Tra7is. N. V. Ac. Set. 166 May 8, 



sphere, flight requires special apparatus and structure and very great 

 muscular power. But insects and birds outstrip all other creatures. 

 Their whole structure is wonderfully adapted to aerial life ; the skele- 

 ton light, the body filled with air, the wings elastic and light, but firm, 

 and the wing muscles exceedingly powerful. Wings of birds are quite 

 similar in principle and structure, hut wings of insects greatly differ. 

 Three classes of insect wings may be noticed : the transparent wings 

 of the bee, fly, etc. : the dense opaque wing covers of the beetles : and 

 the scale-covered wings of the butterfly and moth. 



The wing of tfie bat is an expansion of skin stretched upon the long 

 fingers and from arm to leg. Dragons passed away with the age of 

 reptiles. The so-called flying dragon has only a parachute of expanded 

 skin, by aid of which it can sail downward through the air. 

 The flying squirrel has a similar apparatus. In the flying- 

 fish, the pectoral tins are very large, and serve a similar purpose. 

 Some spiders are able to sail the air by means of kites or sails made of 

 their silken webs. 



Locomotion on Solids. — A much greater variety of organs are used 

 for moving on solids. The leech, star-fish and cuttlefish employ suckers, 

 which are little cupping glasses or diminutive water-pumps. The 

 earth-worn has minute spines, and the serpent large scales or scutes on 

 its belly ; caterpillars have pins rather than legs. The foot of the clam 

 is a fleshy protuberance which can be pushed into the sand, enlarged at 

 the end and then shortened. The snail glides over the surface by sets 

 of short muscles in the under side of its body ; flies adhere by a fluid, 

 exuded by the hairs on the surface of their foot discs. 



Of jointed limbs the millipede has, in some species, two or three 

 hundred. The centipede has thirty or forty legs ; crabs and lobsters 

 have ten legs, but two used as pincers. The spider has eight legs, and 

 the true insects have six legs. The reduction in the number ot similar 

 organs indicates advance in locomotive powers. 



As the skeletons of all vertebrates are built en the same general 

 plan, the four hmbs of all show a certain resemblance ; of course, the 

 similarity is most evident in the legs of quadrupeds. 



The limb bones of fishes are in nearly all cases within the body, as 

 the basis of the paired fins. But in some fishes they are sufficiently de- 

 veloped to be useful for crawling. In reptiles the limbs are generally 

 awkward on land, although some lizards are agile. The legs ot birds 

 are highly specialized. 



While mostly intended for running on land, the mammals neverthe- 

 less exhibit a greater diversity of organs and methods of locomotion 

 than any other class of animals. Here we find the swimming whale 

 and seal, the flying bat, the sailing lemur and squirrel.fthe^^creeping 



