174 ABDOMINAL PATTERN IN TABANIDAE (dIPTERa) 



place, in so far as here the subdorsal spots have also disappeared 

 and only the median spots remain. In Tabanus lineola (fig. 5) 

 we have a type of pattern in which a white dorsal longitudinal 

 band is accompanied by two black bands, and these border again 

 on a white band which in turn is bordered by dark. This pattern 

 may give, as the case may be, the impression of three white bands 

 bordered by black, or of two black stripes on light ground. Fre- 

 quently, as in T. costalis, and in T. taeniola (fig. 6) the lateral 

 dark areas are rather light, so that the pattern is further simpli- 

 fied. The dark pigmentation even of the stripes bordering the 

 median light band may be rather pale, so that the whole effect is 

 one of light coloration. From dark gray with three distinct 

 white stripes bordered by black, we have all transitions to a gen- 

 eral olivaceous yellow color with a pale yellowish white stripe 

 along the median line, bordered by grayish color hardly darker 

 than the surrounding color. 



Another type of modification of the primitive pattern is found 

 when the median light spots have been greatly reduced (T. lasi- 

 opthalmus, fig. 7), or altogether disappeared (T. dilaeniatus, fig. 

 14). In T. lasiopthalmiis rudiments of paired black elements are 

 visible on segments three to five and rudinients of white trian- 

 gular spots on segments three to four. Excepting these spots, 

 the entire median stripe is lacking. On the other hand, the sub- 

 dorsal spots have been retained on most segments and have been 

 greatly enlarged on segments one and two; here they are placed 

 obliquely, following the same tendency as already indicated in 

 T. sufis (fig. 2) ; their outer contours have become indistinct 

 owing to the diffusion of the lateral spots in their dark back- 

 ground. The disappearance of white spots on a dark back- 

 ground frequently has the effect of lighting up the background 

 in grayish or brownish hues of indistinct boundary, and it is in 

 the nature of the reduction process that these indistinct shades 

 are most often encountered on the sides of the segments, while 

 the middle retains a well-marked pattern. 



A third tA'pe of modification is that in which the median white 

 spots alone have been preserved, and, subsequently, undergo 

 reduction in number or in size. We have seen in T. sufis (fig. 2) 

 how the triangular form of these median spots originates: it is 

 due to the tendency of all dark and light spots (or cross-bands) 



