OB^ THE TABANID^ OF THE UNITED STATES. 423 



ones, I have described what I considered as well-defined types, indicating at the same time 

 the aberrant types, but without deciding whether they are species or varieties. The dis- 

 crimination of closely allied species in such cases, is a matter of time and of local ob- 

 servation. 



One of the difficulties one has to contend with in describing species of Tabanus, is the 

 small number of males usually found in collections, and the vmcertainty, in some cases pre- 

 vailing, as to what species they really belong. Among the twelve species of the subgenus 

 Therioplectes I have not seen the males of more than one half, although the females were 

 numerously represented. It would be imprudent to attempt the description of the proba- 

 bly numerous closely allied species of that group, without the knowledge of both sexes. 

 Males are usually the bearers of the most distinctive and most plastic characters, and in the 

 present genus, owing to their scarcity, they do not seem to have been sufficiently studied. 

 A great deal remains to be learned yet from local observation on males and females caught 

 in the same locality. Thus Dr. Zeller, in 1842, separated from the common European T. 

 hovinus the closely allied T. svdeticus, which, in the female sex, shows but slight differences, 

 while in the males a most marked distinguishing character exists : T. bovhitts has the fleets 

 nearly of the same size over the whole eye, while T. sudedcus has the usual dividing line 

 between large and small facets very distinctly drawn. 



It will be noticed that the descriptions are not preceded by diagnoses of the species. I 

 thought that a carefully prepared analytical table, as well as the comparisons of closely 

 allied species, which I have placed at the end of the descrijitions, sufficiently supply the 

 place of diagnoses. 



The most tru,stworthy characters ^ for the discrimination and description of the species of 

 Tabanus, are the plastic characters taken from different parts of the head : the shape of the 

 palpi, of the third antennal joint, of the frontal callosity and the breadth of the front 

 itself; in the male, the shape of the head, and the relative size and distribution of the large 

 and small facets on the eyes. Although not absolutely immutable, these characters are 

 more so than coloring or pubescence ; they have, moreover, the advantage of greater dura- 

 bility, and can be observed even in badly jireserved and old specimens. Some of these 

 characters, of course, do not admit of any other but of a comparative description ; thus 

 when I say " front rather broad," I suppose the reader to be acquainted with the frontal 

 breadth in the allied species. The venation of the wings affords very few available charac- 

 ters; the most useful is the degree of opening of the first posterior cell at its distal end. 

 The differences in the markings on the eyes have been used for the discrimination of certain 

 closely allied European species. These markings are not absolutely the same in all speci- 

 mens of the same species, and Zeller observes in this connection that the position of the 

 crossbands is of more importance than their length or breadth. Sliglitly marked cross- 

 bands, with an indefinite outline, are apt to disappear altogether in some specimens (see 

 Loew, 1. c, p. 575). 



The coloration of the eyes can usually be revived in dry specimens by putting them on 

 moistened sand for a few hours. The experiment, however, does not always succeed, and 



'To those who wish to know more about the characters (Verh. Zool. Bot. Ges., 'Wien, 1858, p.573-GI2) and Zcller's 

 available for ilescribing species of Tabanus, I recomtnend Dr. article in Oken's Isis, 1842. 

 Loew's Zur Kenntniss der Europaisehen Tabanus-Arten 



