76 



AN INTRODUCTION TO ENTOMOLOGY 



b. THE APPENDAGES OF THE ABDOMEN 



In the early embryonic stages of insects, each segment of the 

 abdomen, except the telson, bears a pair of appendages (Fig. 89) . This 

 indicates that the primitive ancestor of insects possessed many legs, 

 like a centipede. But the appendages of the first 

 seven abdominal segments are usually lost during 

 embryonic life, these segments being without appen- 

 dages in postembryonic stages, except in certain 

 Thysanura and Collembola, and in some larvae. 



Reference is made here merely to the primary 

 appendages of the segments, those that are homodyna- 

 mous with the thoracic legs; secondarily developed 

 appendages, as for example, the tracheal gills, are 

 present in the immature instars of many insects. 

 The styli or vestigial legs of certain Thysanura. — In 

 certain Thysanura the coxa of each middle and hind 

 thoracic leg bears a small appendage, the stylus (Fig. 90) ; 

 and on from one to nine abdominal segments there is 

 a pair of similar styli. These abdominal styli are 

 believed to be homodynamous with those of the thoracic 

 legs, and must, therefore, be regarded as vestiges of 

 abdominal legs. 



The collophore of the Collembola. — Although in the 

 postembryonic stages of Collembola the collophore is 

 an unpaired organ on the middle line of the ventral aspect of the first 

 abdominal segment, the fact that it arises in the embryo as a pair of 

 appendages comparable in position to the thoracic legs, has led to the 

 belief that it represents the legs of this segment. The structtue of 

 the collophore is described more fully later in the chapter treating of 

 the Collembola. 



The spring of the Collembola. — The spring of the Collembola, 

 like the collophore, is believed to represent a pair of primary append- 

 ages. This organ is discussed in the chapter treating of the Col- 

 lembola. 



The genitalia. — In most insects there are more or less prominent 

 appendages connected with the reproductive organs. These append- 

 ages constitute in males the genital claspers and in females the ovi- 

 positor; to them have been applied the general term genitalia, they 

 are also known as the gonapophyses. 



The genitalia, when all are developed consist of three pairs of 

 appendages. Writers vary greatly in their views regarding the seg- 



Fig.89.-Em- 

 bryo of Hy- 

 drophilus 

 showing ab- 

 dominal ap- 

 pendages. 



