40 



which orders the mode of development of the wings has been 

 somewhat fully observed. The wings in certain Lepidoptera, and 

 probably in many other insects, begin to form in the embryo before 

 the larva or grub hatches from the egg. They first appear as folds 

 or outgrowths of the hypodermis, and lie in pouches on either side 

 of the body. In those insects that have no distinct pupal period, as 

 the grasshoppers, the wings begin to appear externally in the third 

 stage of larval life, i. e. after moulting has taken place twice ; in the 

 holometabolic insects, as the Diptera and Lepidoptera, the wings are 

 not seen externally until the pupal state is reached. 



Herold was the pioneer of these studies on wing-structure, and in 

 1815 he described the "primitive wings or wing germs" in the 

 caterpillar of the cabbage white butterfly {Pieris brassicce) after the 

 third moult. This observer informs us that the primitive wing germs 

 appear on the inside of the meso- and metathorax, and may be 

 recognised by their attachment to the hypodermis, and by their 

 regular symmetrical form, whilst fine tracheae were observed attached 

 to the wing germs. At the same time Herold discovered the mode 

 of origin of the nervures of the wing, and traced out the mode in 

 which the scales were formed. Earlier observers, Malpighi, 

 Reaumur, Swammerdam, and Lyonet, had observed these wing 

 rudiments in the larva just before pupation under the old larval skin, 

 and although they were observed to be situated in the "fat body" 

 of the larva, without being attached to it or being a consrituent part 

 of it, and that they were fastened to the skin in a deep fold which 

 the skin makes at the point of attachment, yet their true nature was 

 scarcely understood. We might here observe that in all those 

 butterflies with prominently developed pupal wings, the wings are 

 specially well developed in the last larval instar. They are par- 

 ticularly noticeable in the full-grown larva of Euchlo'e cardamiiies 

 and Anthocaris belia. 



Whilst Weismann was conducting his researches into the embry- 

 ology of insects (particularly the Diptera, Musca vomitoria and Sar- 

 cophaga car/iaria) he observed that the larvae of these flies had 

 developed wings previous to pupation, and also that the legs, 

 imaginal mouth appendages, &c, were also undergoing development. 

 He then discovered that these various structures — -wings, legs, &c. — 

 were developed from minute masses of microscopic cells which he 

 called "imaginal discs." Six of these " imaginal discs " were ob- 

 served on the venter of the three thoracic segments, and four lateral 

 discs, one on either side of the meso- and metathorax. The former 

 gave rise to the legs, the latter to the wings of the imago. The 

 "imaginal discs" of the wings were afterwards found to be present 

 in newly hatched larvae. They appear to be from the first in 

 connection with the hypodermis, and are attached to minute 

 tracheae, forming minute folds of the peritoneal membrane of these 

 tracheae. These tracheal enlargements increase in size until at last 

 they become differentiated into a mass that corresponds with the 



