APPENDIX. 709 



sents the month-parts of one of the Mallophaga, Goniodes, to 

 compare with the rndimentary mouth-part of Pediculus ', ibis 

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

 mandibles ; max^ the maxillse ; Z, the lyre-formed piece ; pi, the 

 "plate", and o, the beak or tongue. (This and figs. 658-661 

 are from Melnikow's memoir.) Fig. 662 represents the mouth 

 of Pediculus vestimenti (copied from Schiodte) with the parts 

 entirely protruding, and seen from above, magnified one hun- 

 dred and sixty times ; aa, the summit of the head, with four 

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

 part 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 rudimentary state in the 

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

 isolated portions of the inner layer of skin, attached either to 

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

 in the young Jarva), which 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 embrj'o. Weismann has 

 also satisfactorily shown that in the Diptera the wings arise 

 from similar discs in connection with what he doubtfully re- 

 garded as a nerve. 



More recently, however, Landois has published in Siebold 

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

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

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

 expansions of the peritoneal membrane surrounding a trachea. 

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

 transform into elongated nucleated cells, in which tracheae are 

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

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

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



