12 



BULLETIN 329, U. S. DEPARTMENT OF AGRICULTURE. 



closely arranged cells (PL III, fig. 2). The interior of an egg con- 

 taining an embryo three-fourths developed has a peculiar banded 

 appearance (PL III, figs. 3, 4), but as the time of hatching ap- 

 proaches the most noticeable features are the cephalic eye spots on 

 the embryonic larva, which become more and more distinct. (PL III, 

 fig. '5.) Detailed studies on the embryology of Simulium have been 

 made by Mecznikow, 1 Kolliker, 1 and Graber. 1 



The average period of incubation, from June to October, of the 

 species studied by the writer at Spartanburg, S. C, was about 8.8 

 days ; the minimum period, 7 days ; and the maxium, 12 days. The 

 following table will show the normal variation in that region from. 

 June to October: 



Table showing period of incubation of eggs of Simulium. a 



T,ot 



Species. 



Date of 

 ovipo- 

 sition. 









Num 



aer c 



f days of incubation. 



No. 



1 



2 



3 



4 



5 



6 



7 



8 



9 



10 



11 



12 



13 



14 



1 





June 12 

 June 21 

 July 29 

 Sept. 6 

 Sept. 11 

 Sept. 16 

 ...do 



a 

 a 

 a 

 a 

 a 

 a 

 a 

 a 

 a 

 a 

 a 

 a 

 a 

 a 

 a 



a 

 a 

 a 

 a 

 a 

 b 

 a 

 b 

 b 

 b 

 b 

 a 

 b 

 b 

 a 



a 

 a 

 a 

 b 

 a 

 b 

 b 

 b 

 b 

 b 

 b 

 b 

 e 

 c 

 b 



b 

 b 

 b 

 b 

 a 

 c 

 b 

 c 

 c 

 c 

 c 

 b 

 c 

 c 

 c 



b 

 c 

 b 

 c 

 b 

 c 

 c 

 c 

 c 

 c 

 c 

 c 

 d 

 d 

 c 



c 

 c 

 e 

 c 

 b 

 d 

 c 

 d 

 d 

 d 

 d 

 c 

 d 

 d 

 d 



c 

 c 

 c 

 d 

 b 

 h 

 d 

 h 

 h 

 h 

 h 

 d 

 h 

 h 

 d 



d 

 d 

 c 

 h 

 c 



d 

 h 

 d 



h 



h 









? 











3 



do 



h 



h 



h 



h 





4 







5 



...do 



c 



c 



d 



h 







6 









7 



do 



n 















8 



...do 



...do 















9 



.. do 



...do 

















in 



...do 



...do 

















ii 



...do 



...do 

















r> 



do 



...do 



h 

 h 

 h 



h 















13 



do 



...do 



h 

 h 



h 













14 



...do 



...do 



h 

 h 











It 



do 



...do 



h 





- 













a Letters indicate stage of incubation, shown on Plate I, as follows: a, corresponds to fig. 1; b, corresponds 

 to fig. 2; c, corresponds to figs. 3, 4; d, corresponds to fig. 5; h, date of hatching. 



The eggs seem to hatch equally well in either still or running water. 

 The percentage of eggs that hatch is very high when they are placed 

 so that the water reaches them, the average sterility of a series of 

 lots kept under observation in normal conditions being from 4 to 5 

 per cent. The approximate percentage of egg masses, however, that 

 is deposited under such favorable conditions is certainly relatively 

 small as compared with the total number deposited, and the location 

 of the egg mass seems to be the dominant factor in determining the 

 successful hatching of the larva and the consequent abundance of 

 the insect. 



THE LARVAL STAGE. 



The larvae of the known species of Simulium, as will be discussed 

 it length farther on, are invariably aquatic in their habits and re- 



1 See Bibliography, p. 35. 



