176 ABDOMINAL PATTERN IN TABANIDAE (dIPTERa) 



Another series of pattern transformations are the result of a 

 breaking up of the dark background into dark spots. From a 

 glance at fig. 2 {T. siifis) it will be evident that the same primitive 

 patterns from which we have started can be taken as the origin 

 of a pattern consisting of dark spots on a light background. Such 

 a pattern, consisting of four rows of black spots, we find on the 

 yellowish-olivaceous abdomen of T. ditaeniatiis (fig. 13). That 

 this pattern must have had the same origin is all the more evi- 

 dent as the male of T. ditaeniatus (fig. 14) has a pattern which rep- 

 resents a modification in the opposite direction, and of a similar 

 type as T. lasiopihalmus: suppression of the median row of light 

 spots so that the resulting impression is that of two light sub- 

 dorsal bands on a dark background. By reduction of dark spots 

 on a light abdomen a uniform light coloration like ihat of T. par 

 (fig. 15) may be derived. There is a considerable variety of 

 indistinctly-shaded a))domina' patterns found in species of 

 Tabanidae, of which generally the uniformly light type may be 

 considered an end result. 



Turning to the genus Chrysops, we find phenomena quite anal- 

 ogous to those described in Tabanus. In Chrysops, however, 

 generally the initial stage is that of four rows of black spots on 

 a light (j^ellow or grayish-yellow) background, as exemplified in 

 C. vittatus (fig. 16). The same tendency of pattern reduction 

 beginning from both ends of the abdomen, together with fusion 

 of the remaining elements, takes place in Chrysops. The pattern 

 variations in Chrysops are well known as they serve to distin- 

 guish the species. In C. montanus (fig. 17) the lateral black 

 spots present in C. mttatus have disappeared on segments one and 

 two; on segments three, four, and five they tend to fuse together. 

 In C. callidus (fig. 18) the subdorsal black spots on segments one 

 and two separate in C. vittatus, and beginning to fuse in C. mor- 

 tanus, have fused completeh^, forming a new characteristic ele- 

 ment of pattern. On segments three and four the lateral and 

 subdorsal black spots have fused completel}', leaving only nar- 

 row light areas open in the median dorsal line. On the following 

 segments black has swallowed everything, and even on segment 

 five, which in C. vittatus shows four distinct black spots on light 

 ground, and shows the same spots less distinctly in C. montanus, 

 has turned whollv black in C. callidus. Finallv in C. sackeni 



