2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8o 



INTRODUCTION 



Nowhere has nature more strikingly displayed her mechanical 

 genius than in the thorax of a winged insect ; nowhere else can we find 

 a mechanism so compact, so efficient, so simple, and yet of such varied 

 powers. Locomotion by the coordinated action of three pairs of legs, 

 flight by the unified vibration of two pairs of wings — these are the 

 common functions of the thorax ; but, add to them the powers of 

 leaping, grasping, climbing, digging, swimming and many others of 

 which the legs of various insects are capable, and consider that the 

 Vikings may carry the body forward or backward slowly or with great 

 speed, or keep it hovering almost stationary in the air, while, rubbed 

 upon each other, by some insects they can be made to produce sounds 

 of great volume, and it becomes needless to repeat that the insect 

 thorax is a marvelous bit of machinery. 



If we had but to describe the thorax as it is, the task of the anato- 

 mist would not be a simple one, but it is always necessary to look 

 beyond the facts that confront us and to discover the more funda- 

 mental structures upon which they are reared, an undertaking which 

 requires redoubled efifort, but without which there can be no true 

 morphology. An artist may depict the form and color of a building 

 in a manner pleasing to the eye, but, unless he has understood the 

 framework and the principles of its construction, his picture cannot 

 be convincing to the mind. 



It is certain that insects did not start out to be either six-legged 

 creatures or winged creatures, and that, during their evolution, the 

 thorax has been continually refashioned to adapt it to the new modes 

 of locomotion. The embryonic history of insects shows that the 

 thorax was first differentiated as the locomotor region of the body 

 by the specialization of its three pairs of segmental appendages as the 

 principal organs of progression, this being accompanied by the reduc- 

 tion of the gnathal appendages to feeding organs, and by the suppres- 

 sion of most of the abdominal appendages (fig. i). Walking or 

 running on three pairs of limbs instead of on many, now, not only 

 involved a perfection of these appendages themselves, and a con- 

 siderable amount of reconstruction in the segments bearing them, in 

 order to give the legs better support and their muscles more effective 

 attachment, but it necessitated a remodeling of the general body 

 structure and proportions to effect a proper balance about the newly 

 localized center of gravity. When, at some later period, lateral expan- 

 sions of the tergal plates began to serve, perhaps, as parachutes, and 

 insects became gliders, it is but natural that the tergal lobes which 



