30 



SOME ASPECTS OF MORPHOLOGY 



sclerotised parts of the subcoxa, situated anteriorly and 

 posteriorly to the coxa (Cx), form the pre- and post-coxal bridges 

 (P.B., P'.B.) and the ventral part of that sclerite may form, with 

 the lateral edge of the sternum, a defined element termed the 

 laterosternite (Ls). This, in few words, expresses the salient points 

 in Snodgrass's views. In order to instance how hypothetical the 

 problem is, it may be mentioned that Hansen (1930) rejects the 

 theory as a whole and homologises the eupleuron and trochantin 

 of Snodgrass with the subcoxa (precoxa) and coxa (qoxopodite) 

 of Malacostraca ; he further maintains that the insect coxa is the 



Scx^ 



ABC 



Fig. 16. Simplified diagrams representing the possible origin of the 

 Pterygote thoracic pleurites from the subcoxa, according to 

 Snodgrass (1929). A, hypothetical primitive condition ; B, 

 intermediate condition ; C, stage with a differentiated pleuron ; 

 a and h, dorsal and ventral articulations of coxa (Cx) ; c, 

 secondary articulation with trochantin ; W.P., pleural wing 

 process. For explanation of other lettering, vide pp. 29 and 30, 



homologue of the basipodite. The subcoxal origin of the pleuron 

 is dismissed by him, and it would appear that he regards the 

 episternum and epimeron as independently developed sclerites. 



The foregoing conception of the origin of the insect pleuron is 

 substantiated by following the development of the parts concerned 

 among the lower Pterygota. Thus in the cicada Tihicina the 

 original subcoxa shows in the nymph differentiation into rudi- 

 mentary pleurites with the beginning of a demarcation with a 

 small basal sclerite or trochantin. Roonwal (1937) has followed 

 the development of the parts in question in Locusta and has 

 demonstrated that the subcoxa is the fundament from which the 

 future pleuron is derived. 



