28 



80ME ASPECTS OF MORPHOLOGY 



the insect leg are tolerably clear, those of the 

 proximal parts are highly involved. It is by no 

 means certain whether any exact serial comparison 

 with the generalised biramous appendage is possible 

 in view of the evolutionary changes that have 

 supervened. In the present stage of knowledge the 

 interpretation of Snodgrass is in closer accord with 

 the observed facts than that of other morphologists. 

 A comparison of the views of Hansen and Snodgrass 

 is given in tabular form on this page. 



B. The Pleura. The origin and differentiation of 

 the pleuron have been much discussed by morpho- 

 logists in recent years, and, in this connection, the 

 names of Heymons (1899), Prell (1913), Crampton 

 (1914, 1926), Bonier (1921), Becker (1923, 1924), 

 Weber (1924) and Snodgrass (1929) may be specially 

 mentioned. It is probable that Heymons' general 

 contention that the pleural sclerites are in part, or 

 mainly, derived from the subcoxa is substantially 

 correct. Among the Apterygota the pleural sclerites 

 are so variably developed that they exhibit no 

 obvious indications of having been evolved along 

 any common line of development. As Ewing(1928) 

 has pointed out, in Campodea, Scutigerella and 

 certain Collembola they are represented by a simple 

 subcoxal or basal segment of the leg (Fig. 11). In 

 Machilis a single, although reduced, sclerite is also 

 present. In other cases there is no defined subcoxa, 

 and its place is taken by two or more sclerites 

 which present diverse phases of development. It 

 is possible, however, to group these sclerites into a 

 proximal, and usually arched, plate or trochantin 

 over the base of the coxa, and a distal plate or series 

 of plates forming the eupleuron. Single proximal 

 and distal sclerites are evident, for example, in 

 Acerentulus (Ewing), and in Isotoma and other 

 Collembola (Fig. 15). In Lepisma, Snodgrass (1927) 



