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TEXT-BOOK OF ENTOMOLOGY 



observed that the scales are not all formed at once, but arise one 

 after another, so that on one and the same wing the scales are in 

 different stages of development. 



More recently Schaeffer has stated that the scales and also the 

 hairs are evaginations of greatly enlarged hypodermis cells, and still 

 more complete evidence has been afforded by A. G. Mayer (1896). 

 In the wings of Lepidoptera, about three weeks before the imago 

 emerges, certain of the hypodermis cells, which occur at regular inter- 

 vals, begin to increase in size and to project slightly above the level of 

 the hypodermis ; these are Semper's " formative cells," and are des- 

 tined to secrete the scales. They increase in length, and appear as 

 in Fig. 223. In the next stage observed, the projections are much 



longer (Fig. 224). The hypodermis 

 is now thrown up into a regular 

 series of ridges, which run across 

 the wing. Each ridge, says Mayer, 

 corresponds in position with a row 

 of formative cells, and each furrow 

 with the interval between two ad- 

 jacent rows. The scales always 

 project from the tops of these 

 ridges. The ground or basal mem- 

 brane has not participated in this 

 folding, and the deep processes of 

 the hypodermis (p?r) that once 

 extended to this membrane have 

 largely disappeared. Figure 225 

 represents a more advanced stage 

 almost eight days before the emer- 

 gence of the imago. 

 The scales are originally filled with protoplasm, which gradually 

 withdraws, leaving behind it little chitinous bars or pillars which 

 serve to bind together the upper and lower surfaces of the scales, 

 and finally the scales become " merely little flattened hollow sacs 

 containing only air." As Mayer shows (Figs. 226, 227), from the 

 study of scales examined four days before emergence of the butterfly 

 (Danais), "the striations upon the upper surface of the scale are 

 due to a series of parallel longitudinal ridges," while the under side 

 is usually smooth. 



The mode of insertion is seen in Fig. 227. The narrow cylindri- 

 cal pedicel of the scale is merely, according to Semper, inserted into 

 a minute close-fitting socket, which perforates the wing-membrane, 



Fio. 225. Portion of a longitudinal section 

 through a pupal wing about. eight days before 

 emergence : x y formative scale-cell ; upper , a 

 scale. 



