20 BULLETIN 329, U. S. DEPARTMENT OF AGRICULTURE. 
LARVAL DEVELOPMENT. 
From 12 to 16 hours before hatching the young larvae are in an ap- 
parently free condition within the shell. There is an almost con- 
tinual movement of the head, which is forced against the ectoderm 
by sudden convulsive movements of the body. The shell suddenly 
splits and the larva at once emerges, literally tumbling out. The 
split reaches from the cephalic end of the egg to the middle, generally 
a little to the left. It is evident from the clear appearance of the 
alimentary tract of the newly emerged larva that the ectoderm is 
not eaten away at all. The larva at once clings to the nearest point 
of attachment by its thoracic proleg and waves its anal portion 
around several times, evidently to clear the rectal gills. It then 
attaches itself to the nearest object by the peculiar suckerlike organ 
on its anal extremity and thrusts out its gills at intervals of about 
15 seconds. It at once commences to feed on the matrix surround- 
ing the eggs and the adjacent microorganisms, using its thoracic 
proleg as a scoop, with a constant movement of this organ toward the 
mouth parts. In still water the fans are not used for sweeping in 
the food for some hours after the larvae have hatched, but remain 
appressed to the sides of the head. 
The newly hatched larvae of all the species described in this paper 
measure from two-thirds to three-fourths of a millimeter in length. 
The structure of the larvae in the first instar, of the species examined 
by the writer, is fundamentally the same as in the last, with the 
exception that the head is larger in proportion to the rest of the body 
and the general appearance very transparent, due to the lack of pig- 
mentation and the empty condition of the alimentary tract. The 
black eye spots on the sides of the head are very prominent, and the 
dorsal markings of the head, though not so distinct as in later stages, 
are easily distinguishable. The silk glands are employed almost at 
once after hatching to form a silken thread, which is used to hold the 
larva in position in the current. The structure of the rectal gills 
seems to remain the same throughout larval development. 
LARVAL HABITS. 
MOLTING OF THE LARVAE. 
According to Strickland 1 the larvae do not shed the entire skins 
but only the skeleton of the head. The writer has found that the 
entire skin is shed, as larvae have been found which had partly cast 
off the entire skins, bearing the chitinous hooks of the caudal sucker, 
and which showed the caudal sucker again developed in the new 
larval skin. The entire skeleton of the head is cast off toward the 
1 Strickland, E. H. Some parasites of Simulium larvae and their effects on the develop- 
ment of the host. (In Biol. Bui., v. 21, no. 5, p. 302-338 (p. 303), 5 pi., Boston, 1911. 
