STRUCTURE, DEVELOPMENT, AND BIONOMIOS OF HOUSE-FLY. 511 
be clearly understood by reference to fig. 16. The average 
length of the full-grown larva of M. domestica 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 
seoments, 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 
theanterior segment or “head” as the first, Weismann (1863 
and 1864) considers that the body is composed of twelve seg- 
ments. Brauer (1883) is of the opinion that there are twelve 
seoments, 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 “ 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 larvee 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 
seoment, so that the apparent second post-oral segment (iv), 
that is, the segment posterior to the anterior spiracular pro- 
cesses, 1s 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, 1), 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 may 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 
