APPENDIX . 709 



sents the nioutli-psirts of one ol" the Mallopliaga, Goniodes^ to 

 compare with the rudimentary mouth-part of Pedicuhcs ; lb is 

 the upper lip, or hibrum, situated under the clypeus ; mad, the 

 mandibles ; max, the maxillae ; I, the lyre-formed piece ; pi. the 

 "plate", and o, the beak or tongue. (This and figs. 6o«-GGl 

 are from Melnikow's memoir.) Fig. 6G2 represents the mouth 

 of Pediculus vestimenti (copied from Schiodte) with tlie parts 

 entirely protruding, and seen from al)OAe, magnified one hun- 

 dred and sixty times ; oo, the summit of the head, witli four 

 bristles on each side ; bb, the chitinous band, and r, the hind 

 jiart of the lower lip ; dd, the foremost protruding part of the 

 lip (the haustellum) ; ee the hooks turned outwards ; /', the 

 inner tube of suction slightly bent and twisted ; the two pairs 

 of jaws are perceived on the outside of these lines ; a few blood 

 globules are seen in the interior of the tube. 



Formation of the Wings. — As has already been remarked 

 on p. 64, the genital glands and the muscles of the adult insect 

 were found by Weismann to exist in a rudimentaiy state in the 

 embryo, while the imaginal discs (wliich are minute scales, or 

 isolated portions of the inner laj-er of skin, attached either to 

 a nerve or trachea, and which are readily seen on dissection 

 in tlie young larva), wliich are destined to grow and spread so 

 as to form the skin of the adult, even exist, though in an ex- 

 tremely rudimentary condition, in the emliryo. AVeismann has 

 also satisfactorily'^ shown that in the Diptera the wings arise 

 from similar discs in connection with what he doubtfully ve. 

 garded as a nerve. 



More recently, however, Landois lias published in Siebold 

 and Kolliker's "Zeitschrift " a fuller account of the formation 

 of the wings in the butterflies. They are found to exist in tlie 

 caterpillar, soon after leaving the egg, in the form of minute 

 expansions of the peritoneal memV)rane surrounding a tracliea. 

 This forms a microscopic sac filled with fat cells, some of wliich 

 transform into elongated nucleated cells, in which trachea- are 

 developed. As the bag grows larger, the trachcie enlarge, and 

 project towards what is destined to be the omer edge of the 

 wing, until when the larva is ready to transform into the pupa, 



