20 BULLETIN" 329, U. S. DEPARTMENT OF AGRICULTURE. 



LARVAL DEVELOPMENT. 



From 12 to 16 hours before hatching the young larvse 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 larvse have hatched, but remain 

 appressed to the sides of the head. 



The newly hatched larvse of all the species described in this paper 

 measure from two-thirds to three-fourths of a millimeter in length. 

 The structure of the larvse 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 larvse do not shed the entire skins 

 but only the skeleton of the head. The writer has found that the 

 entire skin is shed, as larvse have been found which had partly cast 

 off the entire skins, bearing the cjiitinous 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. 



