4 BULLETIN 329, IT. S. DEPAKTMENT OF AGEICULTTJEE. 



Toward its caudal end the ileum narrows into what is probably the 

 colon (<?), which leads in turn directly into the sacklike rectum. 

 This is round in shape, transparent, deeply striated, and capable of 

 great contraction and distention. It contains six large oval glands 

 (rg), which are somewhat similar to the rectal glands of Stomoxys 

 calcitrans L. At its caudal end the rectum contracts into the 

 external orifice, or anus (a). 



Female Reproductive Organs. 

 PL I. 



The following is a description of the reproductive organs of the 

 female of Simulium venustum. The two ovaries (lo, to) are situ- 

 ated laterad of the ileum (i). They are small pear-shaped organs, 

 transparent and composed of a thin outer membrane which forms a 

 sack. The interior is filled in newly emerged adults with many 

 small nuclear eggs. (PI. I, fig. 1.) Joining the ovaries to the diver- 

 ticulum and the basal membrane of the outer skeleton of the abdo- 

 men are large trachea?, which branch many times, finally ending in 

 thin threads. The caudal end of each ovary narrows into a tubular 

 oviduct (od). The two oviducts join and form the cephalic end of 

 the tubelike structure known as the utero- vagina (v). The utero- 

 vagina and oviducts lie immediately above the lower end of the rec- 

 tum. Leading from the cephalic end of the utero-vagina on the 

 right side is a very narrow tube which terminates in a round, bulb- 

 like structure, black in color, with a transparent membrane. This 

 is the spermathecum (s) , whose function is that of a reservoir for the 

 spermatozoa injected by the male at the time of copulation. When 

 an egg is deposited fertilization is effected as the egg passes the orifice 

 in the spermathecum. 



Development of the Eggs. 



The stages in the development of the eggs within the ovaries may 

 be described at five different stages, as follows : 



First stage. — The eggs are round, very small, rather transparent, 

 and are present in the ovary in large numbers. As a rule they are 

 rather indistinct. (PI. I, fig. 1.) 



Second stage. — The eggs become denser and generally larger toward 

 the oviduct, more irregular in shape with a tendency toward an oval 

 form. As a rule they are still somewhat indistinct. (PI. I, fig. 2.) 



Third stage. — The eggs are of a perfect oval shape, dense as a rule, 

 and of nearly equal development,^ those near the oviduct possibly 

 larger. These latter have a tendency toward a slight bulge on one 

 side. The shell has become somewhat denser in appearance, and the 

 eggs on the whole are very distinct. (PL I, fig. 3.) 



