34 SOME ASPECTS OF MORPHOLOGY 



3. The theory of Snodgrass is more in accordance with that of 

 Borner than of Crampton. According to him the cardo and 

 stipes, however, are not separate segments, but represent the 

 secondary division of a single original basal segment. The galea 

 and lacinia are endites of this basal segment and the palpifer is a 

 secondarily demarpated portion of the stipes as maintained by 

 Borner. 



The theoretical views of Snodgrass are largely formulated from 

 a study of the basal musculature of the maxillae (Fig. 19), and in 

 this connection it needs to be borne in mind that such muscles 

 may not necessarily retain their original functions or their former 

 dispositions. In accordance with alterations in the basal 

 articulation of an appendage, and in the number of its constituent 

 segments, the muscles concerned may shift their positions or 

 become segregated into secondary groups of fibres. Comparative 

 studies of progressive changes in the muscles are greatly needed 

 before we are in a position to ascribe to such criteria their due 

 morphological values. Snodgrass 's contribution is important in 

 that it discountenances attempts to determine homologies solely 

 upon the basis of a study of the sclerotised parts, without reference 

 to other considerations. 



Since the margin of the basal cavity of the maxilla in the more 

 primitive insects surrounds the bases of the cardo, stipes and 

 lacinia, and the tergal and sternal muscles of the maxilla are 

 distributed to these parts, Snodgrass concludes that the latter are 

 all differentiations of a single primitive basal segment. The galea 

 and lacinia he regards as subdivisions of a single endite carried by 

 the stipes, since they are flexed by muscles having their origin in 

 the latter sclerite. In supporting Borner's contention that the 

 palpifer does not represent an independent segment, he lays stress 

 on evidence afforded by the basal palpal muscles. These muscles, 

 which move the palp as a whole, have their origin within the 

 stipes and pass directly through the palpifer to be inserted with 

 the proximal segment of the palpus ; furthermore, no muscles 

 arise within the palpifer or are inserted upon it. 



While the study of the completed maxillae and their musculature 

 affords scope for more exhaustive investigation, the differentiation 



