2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



evolution of insect structures that unquestionably might be brought 

 to more satisfactory conclusions if entomologists could get over the 

 habit of thinking of insects purely as insects. Since insects are arthro- 

 pods, a wider survey of the general field of arthropod anatomy will 

 often lead to a better understanding of things about insect structure 

 than can be derived from an intensive study of the insects themselves. 

 For this reason the present paper, which has for its object an under- 

 standing of the fundamental nature of insect genitalia, includes a 

 review of the genital organs in the major groups of the Arthropoda 

 and, for good measure, also in the Onychophora. 



The somatic adaptations of animals to the function of reproduction 

 are in general of a superficial nature ; they have not brought about the 

 development of any fundamental organ or system of organs com- 

 parable with the structures that subserve most of the other important 

 activities of the animal, such as locomotion, orientation, alimentation, 

 or blood circulation. This condition evidently arises from the fact that 

 the germ cells, in a physiological sense, are parasites of the soma ; 

 though they are given lodgment within the body, the accommodations 

 for their growth and discharge have been built up by alterations of 

 structures already present for some other purpose, or by the addition 

 of rather haphazard accessories. In the annelid— onychophoran- 

 arthropod series of animals the only common feature of the reproduc- 

 tive system is the inclusion of the primary germ cells in the mesoderm. 

 The germ cells liberated from the mesoderm are never discharged 

 directly to the exterior through the ectoderm, as in some of the 

 Coelenterata ; they are first given off internally, usually into the 

 coelome or confined parts of the latter, where they undergo their 

 development into ova or spermatozoa, and the gametes must then be 

 extruded from the body cavity to the outside through openings in the 

 body wall. The evolution of the reproductive organs in the Arthropoda 

 has been largely a matter of developing special containers for the 

 maturing germ cells and of establishing exit passages for the gametes. 

 External accessories have been added in most groups to assure insemi- 

 nation of the female by the male or to aid the female in the deposition 

 of the fertilized eggs. 



ENUMERATION OF THE ARTHROPOD BODY SEGMENTS 



Because of the great variation in the position of the genital open- 

 ings in the Arthropoda, it becomes highly desirable, in a comparative 

 study of the external genitalia, to be able to identify and briefly desig- 

 nate the corresponding body segments in the several arthropod groups. 



