40 INTRODUCTION TO EVOLUTION 



We are now in position to understand more concerning the significance 

 of the arrangement of insect mouth parts discussed previously (pp. 33-36). 

 It will now be evident, for example, that the mandibles and 1st and 2nd 

 maxillae are paired appendages, modified biramous appendages. In the 

 grasshopper one pair, provided by one metamere of the body, have be- 

 come cutting mandibles. Since a pair of appendages consists of a right- 

 hand appendage and a left-hand one, we can understand why it is that an 

 insect has a right mandible or jaw and a left mandible instead of having 

 an upper jaw and a lower jaw as we have. 



A peculiarity of the sucking tubes or proboscises of such insects as bees 

 and butterflies may have seemed odd to the reader — the fact that they 

 are not tubes having solid and continuous walls but are composed of right 

 and left components held together firmly enough to form a more or less 

 watertight tube. The reason for this type of construction is evident if we 

 conceive that ancestral insects were under the necessity of utilizing the 

 raw materials at hand in evolving these tubes. The raw materials con- 

 sisted of paired, biramous appendages which had already been modified 

 as mouth parts for use with a diet of solid food. These paired elements 

 then received additional modification to form tubes through which liquids 

 could be drawn into the mouth. The resulting "peculiar" tube construc- 

 tions are thus readily understandable upon a basis of descent with modifi- 

 cation, whereas it would be difficult to form a rational explanation for 

 them upon a basis of special creation. 



Vestiges 



Vestigial or rudimentary organs are parts of the body that are relatively 

 small in size and have little, if any, ascertainable function. In every case of 

 importance to the study of evolution they appear to represent useless rem- 

 nants of structures or organs which are large and functional in some other 

 animals. 



The most familiar rudimentary organ in man is the yermiform appen- 

 dix (Fig. 3.12). "Vermiform" suggests its wormlike appearance. The ap- 

 pendix attaches to a short section of the large intestine called the caecum, 

 and the latter is located at the point where the large intestine is joined by 

 the small intestine. The caecum is a short pouch, ending blindly except for 

 the small opening into its extension, the appendix. 



If we study the digestive systems of lower animals we discover that car- 

 nivorous (flesh-eating) mammals have the caecum reduced to a short, 

 blind pouch much like our own. Cats, for example, have a short caecum, 



