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 STRUCTURE, DEVELOPMENT, AND BIONOMICS OF HOUSE-FLY. 511 



be clearly understood by reference to fig. 16. The average 

 length of the fall-grown larva of M. domes tic a is 12 mm. 



The question as to the number of segments which constitute 

 the body of the muscid larva is a debated subject. I have, 

 however, taken as my criterion the arrangement of the 

 somatic musculature. Newport (1839) considered that the 

 body of the larva of Musca vomitoria consisted of fourteen 

 segments, but if the anterior portion of the third segment, 

 that is, my first post-oral segment, is included, there were 

 fifteen, to which view he appeared to be inclined. Counting 

 the anterior segment or " head" as the first, Weismann (1868 

 and 1864) considers that the body is composed of twelve seg- 

 ments. Brauer (1883) is of the opinion that there are twelve 

 segments, but that the last segment is made up of two; 

 Lowne follows this view in his description of the blow-fly 

 larva and considers that there are fifteen post-oral segments. 

 I am unable to accept Lowne's view. Counting the proble- 

 matical cephalic segment, for which I shall use Henneguy's 

 (1904) term f( pseudo-cephalon," as the first segment, I 

 believe that it is succeeded by twelve post-oral segments, 

 making thirteen body segments in all, which is the usual 

 number for dipterous larvae as Schiner (1862) has also pointed 

 out. My study of the somatic musculature, as I shall show, 

 indicates the duplicate nature of the apparent first post-oral 

 segment, so that the apparent second post-oral segment (iv), 

 that is, the segment posterior to the anterior spiracular pro- 

 cesses, is really the third post-oral segment or fourth body- 

 segment. 



The cephalic segment cannot be considered as homologous 

 with the remaining twelve segments, which are true segments 

 of the body as shown by their musculature and innervation. 

 This segment (fig. 9, i), for which Henneguy's term "pseudo- 

 cephalon" is very suitable, probably represents a much reduced 

 and degenerate cephalic segment, its present form being best 

 suited to the animal's mode of life. We nmy consider the 

 greater part of the cephalic segment of the larva as having 

 been permanently retracted within the head; this is shown by 



VOL. 52, PART 4. 39 



