MOSQUITOES AND GNATS 25 



that no one species will serve as typical of the group. In observations 

 conducted by L. O. Howard at Washington, D. C. (1900 Rept.), upon 

 the species Culex pungens it was determined that the eggs were laid upon 

 the water surface in masses of a variety of shapes, often described as 

 boat-shaped because a common form is that of a pointed ellipse (Fig. 10) . 

 The number of eggs in each mass varied from two himdred to four hun- 

 dred, all arranged perpendicularly and in longitudinal rows. The in- 

 dividual eggs are slender, somewhat pointed at the tip, and at the bottom 

 broader and blunt, having a length of 0.7 mm. and a diameter of 

 0.16 mm. at the base. 



It has been demonstrated that imder the advantageous conditions 

 of the warm summer months eggs may hatch in less than a day from the 

 time they are deposited. The larvae, issuing from the under side of the 

 egg mass, are elongate, with head, thorax and abdomen distinct, the 

 head bearing prominent antennaj each consisting of a single segment. 

 About the mouth is a mass of prehensile filaments. The abdomen is 

 segmented, and respiration is by tracheae which open at the apex by 

 means of the anal siphon. They appear to undergo four molts, and, 

 under favorable conditions, may be transformed into pupae in iabout 

 seven days. Studied at a period when the larva is nearly full grown, 

 it is seen to remain near the surface of the water with its respiratory 

 tube at the exact surface and its mouth below receiving food which is 

 directed to it by the rotary movements of the mouth filaments. Occa- 

 sionly the larva descends below the surface, but, by a series of wrigglings, 

 quickly returns. The return is only accomplished by considerable 

 exertion, as, once below the surface, the tendency of the larva is to sink 

 rather than to rise. If, therefore, for any reason it is unable to suffi- 

 ciently exert itself to again reach the surface, it will perish. The efficacy 

 of the film of oil spread upon the water may be thus explained; it not 

 only prevents access to the air, but, by its deleterious effect, renders the 

 larva unable to exert sufficient muscular force to recover the position 

 necessary for respiration and buoyancy. 



The transformation to the pupal stage, occurring under favorable 

 conditions about the seventh day, is marked by a great enlargement of 

 the thoracic segments (Fig. 11). Here the reverse of the just described 

 physical phenomena obtains; the pupa is lighter than water, and, unlike 

 the larva, effort is required to sink rather than to rise. It remains mo- 

 tionless at the surface, when disturbed descending to the bottom by 

 violent wrigglings. As soon as these exertions cease it will again grad- 

 ually rise. The differential structure of the pupa is noticeable in the 

 enlargement of the thorax, and in that the air tubes no longer open at 

 the abdominal apex, but through two ear-like processes on the thorax, 

 the pupa remaining upright at the water's surface instead of head down- 

 ward as in the larval stage. Since the adult insect emerges from its 



