62 



ENTOMOLOGY 



On the wing of a butterfly the scales are arranged in regular rows and 

 overlap one another, as in Fig. 94; in the more primitive moths and in 

 Trichoptera, however, their distribution is rather irregular. 



A scale is the equivalent of a hair, for (i) a complete series of transi- 

 tions from hairs to scales may be 

 found on a single individual (Fig. 95) ; 

 and (2) hairs and scales agree in their 

 manner of development, as shown by 

 Semper, Schaffer, Spuler, Mayer and 

 others. Both hairs and scales arise 

 as processes from enlarged hypo- 

 dermis cells, or formative cells (Fig. 

 96) . The scale at first contains pro- 

 toplasm, which gradually withdraws, 

 leaving short chitinous strands to 

 hold the two membranes of the scale 

 together. 



Uses of Scales. Among Thy- 

 sanura and Collembola, scales occur 

 only on such species as live in com- 

 paratively dry situations, from which 



it may be inferred that the scales serve to retard the evaporation of mois- 

 ture through the delicate integument of these insects. This inference is 

 supported by the fact that none of the scaleless Collembola can live long 

 in a dry atmosphere; they soon shrivel and die even under conditions 

 of dryness which the scaled species are able to withstand. In Lepidop- 



FIG. 94. Arrangement of scales on the 

 wing of a butterfly, Papilio. 



FIG. 95. Hairs and scales of a moth, Samia cecropia. 



tera the scales are possibly of some value as a mechanical protection; 

 they have no influence upon flight, as Mayer has proved, and appear to 

 be useful chiefly as a basis for the development of color and color patterns 

 which are not infrequently adaptive. 



