14 
ENTOMOLOGICAL SOCIETY 
Dixa is the most generalized form and is a surface feeder, 
taking small vegetable particles by the rapid motion of its mouth 
parts. The air tube is well developed, but sessile, and there 
are false abdominal feet to assist the larva in ascending the 
water film at the margin, as is its habit. From Dixa can be 
derived not only all the other Corethridae, but the Culicidae as 
well. Anopheles, for example, is very close to Dixa in many 
characters. 
Except Dixa, all the Corethridae are predaceous, feeding 
largely on the larvae of the true mosquitoes. Next to Dixa 
comes Eucorethra, with its air tube still sessile, but the mouth 
parts modified for its predaceous habits. It is still nearly a 
surface feeder, lying flatly in the water. Corethrella is a fur- 
FIG. 2. Structural details of North American Corethrid larvae: a, ciliated plate of air tube in 
Dixa recens ; b, anal segment of Di.ra recens ; c, the same of D. centralis ; d, leaf-like appen- 
dage of Sayomyia americann ; e, the s.^me of S. trivittata ; f, the same of S. knabi 
ther specialization of this type, the air tube having become 
elongated, allowing the larva to sink lower in the water, while 
the peculiar rapacious antennae are remarkably adapted. 
Corethra shows a development in a different direction. The 
air tube is elongated, but apparently could not be adapted to 
the requirements of the larva, as it became necessary for it to 
sink deeper and deeper in the water, so it has begun to be dis- 
used. The horizontal position is still maintained by the forma- 
tion of air bladders, one pair in the thorax, a second near the 
end of the abdomen. These are enlargements of the tracheal 
tubes and are joined by them to the air tube. It would seem 
that this fortunate arrangement supplied the larva with air- 
reservoirs, so that it is able to stay long below the surface and 
frequent the depths where it finds its subsistence. 
