PEOMACHUS.— EEAX. 197 



Promachus quadratics, Bellardi, loc. cit. p. 27, t. 2. f. 3. 



trapezoidalis, Bellardi, loc. cit. p. 28, t. 2. f. 4. 



jpulchellus, Bellardi, loc. cit. p. 29, t. 2. f. 5. 



truquii, Bellardi, loc. cit. p. 30, t. 2. f. 6. 



ERAX. 



Erax, Macquart, Dipt. Exot. i. 2, p. 107 (1838). 



This genus is numerously represented in North and South America. Owing to the 

 difficulty of drawing good descriptions it is almost hopeless to identify specimens from 

 the existing ones. What is needed are comparative descriptions of the species of a 

 given fauna ; it will be possible on that basis to determine specimens of that same 

 fauna; but it will be impossible from those descriptions, and without comparing 

 types, to establish the specific identity of specimens from a more distant region. 

 The general colouring is not only difficult to describe, but variable ; the yellowish 

 greys, greyish yellows, brownish greys, and greyish browns pass into each other 

 in the same species. The colour of the bristles on all the parts of the body is 

 also variable. 



In E. prolificus, sp. n., which I have attempted to describe from a large number of 

 specimens, the stiff bristles on the upper part of the occipital orbit are either all black, 

 pale whitish, or of both colours mixed. The same is the case with the macrochaetse on 

 the thorax or on the legs of this and many other species. The sexes of Erax are so 

 different in appearance, and the females of different species often look so much alike, 

 that mistakes may easily occur in bringing together the sexes of the same species. I 

 strongly suspect, for instance, after having seen the types, that E. quadrimaculatus, 

 Bell., c? , and E. bimaculatus, Bell., ? , belong to the same species. A thorough study, 

 which this genus has not yet received, may disclose some new characters available for 

 the definition of species. As a slight contribution to that future study, I will advert 

 to two secondary sexual characters existing in this genus : — 1, on the hind side of the 

 front tibiae there are some long bristles, usually " four " in number in the female and 

 only " two " in the male ; in the latter bristles are inserted on the distal third of the 

 tibia : 2, in many species, and principally in the group with a crest of hairs on the 

 thorax, the stump of a vein on the anterior branch of the third vein exists in the female 

 only ; in the male it is either absent or rudimentary. 



It may not be amiss to notice here that the terms " upper and lower," " front and 

 hind side," are applicable to the legs of Diptera in an imaginary position : — e. g. as if 

 the legs were stretched out perpendicularly to the longitudinal axis of the body 

 (compare Mik, Dipterologische Untersuchungen, p. 3). 



