6 Mr. Westwood's Observations on the Genus Derbe. 



I have found it exceedingly difficult to refer the very variable arrangement 

 of the veins of the wings of these insects to a primary type ; although it will ap- 

 pear quite evident, from the figures given in the accompanying plates, that the 

 species of each subgenus agree in the general character of the veining of the 

 vrings, thus proving the value of this character. It appears to me, however, 

 that we are able to trace the mediastinal, postcostal, median and anal great 

 veins in some of these insects, although in others one or more of them be- 

 come so modified as to seem lost, or to have sunk into mere branches of 

 one of the others. The fore-wings of the three species of Mysidia, repre- 

 sented in Tab. I. fig. 3, 4 and 5, agree in the main arrangement of the veins ; 

 and it will be observed that these wings are comparatively short and broad, 

 but the fore-wings of the typical species of Derbe are very much elongated, 

 and are consequently furnished with a greater number of veins ; there being, 

 in fact, six longitudinal branches added. If, however, that portion of the wing 

 of Z). semistriata which is shaded in fig. 1 D, x, were to be cut out and inserted 

 into the wing of Mysidia, in the situation indicated by the mark x, in fig. 3 A, 

 the two wings will be found to be exactly alike in the veining ; in other words, 

 the part of the wing shaded in Derbe semistriata is supplemental, if I may 

 so speak. It is moreover to be observed, that the posterior branch of the 

 postcostal vein of Mysidia, indicated by the letter o (fig. 3 A), seems trans- 

 formed into the true anterior branch of the median vein in Derbe semistriata 

 (fig. 1 D, o). The normal condition of this vein in Derbe is still, however, in- 

 dicated by the small transverse vein (q), which connects the postcostal and 

 median veins ; for on looking alone at the wing (fig. 1 D), it would be imme- 

 diately concluded that the veins o and o o, and the several veins between them, 

 are all branches of the great median vein C, and thus the little transverse 

 vein q is but a supplemental one, giving support to these longitudinal veins. 

 On looking, on the other hand, at the wing of Mysidia pallida (fig. 5), the 

 vein is found to be quite independent of the postcostal vein, forming a 

 branch of the median vein much curved at its base {p), and only wanting 

 the little transverse vein q to identify it with the wings (fig. 3 A and 4 A). 

 On looking, then, at these two wings, we should immediately be led to con- 

 clude that the vein a was a posterior branch of the postcostal vein, the small 

 vein q here becoming oblique, so as to form the true base of the longitudinal 



