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CHAPTER IV. 
ANATOMY AND PHYSIOLOGY. 
THORAX. EXTERNAL SKELETON. CONTENTS. APPENDAGES ‘—-LEGS, 
WINGS. MECHANISM OF FLIGHT. 
THE thorax of insects presents to ‘the entomo- 
logist a problem very like that which comparative 
anatomists have found in the vertebrate skull. Both 
these parts are composed of elements which appear 
in a simpler form in other parts of the body, im the 
rings of the abdomen and the spinal vertebre respec- 
tively. The various forms, however, which the cor- 
responding parts assume are not always easily re- 
ferred to their original types. Their identification 
is often, indeed, a work of great difficulty. Excessive 
development in one, almost complete obsolescence in 
another, and fusion of distinct elements into a com- 
pact form in a third, produce forms im which none 
but the most skilled anatomists can clearly trace the 
homologies of insect or vertebrate structure. 
Entomologists have the more difficult task in 
this analysis, on account of the minuteness of the 
subject of their examination, and of the want of 
much of that help which comparative anatomists 
obtain from observmg the gradual development of 
