DEVELOPMENT OF THE EXTERNAL FORM OF THE IMAGO. 373 



that are occupied by the tracheal ramifications ; here also the blood-fluid 

 circulates, and these lines become transformed later into the network of veins 

 in the wing. In later stages, tracheae are no longer to be found within the 

 veins ; they have either degenerated or, as in Musca, according to WEISMANN, 

 they have been withdrawn out of the veins into the thorax. There, however, 

 remains in the veins a strand, which was discovered by Semper in the Lepi- 

 doptera, and which in early stages accompanies the tracheae ; this we may call 

 the rib-strand ( Semper' s icing-ribs). It resembles a tracheal tube, and consists 

 of an outer matrix and an inner intima which gives off projecting dendriform 

 processes into the lumen. The centre of the lumen is occupied by a longitudi- 

 nally striated strand (a secretion ?). Semper was able to prove the connection 

 of these rib-strands, which, in the adult, are only to be found in the basal half 

 of the wing, which they serve to support, with the tracheal system. These 

 strands must therefore be transformed tracheae. Nerve-trunks are also found 

 in the wing-veins. 



The cuticle of the wing, which does not develop until somewhat late, is 

 considerably thickened on the surface of the veins. The manner in which the 

 two hypodermal lamellae of the wings fuse is of some interest. A " basal 

 membrane" develops on the inner surface of the hypodermis on each side, 

 while the hypodermal cells themselves become pillar-like. The two basal mem- 

 branes become closely applied to one another, fuse, and finally disintegrate, 

 so that, in the adult wing, the hypodermal pillars extend continuously through- 

 out the whole thickness of the wing. 



It should here be mentioned that the facts of ontogeny are not favourable 

 to Adolph's theory of wing-venation. According to this theory, the veins 

 of the fully formed wing are to be divided into convex and concave veins, which 

 differ in their origin, the concave veins being derived from tracheae, while the 

 convex veins develop out of cell-strands into which tracheae can extend only 

 secondarily. The system of convex veins and that of concave veins are origin- 

 ally altogether distinct. But it has been proved by Brauer and Redtenbacher 

 (No. 101) for the wings of the Odonata, and by Grassi for those of the Termites, 

 and more recently by Haase (No. 108) for those of the Lepidoptera, that the 

 branches of one and the same tracheal trunk may be changed partly into convex 

 and partly into concave veins, so that the postulate on which the theory rests 

 is negatived. This theory is also opposed by Van Bemmelex (No. 99), who 

 confirms the observation made by Fe. Mulleb in connection with the 

 Nynyrfialidae, that the system of veins in the Lepidoptera immediately after 

 entering the pupal stage differs in details from that of the adult form. The 

 observation of the development of the venation in the wings has thus a certain 

 phylogenetic significance. 



The hairs and scales of the Lepidopteran wing arise as outgrowths of single 

 hypodermis-cells (mother-cells of hairs and scales, Semper). The characteristic 

 definitive markings develop only after the differentiation of the scales. It must, 

 however, be mentioned that, according to Van Bemmelex, the permanent 

 markings are preceded by transitory markings, which are essentially distinct 

 from the former, but have a few features in common with them. 



Much more complicated ontogenetic processes are met with in the 

 Muscidae. The limb- and wing-rudiments here arise in the same' 

 way as in Corethra. But, in the Muscidae, the whole of the 

 imaginal rudiment is shifted much further into the interior of 



