372 INSECTA. 



When the limb-rudiments increase in size, the peripodal membrane 

 is correspondingly stretched, while the appendage within it assumes 

 a more or less bent position. In consequence of this, the wing- 

 rudiments seem folded, and the rudiments of the legs in Corethra 

 are spirally coiled. The unfolding of the limb-rudiments is brought 

 about by their protrusion from the invagination in which they were 

 at first hidden. As they become more and more protruded, the 

 peripodal depression becomes continually shallower, and finally 

 the peripodal membrane becomes completely evaginated and forms 

 a part of the general hypodermis. 



The internal organs of Corethra, as compared with those of other Holometa- 

 bola, seem during metamorphosis to undergo only unimportant alterations. Of 

 the striking processes of disintegration and of subsequent regeneration, which 

 have been so well established for the Muscidae, nothing is to be observed in 

 Corethra. It deserves to be mentioned, however, that, according to Kowalevsky 

 (No. 112), a disintegration of the larval and the development of the imaginal 

 epithelium of the enteron of Corethra takes place in the same way as in Musca 

 (see below, p. 383). Most of the larval organs pass directly over into the pupal 

 and imaginal stages ; the general musculature also remains unaltered, but the 

 muscles of the limbs and of the wings oome from new rudiments. The latter, 

 according to Weismann, arise in the last larval stage from cell-strands which 

 appeared as rudiments in the embryo. 



When we consider the unimportant character of the internal changes which 

 occur during the metamorphosis of the Tipulidae, of which Corethra serves as- 

 an example, we shall hardly doubt that the conditions here found represent 

 a transition from the incomplete to the complete method of metamorphosis. 

 This is confirmed inter alia by the short duration of the pupal stage and its- 

 capacity for free movement, as also by the early appearance of the compound 

 eye, a character which Corethra has in common with the Hemimetabola. 



We must now describe more in detail the development of the wings, which 

 has been best ascertained in the Lepidoptera by Semper (No. 126), Landois- 

 (No. 114), Pankeitius (No. 120), and Schaffer (No. 124a). The wings, 

 like the other rudiments of extremities, arise as simple outgrowths of the hypo- 

 dermis within a peripodal depression. They thus at first represent a simple 

 fold of the hypodermis, the point of insertion of this fold being connected 

 internally witli peculiar modifications of the fat-body and of the tracheal system. 

 The fat-body at this point shows accumulations of small cells, which have been 

 regarded by Schaffer as formative centres. The tracheae which join the 

 wing-rudiments form a close network of very fine tracheal tubes, which develop 

 as intracellular structures within single large matrix-cells (Landois, Schaffer). 

 These networks of tracheae degenerate after the pupal stage has been entered 

 upon. On the other hand, large tracheal ramifications develop which run into 

 the wings and lead to the development of their venation. When the caterpillar 

 passes into the pupa, the wing-rudiments are evaginated from the peripodal 

 cavity by the action of increased blood-pressure. The wing-rudiments thus 

 become vesicles filled with blood which contain tracheal ramifications. At a 

 later stage, however, the layers corresponding to the future upper and lower 

 surfaces of the wings become closely applied and fuse, except along those lines 



