OF WASHINGTON. 15 
Sayomyia has accomplished a still further specialization along 
these lines. It has dispensed with the air tube entirely and the 
trunks of the tracheal tubes as well, while it floats like a trans- 
parent ghost deep in the pool, carrying its four sacs of air which 
are now never connected with the air above. We suppose that 
the air in these sacs is replenished by diffusion through the body 
walls from the air dissolved in the water; but the character 
appears to us as a very remarkable one which would not have 
been antecedently thought possible. 
The Corethridae, as here limited, include 2 3 species described 
from North America. Of these we know the larvae of 15 more 
or less completely. There are no unknown genera, and it is 
probable that the unknown species of Dixa and Sayomyia are 
similar to the known ones. Indeed some of the species of Dixa 
may be synonymous ; but on the other hand, there are prob- 
ably a number of forms to be discovered. The subject has 
been very little worked. 
The following synoptic table will separate the known larvae : 
1 . Air tube present 2 
Air tube absent, larvae aquatic. . 7 
2. Air tube sessile, larvae at surface of water 3 
Air tube elongate, larvae below surface of water 6 
3. Abdomen with false feet ; antennae not longer than other mouth parts 4 
Abdomen without false feet; antennae long and directed forward 
Eucorethra underwoodi 
4. Ciliated plates of the air tube without a projecting triangular hairless 
apex Dixa clavula l 
These plates with such an apex (fig. 2, a) 5 
5. Anal segment finely haired, the hairs shorter than the stout terminal 
cone (fig. 2, 6) Dixa recens 
Anal segment coarsely haired, the hairs longer than the small terminal 
cone (fig. 2, c) Dixa centralis 
6. Antennae moving in a horizontal plane; larvae flattened dorso-ven- 
trally Corethrella brakeleyi 
Antennae moving in a vertical plane; larvae subcylindrical, 
f Corethra cinctipes "* 
J Corethra lintneri 
I Corethra velutina 
{^Corethra karnerensis 
1 modesta Joh. According to Mr. Johannsen's figure (Bull. 68, N. Y. 
State Mus., pi. 48, figs. 5 and 7, 1903). I have not myself seen the larva. 
The projecting apex belongs to the outer sheath of the plate and it may not 
be shown in Johannsen's figure. 
2 1 am unable to distinguish Corethra cinctipes Coq. and C. velutina Ruthe. 
The latter may not be the European form, but C. karnerensis Felt or C. 
lintneri Felt, which I am likewise unable to distinguish. 
