lu Inti'oduction. 



cubital vein or between tlio branches of tlie latter. The discai vein 

 in our terniiiioiogy is in the basal part formed by the junction of tvvo 

 concave veins (IV and VI), it then divides, and the two veins enclose 

 the discai cell, from this often some veins go to the wing margin; the 

 discai vein is, as formed of two concave veins, concave itself; between 

 the two veins forming it. should lie a convex vein, the vein V, but it 

 is generally absent, or only some end branches are left. For con- 

 veniences sake we use the name discai vein for the whole vein with 

 its branches, bul it must be born in mind that when tliere. among 

 these latter, are convex veins, these originally belong to vein V, the 

 rest of which has disappeared. The discai vein may also be simple 

 and unbranched. Between the discai vein and the cubital vein lies 

 the medial cross-vein, and between the discai and the postical veins 

 lies the postical cross-vein; these cross-veins or one of them may 

 sometimes be absent. The postical vein is ahnost ahvays branched, 

 the lower branch generally uniling with the anal vein for a shorter or 

 longer distance. The anal vein is unbranched; the axillary vein is 

 generally short and often absent or quite rudinienlary, only in the 

 TipuHdæ it reaches the margin. The first posterior cell lies between 

 the convex cubital vein and the concave discai vein, and is bordered 

 towards the base by tiio medial cross-vein; the other posterior celis 

 lie between the branches of the discai vein. the fifth ahvays in the 

 fork of the postical vein and thus ahvays bordered both above and 

 below by convex veins (in the Muscidæ the lower branch of the 

 postical fork is short, more or less vertical and uniting the anal vein 

 rather near to its base, the fiflh posterior cell is then bordered below 

 by the concave anal vein). The anal cell lies between the convex 

 postical vein and the concave anal vein; the space below tlie anal 

 vein is lermed the axillary cell. The Iwo basal cells lie towards the 

 base of the wing and are separated from one another by tlie basal 

 part of the discai vein. 



Many alterations, chiefly reductions, in the venation may take 

 place, and these will be considered in the particular piaces; I shall 

 only here point out some of the more important. F. inst. some veins may 

 totally disappear; the vein V as mentioned is generally almost or quite 

 wanting, and this is characteristic of the Diptera, yet in some flies, 

 especially Mycetophilidæ, it is present to a greater degree. As already 

 noticed the discai vein may be simple and unbranched (fig. 3), and 

 then it does not enclose a discai cell ; this is the case in all cyclor- 

 rhaphous and some orthorrhaphous flies; in this case there is no genuine 

 discai cell but this is confluent with the fourth posterior cell, yet for 

 convenience in the descriptions it may be termed the discai cell, but 



