NO. 2 THORACIC MECHANISM OF A GRASSHOPPER — SNODGRASS 3 



or lost. This usage has convenience for descriptive purposes, but it is 

 likely to confuse our morphological conceptions, since an anatomical 

 part is the same thing regardless of the nature of its surface covering. 



I. GENERAL DISCUSSION 



If all that has been written about the thorax of insects were true, 

 or could be made to fit with our present knowledge of insect struc- 



Scx 



- -^Eutn 



Fig. I. — Diagram of the theoretical structure of a primitive thoracic segment. 



The tergum (T) includes the segmental and preceding intersegmental sclero- 

 tization of the dorsum; the ventral sclerotization consists of a primary seg- 

 mental sternal plate (Stn) and intersegmental intersternites (Ist) ; the pleural 

 area is occupied by a basal subsegment of the leg, the subcoxa (Sex), divided 

 dorsally into a eupleuron {Etipl) and a eutrochantin (Eutn). 



acs, antecostal suture ; c, d, dorsal and ventral subcoxo-coxal articulations ; 

 Eupl, eupleuron; Eutn, eutrochantin; Isg, primary intersegmental line; 1st, in- 

 tersternite ; Mb, secondary intersegmental membrane ; Pc, precosta ; Sex, sub- 

 coxa ; Stn, primary sternite ; T, tergum. 



ture, there would be little need of prefacing a special description of 

 the thoracic skeleton and musculature of the grasshopper with a gen- 

 eral discussion. Science, however, is not a collection of facts but a 

 concept in which to hold the facts. As our collections of facts become 

 larger, our concepts must be altered and enlarged from time to time. 

 Moreover, we often think that we have nicely fitted a fact into a 

 mental container, only to discover presently that it does not fit at all, 

 or that an important part of the fact has been left out. There is noth- 



