14 
BULLETIN' OF THE BUREAU OF FISHERIES. 
air tubes or tracheae, by means of which the larva breathes. The anterior and 
posterior margins of this groove form a pair of lips, which can be opened or closed 
at will and thus control the breathing. The posterior lip extends diagonally up- 
ward and backward to the level of the dorsal surface of 
the last abdominal segment, but though it thus appears 
in a dorsal view it really belongs to the ventral surface. 
At its center is a narrow chitinous flap extending from 
the margin of the lip back along the outer surface (fig. 
5, ft). The lateral edges of the flap are turned forward, 
giving it the shape of the letter U, and it forms the 
posterior or ventral cover of the anal opening. Inside 
of this flap the end of the intestine (i) projects a little 
from the floor of the groove and the anus (a) opens 
halfway from the floor to the edge of the lower lip. In 
front of the anus is a fingerlike papilla on either side, 
tipped with a long bristle. When the lips of the groove 
are closed, these papillae (p) come together in front of 
the anus and form an anterior or dorsal cover for the 
anal opening. In this way the anus can operate when 
the lips are closed, and whatever passes out of it is 
kept away from the tracheal openings. The central 
portion of the anterior lip is also somewhat chitinized 
fig. 5,-Transverse groove on dorsal sur- over a trapezoidal area extending from the edge of the 
c^cercif^^hitL^^p n^ntes- lip forward along the dorsal surface the full length of 
tine; p, papillae; t, openings of tracheal ^g fast segment. 
On either side of the intestine, in the deepest por- 
tion of the groove, is the opening of the tracheal air tube (f), which is elliptical 
in outline, with the long diameter inclined at an angle of 45° to the body axis. 
Each opening is covered with a membraneous valve 
slit along the outer margin of the ellipse, which effec- 
tually closes the opening to the entrance of water. 
On the ventral surface of the last abdominal seg- 
ment are the cerci, two slender cylinders from 3 to 4 
mm. in length in a full-grown larva and about 0.5 mm. 
in diameter. They are more or less wrinkled trans- 
versely and normally are naked, but often become cov- 
ered with growths of algae and Protozoa. There is no 
central lumen in these cerci, but the entire space is 
filled with tissue and contains many muscles. When 
the larva rises to the surface of the water to breathe, 
the cerci naturally flatten themselves along the surface 
film at right angles to the longitudinal body axis. This 
not only furnishes the larva with a secure hold upon the surface film, but it also 
opens the lips of the transverse groove and holds them open as long as the larva 
remains at the surface. When the hold upon the surface film is released, the cerci 
trail backward in the water and close the transverse groove. In front of the 
Fig. 6.— Side view of last segment of 
abdomen, showing depth of dorsal 
groove and the papilla on the ventral 
surface. 
