14 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, 85 



The frequent occurrence of styli on abdominal segments of insects 

 in which the sterna are undivided plates leaves little doubt that the 

 definitive abdominal sterna of insects in general are composite plates 

 including the limb bases as integral parts of their areas. Evidence 

 of the inclusion of the limb bases in the adult abdominal sterna might 

 be derived also from other sources, as in the Lepidoptera, where 

 the abdominal appendages of the larva at the time of pupation are 

 flattened out in the form of discs, and merge into the ventral areas 

 from which later the adult sterna are produced. 



A definitive sternal plate that includes the primitive sternum and 

 the bases of the adjacent limbs is sometimes called a " coxosternum,"' 

 but, as will be shown later, there is a question as to whether the 

 abdominal limb bases represent the coxae or the subcoxae, or include 

 both of these usual basal elements of the appendages. A composite 

 sternal plate, therefore, is more appropriately distinguished from a 

 primitive sternal plate by the term zygosternuni proposed by Prell 

 (1913). For the same reason the name " coxite," often given to the 

 limb base element of the zygosternuni, is objectionable as being more 

 specific in its meaning than is warranted by the known facts of the 

 origin of the part in question. Besides this, the suffix itc implies that 

 a structure so-named is '' a part of " a coxa, and this implication is 

 clearly not intended. 



II. THE ABDOMIN.\L SEGMENTS 



Entomologists sometimes nominally distinguish the segments of 

 the insect abdomen frOm those of the thorax as " urites," a term 

 perhaps recommended by its brevity, but one which, by inference, 

 reduces the entire abdomen to the status of a " tail." Consistent with 

 this usage, the abdominal appendages would all be " uropods," but 

 the custom of carcinologists in applying the latter term only to the 

 terminal pair of appendages has better anatomical sanction. (I^an- 

 kester, 1909; Sedgwick, 1909.) 



From embryological evidence there appears to be little doubt that 

 the primitive number of abdominal segments in typical insects is at 

 least 12 (fig. 5 A). Twelve segments are actually present in adult 

 Protura (B), each having distinct tergal and sternal plates, but tlie 

 tenth and eleventh are said to be added by " epigenesis," that is, they 

 are developed during postembryonic growth. In many of the Aptery- 

 gota and in the lower Pterygota, 1 1 segments are present without 

 question, while in some forms there are possible rudiments of a 

 twelfth segment. The twelfth or primitive terminal segment is the 



