TRANSFORMATIONS OF BUFFALO GNATS. 5 



Fourth stage. — The eggs are fully as large as those newly laid on 

 grass blades, of the same triangular form, very dense, the shell very 

 strong, and not differing from eggs just laid, with the exception that 

 occasionally they may be slightly smaller. (PL I, fig. 4.) 



Fifth stage. — One or two large eggs are usually left in the ovaries 

 near the oviduct, and the rest of the ovary is entirely filled with the 

 typical round eggs of the first stage. This stage is found after 

 adults have finished ovipositing, and also in specimens taken actually 

 engorging, concerning which more will be said later. (PL I, fig. 5.) 



The Genitalia of the Male. 

 PI. II, figs. 1-5. 



The structure of the genitalia of the male shows remarkably con- 

 stant specific characters. Their systematic value may be seen from 

 the striking differences in the illustration. The determination of 

 the species based on color alone is at best unsatisfactory owing to the 

 amount of variation in individuals of the same species and fading 

 of color in dry mounted specimens. The appearance of the structure 

 of the genitalia does not change when the specimens are preserved in 

 alcohol, and in dried specimens the original shape can be completely 

 restored by the following method: 



The last segment of the abdomen of the male ^fter being re- 

 moved with sharp scalpel or needle is placed in 6 per cent potassium 

 hydroxid (KOH) and left until the soft parts are removed, the 

 chitinous portions remaining intact. The material is then washed 

 with water, run through alcohol in the usual way up to 100 per 

 cent, transferred to xylol, and mounted in balsam, so as to give a 

 direct caudal view as shown in Plate II, figures 1-5. Some of the 

 European species have been studied in regard to the genitalia of the 

 males by Liindstrom, but nothing has been done so far on American 

 species. 



The segments to which the organs of the genital armature are 

 attached are collectively designated under the name hypopygium. 

 The most prominent organs of the hypopygium are the two claspers 

 (c c) and the basal pieces to which they are attached. The claspers 

 may be either large, fingerlike, as in S. venustum, or greatly reduced 

 and somewhat hook shaped as in S. bracteatum. They sometimes 

 bear a spine at the distal end as in S. venustum, a single chitinous 

 fingerlike process as in S. bracteatum, or three or four such processes 

 as in S. vittatum. The basal pieces (b, b) to which the claspers are 

 articulated are divided by a suture near the base. They are very 

 large in S. bracteatum and are reduced in S. venustum. The relation 

 between the size of the claspers and basal pieces seems to be very 



