20 BRITISH LEPIDOPTERA. 



the dorsal part of the larva ; this corresponds with the heart of the 

 higher animals. The other organs gradually undergo differentiation, 

 and the mouth organs also become developed. At this period of 

 development faint pulsations of the dorsal vessel are discernible. 

 The separation of the alimentary canal into an oesophagus, a widened 

 sac or stomach, and another contracted tube or intestine is clearly 

 discernible, whilst the outer proteid part of the egg-contents is 

 probably absorbed by cutaneous endosmosis. The tracheae are developed 

 from the spiracles inwards, but do not become visible until injected 

 with air. 



Such are the broad outlines of the larval development within the 

 egg. From a tiny mass of protoplasm in the yelk of the egg, we get 

 a larva produced such as we know it when newly-hatched. The egg- 

 shell of most of our larger species is too opaque to allow these changes 

 to be seen, but they can be readily observed, as we have already stated, 

 in the eggs of Tortricids or Pyralids, owing to the thinness of the 

 walls of the eggs in these groups. 



During the first stages of embryonic development, the ventral side 

 of the embryo is external, or lies along the inner concave side of the egg, 

 development commencing (as is usual in the Articulata and Vertebrata) 

 on the ventral side of the insect. As development proceeds, the embryo 

 changes its position, on account of the turning of the anal segment 

 and its gradual upward movement, and that of the growing segments 

 behind it, along the venter. In this manner the ventral part of the 

 embryo gets turned towards the centre of the egg, whilst the dorsal 

 part is turned towards the outside. 



Our observations of these movements were made on the embryo of 

 Peronea (Tortrix) ferrugana. We found that when the embryo begins 

 to show traces of segmentation, the thoracic segments are seen to develop 

 three pairs of jointed buds or legs. At this time the embryo occupies 

 a somewhat curved position, with the head slightly bent round 

 towards the anal extremity, but with the legs outside, i.e., the larva is 

 bent back on itself so as to form a curve agreeing roughly with the 

 curvature of the shell, with what afterwards becomes the ventral sur- 

 face of the larva outside, and the dorsum towards the centre. The 

 embryo then gradually changes its position, the anal segment curling 

 round and being pushed by the growth of the preceding abdominal 

 segments, slowly up the ventral surface of the larva; whilst the 

 dorsum gets pushed out, as it were, towards the centre of the egg. 

 During this process the embryo becomes shaped something like the 

 letter S> the movement continuing until a complete reversal of the 

 embryo has been effected. The next stage is that in which the head 

 and anus are in contact, each half running almost parallel, and this 

 again is followed by an almost circular position, in which the dorsal 

 area is now outside, and the ventral surface (with the legs) on the inside. 

 The head, during all this time, scarcely changes its position. Very 

 little further change in position takes place, the embryo, by this time, 

 occupying all the available space in the egg. 



With regard to the change in position that the embryo undergoes 

 in the egg, Chapman says that at the time that the ventral surface is 

 towards the margin of the egg, the dorsal surface, or rather dorsal 

 aspect, is still applied to the yelk-sac. At this time the dorsal sur- 

 face is still broken by the umbilical opening, but, when the latter closes, 



