OUTLINES OF ENTOMOLOGY. 



453 



CHAPTER II. 



EXTERNAL STRUCTURE OF INSECTS. 



Ichneumon fly ( Ophion macrurum , L ) 

 after Riley. 



In our introductory chapter were 

 noted some of the differences between 

 )insects and vertebrate animals, and also 

 between insects and other articulates. 

 Now in order to comprehend these dis- 

 tinctions still more clearly, and to under- 

 stand how the various parts and organs 

 are modified to produce the almost 

 innumerable diverse forms, it becomes 

 necessary to examine the general struc- 

 ture still more critically. 

 The external crust or skeleton of an 

 insect forms a many-jointed tube in which, as has already been men- 

 tioned, are enclosed and protected all the softer vital parts, such as 

 the muscles, nerves, the air and blood vessels, etc. The most common 

 form of this tube is more or less cylindrical, but in some insects it is 

 shortened and flattened so that the outline is oblong, oval or nearly 

 circular. This body-wall is composed of fourteen ring-like sections or 

 segments which are more or less closely connected. Of these joints 

 the first, forming the head,* is most distinct. The three joints forming 

 the thorax have the appearance of being firmly soldered together, and 

 it is often difficult to trace the divisions In the abdominal region, on 

 the contrary, the rings are loosely connected by a flexible and elastic 

 membrane, which allows them to move freely in any direction. Two 

 or three of the terminal joints of the abdomen are changed from their 

 original shape to form and support stings, piercers, forceps, and the 

 like, so that we are seldom able to distinguish more than six or seven 

 distinct rings in this part of the body. 



It must not be supposed that the fourteen joints are the only divi- 

 sions of the insect skeleton. If this were the case we should never have 

 occasion to admire the grace of the butterfly's flight or the surprising- 

 agility of various beetles and bugs. The fact is that each joint, although 

 it may appear like a simple ring, is composed of from six to nine vari- 

 ously shaped pieces, each piece, however closely fitted to the others, 

 being capable of independent motion by the muscles within. Otherwise 

 an insect would never be able to move a wing or leg or other organ 



*Dr. Packard considers the head as composed of four joints. 



