540 THEREVID.E 



" belong ; he sees that species are often sharply and surely distinguished 

 " by trifling differences luit finds little or no information upon such 

 " distinctions in the existing descriptions; he discovers that in the genus 

 " Thercva essentially distinct species may agree in all points of structure 

 " and be only distinguished by the difference of coloration, and at the same 

 " time that the colour of many species is exceedingly variable; these 

 " circumstances compel him to study afresh for a long time before he 

 " arrives at a definite result." After writing these words in an intro- 

 duction, Loew proceeded to describe thirty-three European species, to 

 twenty-one of which he gave new names, and it is disconcerting to find 

 that two of his names have sunk as synonyms and that no less than 

 twelve out of the remaining nineteen new species have never been 

 recognised (or at any rate recorded) by subsequent writers. The 

 distinctive specific characters in the restricted genus Thercva {sensv meo) 

 lie in the colour of abdominal ground markings (probal)ly the best char- 

 acter), the colour of the pubescence on frons, face (middle as distinguished 

 from sidemargins), jowls, lower, middle, and upper parts of the back of 

 the head, Imsal antennal joint, thorax and abdomen especially, back of 

 front femora, etc., while in the females distinctions may be found in the 

 shape of the shining black callus on the frons (a character not absolutely 

 invariable), and the ground colour and pubescence of the last abdominal 

 segments. I have not the slightest doubt that there are many perfectly 

 good species distinguished by these apparently trivial characters, but the 

 difficulty lies in fixing the absolute distinction ; perhaps under microscopic 

 examination characters will ultimately be found in the genitalia, but at 

 present the eye distinguishes our species better than the lens. The charac- 

 ter of the presence of two pairs of praescutellar bristles is usually useful 

 for distinguishing T. fidva, annulata, and lunulata, but is not infallible, 

 while the wide open fourth posterior cell is characteristic of T. lunulata. 

 To add to our difficulties, specimens of British species seldom look exactly 

 like specimens of the same species from the continent, so that even after 

 identification by description doubts may easily re-arise upon comparison. 

 Schiner says the species are as a rule " Eobber Flies," which have a some- 

 what cunning and uneasy cat-like demeanour, in that they lurk between 

 the leaves of low shrubs, and know how to conceal themselves like lightning 

 without flying away. I have never seen them acting predaceously myself, 

 though I admit that their actions appear very suspicious, and it is worthy 

 of note that Professor Poulton has not been able to obtain any record for 

 his paper on predatory Diptera ; Walker however states that they some- 

 times prey on other insects, and Williston says, "Their food is chiefly 

 " other diptera, for which they lie in wait upon leaves and bushes, or upon 

 " the bare ground." 



Synonyiny. — I consider the spelling Thereva to be both more correct and more 

 euphonious than Thereua ; it is also the original spelling proposed by Latreille, and 

 I cannot find the spelling Thereua until Agassiz incorrectly stated in 1846 (Nomenclator 

 Zoologicus) that it was Latreille's spelling of 1796. 



