138 Orthorrhapha brachycera. 



wood. Mr. Schlick and I have bred T. bipunctata from a pupa found 

 in sand, T. nohilitata from larvæ and pupæ found in eartii and in 

 decaying wood in an oak and T. circumscripta from larvæ and 

 pupæ found in earth and also in a decaying oak. Finally Psilocephala 

 melaleuca was bred (Frauenfeld, Verh. zool. bot. Geseli. XVI, 1860,449) 

 from a larva found in decaying wood. Westwood (Proc. entom. Soc. 

 London, 1859, 59) mentions the larva of an undetermined species 

 which attacked the pupæ of Aleucis pidaria and Sphinx ligustri. 



The species of Thereva are very characteristic flies on account 

 of their whole shape and appearance; they occur in the herbage on 

 the outskirts of woods and in similar localities, some species especially 

 on dry, sandy piaces often at the shore; several species are especially 

 found on Urtica. Schiner records that he has seen the males of T. 

 anilis swarming in sunshine around bushes, The species are generally 

 considered as predaceous, though I am not avare that any direct 

 observation has been made. I am inclined, however, to think this 

 correct, as their whole behaviour seems to point in this direction; 

 they are for example often seen sitting on the leaves in a way which 

 gives the impression that they are watching for prey; I think the 

 prey is small flies and the like. I may here mention the faet that, 

 during the last summer (1907) which was very cold and rainy, the species 

 of Thereva were very common and present in great numbers in con- 

 trast to what is generally the case; both I myself and also my ento- 

 mologicai friends made this observation. 



Of the genus 66 species are known from the palæarctic region; 

 10 have hitherto been found in Denmark. 



The Thereva species are generally, and also here, divided into 

 two groups in accordance with the fourth posterior cell being closed 

 or open (the latter group forming the genus Dialineura Rondani), but 

 it must be remarked, that in the group with the cell closed individual 

 aberrations, with an open cell, are found, and not even rarely, some- 

 times only one wing shows the aberration, or the extent of the 

 opening is different in the two wings; generally it will not give rise 

 to confusion. — The species are somewhat difficult to distinguish, 

 especially the males of the species grouping round T. nobilitata, as 

 there are no plastic characters and the colour of the pilosity is some- 

 what variable; by carefull use of the foUowing analytical table and 

 descriptions I think it possible to determine the Danish species with 

 certainty. 



Table of Species. 



1 . Fourth posterior cell closed 2. 



— Fourth posterior cell open 8. 



