92 MOSQUITOES OF NORTH AMERICA 



there are in place of lateral plates series of large simple spines such as are present 

 in some larvae of Aedes. 



As before stated the tracheas 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 withdrawn far beneath it.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 larvae. From it the breathing tube of the other larvae is 

 evolved and the components of the closing apparatus are homologous with the 

 six pieces present in Anopheles. In Culex and other larvae the stirrup-shaped 

 piece within the tube, through which the tracheae 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. 



