42 



The application of the term Lepidoptera, or " scale-winged," to the 

 order that includes the butterflies and moths has begotten the popular 

 but erroneous idea that only these insects have scales. Scales are 

 only modified hairs, and Semper has proved beyond question that 

 their mode of origin is quite identical, whilst even in some 

 larvae the hairs become flattened and scale-like. True scales are 

 characteristic of the Synaptera and Lepidoptera, yet they also occur 

 in Trichoptera, in Psocidae {Amphentomum), in many Coleoptera 

 (Curculionidce, Cleridce, Ptinidce, Dermestidce, Scarabccidce, Cerambi- 

 cidce, &c), in the Culicidae, and a few other Diptera. At the same 

 time it may be well to notice that whilst many species in all these 

 latter orders are without scales, not a single species of the Lepidoptera 

 is known in the imaginal stage to be without them. 



When the pupal wing is undergoing development certain large 

 nucleated hypodermal cells in the wing may be observed to undergo 

 a certain amount of elongation, the elongations projecting beyond 

 the surface of the wing. The cells thus developed are placed at 

 regular intervals on the surface of the wing, and the prolongations at 

 last form regular rows on the surface. It is at this time that the 

 hypodermis of the wing is thrown up into a series of regular ridges 

 which run across the wing. Each of these ridges carries on its 

 summit a row of the prolongations, or primitive scale-cells as they 

 really are, whilst the furrow between two adjacent ridges repre- 

 sents the interval between two rows of scales. The scales always 

 project from the tops of these ridges. 



The scale-cell increases rapidly in size, flattens out, and finally 

 assumes the outward shape of the mature scale. A layer of chitin is 

 then secreted over its entire outer surface, so that the scale becomes 

 a thin, flat, chitinous bag, filled with protoplasm, the chitin upon the 

 upper surface being striated, the lower surface smooth. Many scales 

 have two sets of striae, a well-developed longitudinal set and a finer 

 transverse set. These striations diffract the light, and give rise to the 

 iridescent colours that many scales, particularly those of Lepidoptera, 

 exhibit. 



So long as the scales remain filled with protoplasm they are quite 

 transparent ; but the protoplasm afterwards becomes coarsely granular, 

 and appears to give place to a secretion from the haemolymph which 

 contains the necessary material for the elaboration of pigment, the 

 white coloration being quite different from the opaque whiteness 

 noticeable in some air-filled scales. This secretion from the haemo- 

 lymph does not appear to enter the scales which will be finally white 

 (due to air contents), but does in many species enter other scales 

 without undergoing any further differentiation. These scales remain 

 white, although under the influence of certain chemicals — ammonia, 

 &c. — the white is readily changed to a cream or yellow tint, e. g., 

 the white of Mchinargia galathea, of Polyommatus corydon, &c. 

 This material has been designated "pigment-factor." by Riding. 

 In those pigmented scales which do not remain white, the secretion 



