1919] Crampfon — Terga and Wing-Bases of Insects 59 



reduced to a narrow transverse band in the tergal region of the 

 Diptera, I have restricted the present discussion to the mesonotum 

 alone, since it is usually well developed in most of the insects of 

 this group. 



In the mesonotum of the insects here discussed, there is a tend- 

 ency for the prescutum ("psc," Figs. 1, 3, 5, and 6) to become pro- 

 longed further backward into the scutal region "sc," thereby be- 

 coming longer than broad, while in the lower insects it is frequently 

 broader than long. On the other hand, the scutellum, "si," which 

 may become very narrow and prolonged far forward into the scutal 

 region, "sc," in such lower insects as the Blattidse, etc., in the 

 insects here discussed tends to become somewhat broader than 

 long. The mesothoracic postscutellum, "psl," is usually unde- 

 veloped in such lower insects as the Blattidae, Mantidse, Acrididse, 

 Dermaptera, Coleoptera, etc., but in the Embiidae and Plecoptera 

 it is quite well developed, as in the insects under consideration. 

 The mesothoracic tegula "t" is frequently quite large in the 

 insects under discussion, and is usually developed only in the 

 mesothorax, while in the lower forms, it is usually small, or not 

 developed in the mesothorax, although in some cases it may be 

 developed in both meso and metathorax in the lower insects. 

 The sclerite labeled "a" in Figs. 1,2, 4, etc., is usually not well 

 developed in lower insects, while in many of the higher insects it is 

 quite large, its better development in the latter insects being 

 probably correlated with the superior powers of flight in the 

 higher forms. I have not observed the middorsal suture "ms" 

 in many lower insects (excepting the Plecoptera and Embiidse), 

 while it appears to be present in many of the higher insects, 

 although it is not present in all of them. 



As may be seen from the accompanying diagrams (Plate II) the 

 general plan of the sclerites is relatively simple, and is adhered to 

 quite closely by most of the insects here shown. It is thus a much 

 simpler matter to compare the different insects together in at- 

 tempting to trace their paths of development than is the case with 

 the wing venation, where the complex and intricate patterns, with 

 their bewildering array of modifications, make it very difficult to 

 trace out the paths of development followed by the different groups 

 of insects, unless one has practically all of the intermediate stages; 

 and even then he may be led astray by the study of only one set of 



