INTRODUCTION. 33 



troinyia, Dicranota, and Rliaphidolahis. In those genera where 

 the discal cell is open in some species only, or in some specimens 

 of certain species, the same rule prevails ; it coalesces with the 

 second posterior cell, when there are four such cells, and with the 

 third, when there are five (compare the genus Dicranomyia). 

 Cases, where the anterior branch of the fourth vein is forked and 

 the posterior not ; in other words, where, with four posterior cells, 

 the discal cell coalesces with the thii'd posterior cell (as in Tab. 



I, fig. 15) ; such cases are rare, and occur more commonly only 

 in the section Erioptei'ina (compare the general remarks on 

 this section) ; outside of it, the genera Thaumadoj^tera and 

 Elliptera (Tab. I, fig. 10) only possess t-his character. In Dic- 

 ranomyia pubipennis 0. S., also, when the discal cell is open, it 

 coalesces with the third posterior cell ; a singular exception 

 from among all the Dicranomyise, Outside of the Tipulida; 

 brevipalpi, this structure may be observed in Pfychoplera (Tab. 



II, fig. 19). The occurrence of five posterior cells, without any 

 fork on the posterior branch of the fourth vein, can take place 

 only when the anterior branch of this vein has a double fork. 

 This is the case with Dolichopeza ; but I have not met with any 

 instance of this kind among the Tipulidse brevipalpi, except in 

 the Limnophilina. It is worthy of notice, that in this section 

 where the discal cell is, as a rule, always closed, whenever an 

 abnormal specimen is met with, where this cell is open, the 

 branching of the fourth vein is very apt to appear like that of 

 Dolichopeza. 



The fork of the anterior branch of the fourth vein is formed by 

 the insertion of the vein which Mr. Loew calls the anterior inter- 

 calary vein (u in Monogr. I, p. xxiv, fig. 3). It is the addition 

 of this vein which raises the number of posterior cells to five. 



The small cross-vein usually forms the inner end of the first 

 posterior cell. In some rare cases the inner end of the sub- 

 marginal cell is in immediate contact with the discal cell (as in 

 the wing of Triogma, Tab. I, fig. 7), and in such cases there is, 

 of course, no small cross-vein. This structure characterizes the 

 genera Triogma and Paratropeza Schiner ; it also occurs in most 

 specimens of the North American Cylindrotoma nodicornis and 

 adventitiously in the genus Rhamphidia. 



I call prsefurca (a term which has been used by Mr. Ilaliday 

 in Walker's Ins. Brit. Dipt. Ill, p. 304) the portion of the second 



3 June, 1868. 



