No. 2.] LARVA OF CTENOPHORA ANGUSTIPENNIS. 543 
ical value must be slight. The prothoracic segment, the one 
just posterior to the head, appears at first sight double, as it is 
divided transversely by a distinctly-marked fold. This fold, 
however, is merely the result of the very frequent retractions 
of the head, which may be completely withdrawn within the 
first segment. 
Indeed, it is generally so withdrawn, except when the 
animal is moving or eating. The head is thus surrounded 
by a fold of the body wall which greatly facilitates the retrac- 
tion and protrusion of it. The top of the head (Fig. 2) tis 
defended by a strong chitinous shield. The occipital region 
of this, as is the usual case in the retractile head of the 
Dipterous larva, is imperfectly chitinized and is posteriorly 
excavated by two deep notches. The antenne (Tig. 3) are 
single jointed, projecting a little from the side of the head. 
No eyes are visible. 
The extreme posterior segment (Figs. 4, 5 and 6) is 
modified as usual. The anus opens at the apex of this seg- 
ment. Just above the anus and on each side of the median line 
are the two large oval stigmata (Fig. 7). The elliptical 
central core or plug looks coal black, while the surrounding 
ring of irregular chitinous threads is of a deep brown. The 
stigmata are of the primitive or generalized type, and are, 
therefore, without lips. The aperture can be closed by bring- 
ing down the surrounding lobes. The spiracles are sur- 
rounded by six backward projecting flexible lobes, four of 
these are in a line above the spiracles, while the others are 
just below and on each side of the anal opening. When these 
are contracted they serve to protect the stigmata, and are 
strengthened by the presence of small chitinous patches on 
the posterior tips. 
As the larval skin is quite transparent, the main body 
systems can easily be seen through the skin. If the fat-bodies 
are well-developed, as they are just before pupation, they 
completely envelop the alimentary canal and therefore the 
larva appears white and opaque. When the live animal is 
observed the heart can be seen as a delicate pulsating tubule, 
