172 ABDOMINAL PATTERN IN TABANIDAE (dIPTERA) 



as a derived condition, if it can be shown that existing traces or 

 rudiments of pattern are themselves the remainders of a derived 

 type, so that the possibiHty is excluded that they might be a sort 

 of pattern in statu nascendi. If patterns appear de novo, it should 

 be assumed that they will appear in places where the physiologi- 

 cal conditions for their appearance are in evidence; it would be 

 astonishing if a pattern would appear as the result of a number 

 of sinmltaneous but otherwise entirely unrelated changes, while 

 by a single change in the germ plasm either a homogenous colora- 

 tion, or, if any pattern, a more or less isotropic pattern will be 

 developed, showing at most a relationship to the borders of the 

 segments or other preferential localizations. 



These principles are sufficient to classify the abdominal pat- 

 terns of Tabanidae as to their phylogenetic value or relative 

 degree of evolutionary specialization. It should be kept in mind, 

 however, that here no classification of species is intended; this 

 would be entirely impossible on such basis, as often male and 

 female of the same species represent divergent types of pattern 

 of very different degree of specialization. For the larger groups, 

 subgenera, etc., often a certain degree of specialization is char- 

 acteristic, but not unfrequently one or two species, or one sex of 

 a species, will show a pattern which is more frequently encount- 

 ered in another systematic category, lower or higher in the system, 

 as the case may be. 



The most primitive tj^pe of abdominal pattern which I was 

 able to find in Tabanidae, is found in the genus Haematopota and 

 in a number of species of Tabanus which are evidently related to 

 Haematopota. Tins pattern consists of alternating dark and 

 light elements of medium size, usually five light spots separated 

 by dark on each segment, so that a light spot occupies the middle 

 of the dorsal side of each segment, while two others, which we 

 may call the sub-dorsal and lateral spots, occupy the sides. One 

 will not expect to find everything perfect and to one's wishes in 

 nature; in fact, even in the most generalized forms of which I 

 found illustrations, this ideal pattern is shown slightly modified, 

 and so, especially on the first and again on the last segments of 

 the abdomen in the example of Haematopota hrunnescens (fig. 1), 

 an African species, show that the lateral spots are preserved only on 



