2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, 85 



the thorax. The series of segmental appendages is continuous, and 

 uniform except for minor adaptational modifications, from the head 

 to the metathorax, inclusive ; on the abdomen the appendages abruptly 

 cease, or are greatly altered in form. The musculature of the abdomen, 

 consequently, is sharply differentiated from that of the thorax. Even 

 the respiratory mechanism attests that the thoracico-abdominal bound- 

 ary marks some deep-seated change in the body organization, since 

 the closing apparatus of the spiracles is almost invariably different 

 in one way or another before and behind the intersegmental fold 

 separating the thorax from the abdomen. The definitive head contains 

 three segments that have been, comparatively speaking, but recently 

 transferred from the body to the cephalic region ; the waist line of 

 the insect has long been definitely established, and only in a single 

 order has an abdominal segment been given over to the thorax. The 

 abdomen is distinctly the visceral region of the body, and its major 

 active functions in adult insects are those of respiration, copulation, 

 and oviposition. 



Yet, notwithstanding the functional and structural differences that 

 have come to separate the insect body into cephalothoracic and ab- 

 dominal regions, we can not avoid the assumption that modern in- 

 sects are derived from polypod ancestors, and that the abdominal 

 segments at some time in the past history of the Hexapoda had the 

 same essential structure as that of the primitive thoracic and gnathal 

 segments. In studying the morphology of the abdomen and its ap- 

 pendicular organs, therefore, we must attempt to find in the modern 

 structure a basic plan of organization the same as that we are led to 

 believe exists in the cephalic and thoracic regions from a* study of the 

 segments, the appendages, and the musculature in the preabdominal 

 parts of the body. Considering, then, the nature of the task, it is not 

 surprising that students of insect morphology find in the abdomen 

 problems far more difficult of solution than are those encountered in 

 the head or thorax, and that there are many fundamental problems 

 in the abdomen which are still unsolved. 



To the systematist in entomology the study of the abdomen, or 

 particularly of the genital appendages, is becoming of ever increasing 

 importance, and specialists are coming to feel acutely the need of a 

 fundamental understanding of the organs that have been found in so 

 many cases to give the best characters by which species may be dis- 

 tinguished. Unfortunately, however, no investigator has yet discovered 

 a means for determining with certainty the homologies of the organs 

 most useful for diagnostic purposes. In truth, we may say that the 



