92 MOSQUITOES OF NORTH AMERICA 
there are in place of lateral plates series of large simple spines such as are present 
in some larve of Aédes. 
As before stated the traches open separately. The spiracles are situated well 
back on the dorsum of the eighth segment, in a slight depression surrounded by 
chitinous plates. When the larva is at the surface of the water these plates are 
spread out in such a manner that they support the larva and at the same time 
expose the spiracles to the air. When the larva wishes to go down the plates are 
folded together and form a nearly closed cavity over the spiracles. The air held 
in this cavity prevents the water from entering the spiracles. 
The mechanism has been described by Nuttall and Shipley and by Imms, but 
neither of these descriptions are clear or wholly correct. There are six chitinous 
elements in this closing mechanism. These range themselves about the spiracles 
as an anterior piece, a pair of lateral flaps and three posterior pieces. Just in 
front of the spiracles and overshadowing them is a small median transverse 
plate, the “ anterior fan-shaped plate” of Nuttall and Shipley. This plate is 
so hinged that it can be turned back over the spiracles while these latter are 
withdrawn far beneath it. Close to each spiracle outwardly, is a leaf-like 
fleshy flap which is strengthened distally by a small chitinous strip sur- 
mounted by a sensory seta. When the larva is at the surface of the water these 
flaps are directed outward and prevent the water from entering the spiracular 
depression from the sides. When the larva goes down these flaps are turned up- 
ward and inward and thus help to form the air-containing cavity over the 
spiracles. The structures behind the spiracles consist of a very large, median, 
roughly triangular plate and two smaller plates which support it. The acute 
anterior end of the large plate intervenes between the spiracles and its apex is 
strengthened by a chitinous peg which is attached keel-like beneath. Nuttall 
and Shipley, and also Imms, claim that this chitinous peg is connected with the 
anterior transverse plate, but careful study has convinced us that this is not the 
case. The two rounded hind angles of the plate project well beyond the end of 
the segment and the plate slopes upwards towards these angles. Medianly the 
plate is flexible while the sides, and particularly the angles, are well chitinized ; 
anteriorly there is a median suture. Powerful muscles attached to the middle 
portion of the plate beneath, and to the anterior peg, can draw the plate down- 
ward and forward. When this is done the posterior angles are bent broadly up- 
ward and inward and constitute a pair of flaps which close in the spiracular 
depression behind. The supporting plates support the projecting hind angles of 
the median plate. They are irregular in outline, almost quadrate, and rest with 
their proximal basal portions upon the thickened rim of the lateral serrate plates. 
The breathing apparatus of Anopheles, just described, is the most primitive 
found among mosquito larve. From it the breathing tube of the other larve is 
evolved and the components of the closing apparatus are homologous with the 
six pieces present in Anopheles. In Culex and other larve the stirrup-shaped 
piece within the tube, through which the trachese communicate with the outer 
air, is homologous with the large triangular plate of the Anopheles larva; the 
five flaps surrounding the rim of the tube correspond with the five other pieces. 
