550 PEOCEEDiyG^ OP THE NATIONAL MUSEUM. vol. xxxvi. 



solidated primary segments, with the appendages of the following 

 or neck segment attached to it forming the labium, and sometimes 

 also with the sternum of this segment intimately fused into its 

 Aentral surface forming a gular sclerite. 



2. The microthorax is formed from the embryonic segment imme- 

 diately following the last of the consolidated head metameres. It is 

 the segment of the neck of the adult. Its sclerites form the cervical 

 sclerites of the neck, often reduced or rudimentary, and the gular 

 plate of the head when such a plate is present. Its appendages 

 always fuse with each other, and become closely associated with, gen- 

 erally attached to, the base of the head, and constitute the labium. 



3. The thorax proper consists of three segments, or of three with 

 the tergum of the first abdominal segment added in the Hymenop- 

 tera. These three segments are primary metameres, and there is no 

 real evidence of each having been formed through a fusion of two or 

 more primitive segments. The original thoi'acic region may have 

 consisted of more than three segments, but if so, the extra segments 

 have disappeared and have taken no part in the formation of the 

 thoracic sclerites in modern insects. 



4. The thoracic sclerites in all insects conform to one definite plan 

 represented diagrannnatically in fig. 3. The sclerites are subdivi- 

 sions of the wall of one primitive segment, and the apparent double 

 nature of each segment is secondary'. Characters that have been 

 urged as sj^ecial evidence to the contrary, such as the ecpiivalence of 

 the episternum and ei)imerum and the doubk' structure of the coxa 

 in some orders, lose their significance when nymphal, larval, and 

 pupal forms ai'e examined. The episternum and the epimerum are 

 subdivisions of one original ])late on the side of the segment, and the 

 posterior segment of the coxa shown by some orders is simply a 

 detached i)iece of the epimerum fused upon the coxa in the adult 

 stage. 



5. The i:)rimitive tergum is a single undivided plate froin the entire 

 lateral margins of which the wings arise. This simple tergal struc- 

 ture is shown by the nymphs of all the lower insects antl by many 

 larva' and ])upa' and by the adults of Orthoptera. In the adults of 

 all the other principal orders, however, there is present behind this 

 wing-bearing plate or true notum a second tergal jDlate, the postnotum 

 or pseudonotum, developed in the intersegmental memljrane of the 

 nymph or pupa, having no connection with the wings, but attached 

 to the upper ends of the epimera. 



Tlie notum presents three fundamental ridges on its ventral sur- 

 face as shown in text fig. 2. Its subdivisions do not, in most cases, 

 closely conform with these ridges, nor do they strictly correspond 

 in all the orders. They are in genei-al similar but not necessiirily 

 homoloodiis. 



