings are mem- 

 branous and adapted for flight ; the front pair being, 

 however, considerably the larger of the two. In the 

 Caddis Flies and other Neuroptera, both pairs of wings 

 are likewise fully developed and membranous in structure, 

 although diSering in the mode of arrangement of their 

 veins. jNIoreover, the hinder pair are frequently nearly 



as large as the front 

 ones, a circumstance 

 which seems to indicate 

 that the whole group 

 is a more generalised 

 one. The development 

 of the well - known 

 minute scales on both 

 pairs of wings in the 

 Butterflies and Jloths 

 readily distinguishes 

 the Lepidoptera from 

 all other insects, and 

 likewise suggests that 

 they form a much spe- 

 ciaU-sed modification of 

 the class. Still greater 

 .specialisation as re- 

 gards their organs of 

 flight is, however, pre- 

 sented by the Flies and 

 Gnats, constituting the 

 order Diptera, in which, 

 while the front pair of 

 wings are large and 

 membranous, or hairy, 

 the second pair are reduced to small, drumstick- 

 like processes termed balancers, or Imluns, which are of 

 no sort of use in flight, and are typical rudimentary 

 organs. The specialisation of the wing-structure in this 

 group is, therefore, exactly the opposite of what occurs 

 among the r>eetles, where, as we have already seen, it is the 

 first pair of wings which takes no part iu flight. In the 

 Cicadas and Bugs (Fig. 1), constituting the order Khynchota, 

 the wings, when present, arc tour in number, ^it the first 

 pair may be converted into horny wing-covers, as in the 

 Beetles. Like those of the next order, all the members of 

 this group difl'er from the insects mentioned above in that 

 they do not undergo a complete metamorphosis before 

 attaining their final perfect state. The last order that we 

 have to notice is the Orthoptera, in which are grouped the 

 Cirasshoppers, the Cockroaches, the Karwigs. the Dragon- 

 flics, etc. Except in a few parasitic and some other 



Fio. 1. — Enlabged View of a Flt- 

 «G Bug, with the Wings closed. 



