Jan.— Feb. iSSj.^ 



PSYCHE. 



ties or interstices of the pith-like por- 

 tion open into the central canal of 

 the hair, hut are prevented from direct 

 communication with the outer air by 

 the thin sheath of chitin which forms 

 the outer covering of each hair. Each 

 hair is closed at the apex (fig. i, c) 

 by this outer, impervious chitin layer 

 and at its basal end (fig. i, e) by a thick- 

 ening of the chitin sheath to such an 

 extent as to entirelv close the central 

 canal, with its surrounding pith-like 

 substance, from the outer air. This 

 structui'e, together with the mode of 

 closing of the basal end, leaves the 

 nature of the development of the hair 

 verv apparent. It is a closed sacciform 

 appendage of the external chitinous cov- 

 ering of the insect, as are the scales of 

 lepidoptera, coleoptera and diptera. 

 The knob formed bv the basal end (fig. 

 r, (') of the hair is inserted into a pit 

 in the chitin covering of the insect, 

 almost exactly as the corresponding 

 part of the scales of other insects are 

 attached. 



The structural points above described 

 were obtained in two ways ; first, by 

 sectioning the hairs \\ ith the microtome, 

 and, second, by carefull}- observing 

 imder the microscope the action of dif- 

 ferent reagents on tiie hairs. I obtained 

 a few good transverse sections of hairs 

 taken from the sides of the thorax, and. 

 by examination of these sections, veri- 

 fied the existence of an open central 

 canal. Fig. i, (/, shows one of these 

 sections vviiich was about o.oi mm. in 

 thickness anil about 0.02 mm. in aver- 

 age diameter, drawn with the camera. 



The expulsion of the air from these 

 hairs, when they were broken at any 

 point, was readily efiected h\ chloro- 

 form or alcohol, and scarcely less readily 

 by glycerin, bv turpentin. or even bv 

 water, but if the liair had not been in- 

 jured at any point the air was not driven 

 out by any of these reagents, even after 

 several days action. When the air has 

 been expelled from a hair, the latter be- 

 comes transparent, and is colorless with 

 directly transmitted light in all parts 

 except where the chitin is thickened to 

 close the base ; this chitin is slightlv 

 brown. By obliquely transmitted light, 

 obtained by Aube's illuminating appar- 

 atus, the hairs which have been deprived 

 of air exhifiit a slightly bluish shade. 

 Fig. I, h, represents a hair of which the 

 distal end has l)een deprived of air, the 

 part from .v to v is partly deprived of 

 air and the basal portion is still filled 

 with air. Sometimes, especially when 

 glycerin or turpentin is used to expel the 

 air, a part of the air will remain col- 

 lected in the central canal ( fig. 1 , .v to r) 

 quite a while after it has left the cavities 

 of the pith-like portion. The clearly 

 defined outline of this colimin of air led 

 to the suspicion of an open canal, the 

 presence of which was later proved by 

 sections. 



The hairs from difterent parts of C. 

 dorsalis vary little in form and size. 

 Those from the thorax and from the un- 

 der side of the abdomen are club-shaped 

 (fig. I, a and /;), are from 0.28 to 0.35 

 mm. in length, and from 0.015 to 0.025 

 mm. in diameter. They taper gradually 

 and slightlv from the middle toward 



