30 FLIGHT OF BIRDS AND INSECTS. 
lightness and strength to the wing. The wings are moved by pow- 
erful muscles of flight, filling up the cavity of the thorax, just as 
the muscles are largest about the thorax of a bird. Moreover in 
the body of insects that fly (such as the bee, cock-chafer and dragon 
fly), as distinguished from those that creep exclusively, the air 
tubes (tracheæ) which ramify into every part of the body, are di- 
lated here and there, especially in the base of the abdomen, into 
large sacs, which are filled with air, when the insect is about to 
take flight, so that the specific gravity of the body is greatly di- 
minished. Indeed, these air sacs, dilatable at will by the insect, 
may be compared to the swimming bladder of fishes, which enables 
them to rise and fall at will to different levels in the sea, thus 
effecting an immense saving of the labor of swimming. In the 
birds, as everybody knows who has eaten a chicken, or attended 
the dissection of a Thanksgiving turkey, the soft parts are exter- 
nal, attached to the bony framework comprising the skeleton, the 
wing bones being directly connected with the central back bone; 
so that while these two sorts of animated machines are so differ- 
ent in structure, they yet act in much the same manner when on 
the wing. The differences are clearly stated by Marey, some of 
whose conclusions we now give almost word for word. 
The flight of butterflies and moths differs from that of a bird, 
in the almost vertical direction of the stroke of their wings, and 
Fig. 9. in their faculty of sailing in the air without 
making any movements; though sometimes in 
the course they pursue, they seem to resemble 
birds in their flight. 
The flight of insects and birds differs in the 
form of the trajectory of the wing in space; in 
the inclination of the plane in which the wings 
| beat; in the réle of each of the two alternating 
(and in an inverse sense) movements that the 
wings execute ; as also in the facility with which 
the air is decomposed during these different 
movements. As the wings of a fly are adorned 
with a brilliant array of colors, we can follow the trajectory, 
‘figure, that each wing writes in the air; it is of the form of 4 
figure of eight (Fig. 9), first discovered by Professor J. Bel 
Pettigrew of Edinburgh. 
By an ingenious machine specially devised for the purpos®s 
