546 - THEREVlD^ 



widely open in a number of species whicli many authors have placed in Dialineura. 

 The metamorphoses of scarcely any species are well known, but the larvae are 

 very active in their earlier stages and are said to live in damp earth and in fungi, 

 while the pupae have been found beneath stones _; there is however reason to believe 

 that the larv« are (like the Bomhylidce) parasitic on Lepidopterous (JVoctuce) larvae 

 as I have noted on page 539. 



This genus is well distinguished if limited to the densely hairy- 

 faced species which have the basal joint of the antennae not dilated 

 and the fourth posterior cell distinctly contracted towards the wing- 

 margin even if not quite closed. Dialineura can then be distinguished 

 by the dilated basal joint of the antennae, the bare face, and the widely 

 open fourth posterior cell, but this last character is only subordinate value 

 as it occurs in many species of Thereva. Psilocephala may be at once 

 distinguished by its bare frons and face and its more oblong abdomen. 



Thereva is by far the largest genus in the family, and more than fifty 

 species (in my interpretation of the genus) have been described from 

 Europe. They occur on the leaves of shrubs and on large leaved plants 

 (such as nettles) and in hedge-rows, where they sit very warily and are 

 not at all easy to capture, while some species sit on hot dry sand or bare 

 places ; I have seen the males of T. annulata executing a frantic dance 

 over hot white sandy places in small numbers in the hottest sunlight, and I 

 think I have also seen the males of T. nohilitata acting in a similar 

 manner, while other species are said to dance in small swarms (like some 

 species of Empidce) round about an individual bush. I cannot help 

 suspecting that hybrids occur in this genus, as I have seen a female 

 taken at Padstow in Cornwall in July 1902 which gave me the impression 

 of being a hybrid between T. hipunctata and T. nohilitata, which both 

 occurred there at that time, but it may have been a female of T. marginula 

 which however has not yet been recognised as British ; Colonel Yerbury 

 has taken T. annulata {$) and D. anilis (?) in cop., and I have seen 

 specimens of T. annulata with a most suspiciously brownish pubescence 

 on the thorax; Zetterstedt has recorded T. hipunctata 6 m cop. with 

 T. annulata ? , and Eossi (Faun, etrusc, vol. ii., p. 274) stated that he 

 had taken his B. marginata (which is consequently a Thereva, and possibly 

 T. margimda Meig.) in cop. with T. plch&ia ; Eossi's B. marginata was 

 described between his B. pleheia and B. anilis, of which the latter seems 

 to be T. annulata. Numerous species of Thereva occur in North and 

 South America, South Asia, and Australia, while three species each are 

 recorded from South Africa and New Zealand. 



Synonymy.— 1 have preferred the original spelling of Thereva to the emendation 

 Thereua,k^ mentioned in the synonymy of the family. 7?i62^o Fabricius (1781-1787) 

 is practically a synonym of Thereva but is quite distinct from Bibio Geoffroy (1764). 



Tahle of the Males. 



1 (14) Pubescence not silvery white (even though mainly whitish is 



T. hipunctata). 



2 (13) Pubescence of abdomen mainly tawny, yellowish grey, or black 



(not whitish or pale yellowish grey). Wing-veins normally 

 distinct. 



