THE LARVA. 



All mosquito larvae are aquatic. They possess a complete, well-chitinized head 

 with the typical mouth-parts of mandibulate insects. The thorax is broad and 

 its three segments are fused. The abdomen is long and slender with nine well- 

 defined segments. There are no ambulatory appendages. The anal opening is 

 at the apex of the ninth segment and is surrounded by four more or less de- 

 veloped tracheal gills. The tracheal system is metapneustic with the openings 

 dorsally on the eighth segment, usually through a chitinous tube of greater or 

 less length. The larvae are active and move backward through the water by 

 jerking the body from side to side. They also can move slowly forward by the 

 action of the mouth-brushes. By means of a mechanism at the end of the respir- 

 atory tube the larva can suspend itself from the surface film in order to take 

 in air. Easchke has pointed out that the breathing tube has the further function 

 of an hydrostatic apparatus. It will be most convenient to discuss in detail the 

 structures of the larva of Culex and afterward point out the various modifica- 

 tions that occur in other mosquito larvae. 



THE HEAD AND MOUTH-PARTS. 



The head of the Culex larva is broad, rounded, and somewhat flattened. It 

 is inserted upon the thorax by a narrow neck, the opening into the head being 

 surrounded by a heavy chitinous collar, the foramen. The antennse are promi- 

 nent and inserted anteriorly upon the outer angles of the head. Between the 

 antennae the head is arcuate and a narrow clypeus forms its anterior margin. 

 In front and beneath is the labrum with its appendages. The median free lobe 

 of the labrum is attached between two long, stout spines which project forward 

 and downward from the anterior margin of the clypeus. The other mouth 

 organs are upon the ventral side of the head and the anterior part of the head 

 may be considered a broad rostrum which projects over them. The eyes are 

 placed laterally and consist of two pairs, the large compound eyes, and behind 

 them the accessory eyes. 



The antennae consist of a single chitinous piece and are cylindrical and 

 slightly curved. About two-thirds from the base the shaft is suddenly narrowed 

 and at this point is inserted a fan-shaped tuft of long feathered hairs. The 

 slender apical third is obliquely truncate at the tip. Upon the tip is inserted a 

 conical sensory organ surrounded by three long setae and a shorter spine-like 

 seta. Easchke interprets the sensory cone as an olfactory organ. It has a 

 chitinous cylindrical basal portion with a minute spinous process upon its side 

 and a very delicate conical sensory portion at the apex. The antennal shaft is 

 beset with numerous small spines. The antennae have but little motility. They 

 are controlled by a single rather weak muscle, inserted at the base and running 

 backward to the hind margin of the head. The antennae are connected with the 

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