70 ENTOMOLOGY 



Collembola, several families of Coleoptera (including Dermestidae, 

 Cerambycidae and Curculionidae) , a few Diptera and a few Psocidae. 



Though diverse in form (Fig. 94), scales are essentially flattened sacs 

 having at one end a short pedicel for attachment to the integument. 

 The scales usually bear markings, which are more or less characteristic 



of the species; these markings, always 

 minute, are in some species so exqui- 

 sitely fine as to test the highest powers 

 F IG . 95. cross-section of scale of of the microscope; the scales of certain 

 Anosia. Alter MAYER. Collembola (Lepidocyrtus, etc.) have long 



been used, under the name of " Podura ' ' scales, to test the resolving power 

 of objectives, for which purpose they are excelled only by some of the 

 diatoms. Butterfly scales are marked with parallel longitudinal 

 ridges (Fig. 94, C), which are confined almost entirely to the upper, or 

 exposed, surface of the scale (Fig. 95) and number from 33 or less 

 (Anosia) to 1,400 (Morpho) to each scale, the striae being in the latter 

 genus from .002 mm. to .0007 mm. 

 apart (Kellogg) ; between these longi- 

 tudinal ridges may be discerned 

 delicate transverse markings. Inter- 

 nally, scales are hollow and often 

 contain pigments derived from the 

 blood. 



On the wing of a butterfly the 

 scales are arranged in regular rows and 

 overlap one another, as in Fig. 96; in 

 the more primitive moths and in Tri- 

 choptera, however, their distribution 

 is rather irregular. 



A scale is the equivalent of a hair, 



for (l) a Complete Series of transitions FlG - 96. Arrangement of scales on the 



wing of a butterfly, Papiho. 



from hairs to scales may be found on 



a single individual (Fig. 97) ; and (2) hairs and scales agree in their man- 

 ner 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. 98). The scale at first contains 

 protoplasm, which gradually withdraws, leaving short chitinous strands 

 to hold the two membranes of the scale together. 



Uses of Scales. Among Thysanura and Collembola, scales occur 

 only on such species as live in comparatively dry situations, from which 



