PROC. ENT. SOC. WASH., VOL. 20, NO. 3, MAR., 1918 51 



adherent to the thorax." The embryological investigations of 

 Heymons support this view, and Boerner adopts it in his later 

 publications. 



In the early embryonic stages, there is no clear line of demarca- 

 tion between the base of the leg and the pleural region, and this 

 may have given rise to the idea that the pleural plate represents 

 the basal portion of the leg. At any rate, a further study of the 

 embryological development of the region in question should be 

 carried out before this point can be definitely decided; and even 

 if it should prove to be the case that the pleural plate represents 

 a detached basal portion of the leg, it is none the less true that 

 the pleural plate occurs as a separate and distinct plate in the 

 lowest Apterygotan insects, and in many of the immature Ptery- 

 gotan forms, and must therefore have occurred as a distinct 

 plate at a very early stage in the evolution of insects. On this 

 account, I formerly maintained that it probably occurred as a 

 distinct plate from the very beginning, having been formed by 

 the greater deposition of chitin and pigment, due to the stimulus 

 of muscle stress, friction, or other mechanical causes. 



Although the plural plate occurs as a separate and distinct, 

 plate in many immature insects, this is not the case in most 

 adults, and the question naturally arises as to whether the epi- 

 sternum es and the epimeron em of a larval insect, such as that 

 shown in figure 18, really represents the episternum and epi- 

 meron of the adult. A comparison of the larval and adult stages 

 of the same insect would indicate that the pleural plate of the 

 larva merely increases in size (encroaching more and more on 

 the membranous region of the segment) to form the correspond- 

 ing regions of the adult, and that a union of plates originally 

 distinct frequently take place by the further chitinization and 

 pigmentation of the membranous region between them. In 

 some cases, new lines and sutures are formed in the adult stages, 

 thereby tending to mask the original condition, but the principal 

 landmarks usually remain but little changed, and the parts can 

 be homologized by referring them to these landmarks in both 

 adult and larval stages. It may be further remarked, that the 

 larvae usually retain the plates in a condition approximating 

 that found in the lower Apterygota, much more closely than the 

 adults do, and, in most cases, the condition found in the larvae 

 must therefore be considered as much nearer the original one, 

 although the more primitive, rather than the more highly special- 

 ized larvae, should be considered in this connection. 



The pleural plate in some cases is connected with the basi- 

 sternum, bs (figs. 4, 5, (i, 7, etc.) by a continuous chitinous 

 region forming a connecting bridge between the sternal and 



