608 DERMATINA 



6b Third posterior cell ; united witli Q^ in fig. 334, and with 7 in fig. 332, but 



present in fig. 333. 

 6'' Fourth posterior cell, always closed in Afydaidoe. 



7 Postical {or bth posterior) cell {or postical Jork-cell). 



8 Axillary cell. 



9a Upper {or \st) basal cell. 

 9b Second {or middle) basal cell. 

 9° Anal {or Zrd basal) cell. 

 10 Alula. 



Notes on the Venation of the Mydaid^. 



The venation is specially notable in (1) the very long mediastinal and subcostal 

 veins, (2) the remarkable curving up of the cubital and discal veins so that they end 

 in the subcostal vein or at any rate in the costal vein before the tip of the wing, 

 (3) the position of the discal cross-vein which is similar to that in the JS^emestrinido}, 

 and (4) the very short prsefurca. 



(1) The Mediastinal Vein is longer than in any Diptera except the Neme- 

 strinidw, and the length of The Subcostal Vein is only approached by the 

 Nemestrinidce, Apiocerida^., and some Asiltdce. 



(2) The curving up of the two branches of The Cubital Fork and the end 

 portion of the Discal Vein (whether single or duplicate) and their ending in the 

 long subcostal vein are not indicated in any way in the Nemestrinidce, but are 

 represented in the Apioceridce and to a certain extent in the Asilidoi, though in 

 neither of those families to such a remarkable extent. 



(3) The position of The Discal Cross-vein is rather similar (except in Megascelus) 

 to that in the Nemestrinidoi {conf. also Cyrtidce), but altogether distinct from that 

 in the Apioceridce and Asilidce, its position near the end of the discal cell in con- 

 junction with the tying of the veins near the wing-tii? to the subcostal vein evidently 

 aims at strengthening the wing power in a similar manner to that in the Nemestrinidce, 

 but it is almost certain that the similarities which occur in the venation of the two 

 families do not indicate the slightest relationship, and are merely the development of 

 the venation in a somewhat similar manner in order to obtain a similar result. 

 Just as the tying-together of the long-veins in the Nemestrinidce through the cross- 

 veins has caused the pulling of the long-veins out of their normally straight course, 

 so in some genera of Mydaidce {Triclonus, etc.) the discal cross-vein has pulled the 

 discal vein quite out of its normal course. 



(4) The Pr^furca is much shorter (except in the abnormal genus Megascelus) 

 than in any family of the Brachycera except the Stratiomyidce, but is very 

 differently placed from where it is in that family as it commences almost opposite 

 the middle of the second basal cell. The continuous line of veins in the Nemestrinidce 

 which is known as the " diagonal vein " is somewhat indicated in the Mydaidce, but 

 only extends from the base of the jprsefurca to the end of the discal cell, because 

 in the Mydaidce the lower veinlet from the discal cell (if existing at all) is 

 recurrent. 



There are numerous other details in the venation of the Mydaidce which are 

 remarkable, but perhaps the strongest one not already mentioned is in the discal 

 vein ending (with the exception of the lowest veinlet from the discal cell) distinctly 

 before the wing-tip ; in the Apioceridce the second veinlet from the discal cell is 

 always present and ends considerably below the wing-tip. The only other family 

 in Avhich the discal vein ends before the wing-tip is the Scenopinidce, and con- 

 sequently, although the venation of the two families appears at first sight to be 

 completely distinct, they have one important feature in common. The peculiar 

 shape of the discal cell and the closed (normal) fourth posterior cell are noteworthy, 

 and the anal cell is always closed well before the wingmargin. 



Table of the Palcearctic Genera of MYDAiDJi;. 



1 (2) Third antennal joint remarkably large and " monstrous." 



Perissocerus. 



2 (1) Third antennal joint with an elongate narrow base and a droop- 



ing club. 



