ANATOMY AND PHYSIOLOGY 



6l 



pollen grains; in short, these simple structures exhibit a surprising va- 

 riety of adaptive modifications, many of which will be described in con- 

 nection with other subjects. 



A hair arises from a modified hypodermis cell (Fig. 90), the contents of 

 which extend through a pore canal into the 

 interior of the hair (Fig. 91); sometimes, to 

 be sure, as in glandular or sensory hairs, the 

 hair cell is multinucleate, representing, there- 

 fore, as many cells as there are nuclei. The 

 wall of a hair is continuous with the general 

 cuticula and at moulting each hair is stripped 

 off with the rest of the cuticula, leaving in 

 its place a new hair, which has been forming 

 inside the old one. 



Scales. Besides occurring throughout the 

 order Lepidoptera and in numerous Trichop- 

 tera, scales are found in many Thysanura and 

 Collembola, several families of Coleoptera 

 (including Dermestidae and Curculionidae), a 

 few Diptera and a few Psocida?. 



Though diverse in form (Fig. 92), scales 

 are essentially flattened sacs having at one 

 end a short pedicel for attachment to the 

 integument. The scales usually bear mark- 

 ings, which are more or less characteristic of 

 the species; these markings, always minute, 



are in some species so exquisitely fine as to test the highest powers of 

 the microscope; the scales of certain 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. 92, C), which are confined 

 almost entirely to the upper, or ex- 

 posed, surface of the scale (Fig. 93) 

 and number from 33 or less (Anosia) 

 to 1,400 (Morpho) to each scale, the stride being from .002 mm. to .0007 

 mm. apart (Kellogg) ; between these longitudinal ridges may be dis- 

 cerned delicate transverse markings. Internally, scales are hollow and 

 often contain pigments derived from the blood. 



FIG. 92. Various forms of 

 scales. A, R, thysanuran, 

 Machilis; B, beetle. Anthrenus, 

 C, butterfly, Picris; D, moth, 

 Li in a codes. 



FIG. 93. Cross-section of scale of 

 .1 Ho.v/ii. After MAYER. 



