FILIPPI'S GLANDS 



345 



more or less lobed, and the larger the cells the more numerous are the 

 branches of the nucleus. Gilson 1 finds that those of Trichoptera, 

 Lepidoptera, Diptera, and Hymenoptera ordinarily consist of a 

 small number of cells; and it is 

 quite common, he says, to find only 

 two cells in a transverse section 

 (Fig.. 338, A). In the Tenthredi- 

 niike, however, "the organ still con- 

 sists of a tube, the Avail of which is 

 composed of flat cells, but in addi- 

 tion to that, two series of spheroidal 

 cells are attached to the sides. Each 



P FIG. 339. Branching nucleus of spinning 

 OI these Cells COntaillS a System Of pland of Pieris larva. After Korschelt, from 



tiny canals running through their 



cytoplasm (B, i. d). These cells are the secreting elements ; they 

 continually cast the silk substance into the tube." A peculiarity 

 of the tunica intima is its distinct transverse striation. 



The lining of the 

 glands and of their com- 



f \ mon duct is moulted 



when the caterpillar 

 casts its skin, and this, 

 as well as the mode of 

 development, shows that 

 the glands are invagina- 

 tions of the ectoderm. 

 Gilson finds that the 

 silk - glands and silk- 

 apparatus of Trichoptera 

 are very similar to those 

 of caterpillars, and that 

 the silk is formed in the 

 same way. 



Appendages of the silk- 

 gland (Filippi's glands). 

 In most larva? there 

 is either a single or a 

 pair of secondary glands 

 which open into the spinning glands near their anterior end. They 

 are outgrowths of the gland provided with peculiarly modified excre- 



Fn. 340. Flllppi's glands (G} isolated and seen from 

 above : c. <-. its lobules ; </. its excretory canal ; E. silk-duct ; 

 t\ common canal; c, upper spinning muscle; t>. lower inus- 

 i'lc ; a. lateral muscle ; 7 1 , spinneret. After Blanc. 



1 On cytological differences in homologous organs. Report 63d meeting of British 

 Assoc. Adv. Sc. for 18U3. 1894. p. U13. 



