264 PHYSIOLOGY OF THE INVERTEBRATA. 



protecting themselves, more or less, from the attacks of 

 various enemies.* 



(6) There are two pairs of salivary glands of the larval 

 Lcpidoptcra (see Fig. 7). The posterior or second pair secrete 

 the viscous substance, which hardens on exposure to the 

 atmosphere and forms silk. This silk is the material in 

 which the larvae or caterpillars invest themselves. The vis- 

 cous substance from these glands is made into threads and 

 spun into cocoons by means of a slender tubular organ called 

 a spinneret, which is situated on the labium. 



Most caterpillars spin silken threads to secure themselves 

 from falling, and many of them, as already stated, spin a 

 cocoon in which to pass the pupal state. 



In Myrmecoles and the Hcnierohidcc the silk is furnished 

 by the rectum. 



(c) The glow-worm, or Lampyris sjjlcndidida, and many 

 other insects have the power of emitting light. According 

 to Schulze,t the males of the glow-worm have a pair of 

 photogenic organs, " which lie on the sternal aspects of the 

 penultimate and ante-penultimate abdominal somites. Each 

 is a thin, whitish plate, one face of which is in contact with 

 the transparent chitinous cuticula, while the other is in rela- 

 tion with the abdominal nerve-cord and the viscera. The 

 sternal gives out much more light than the tergal face. The 

 photogenic plate is distinguishable into two layers, one 

 occupying its sternal and the other its tergal half. The 

 former is yellowish and transparent, the latter white and 

 opaque, in consequence of the multitude of strongly refi-act- 

 ing granules which it contains. Trachea and nerves enter 

 the tergal layer, and for the most part traverse it to 

 terminate in the sternal layer, which alone is luminous. 



* For further information on the defensive fluids and the eversible 

 glands of Lepidopterous larvae, see the papers by Mr. E. B. Poulton, F.R.S., 

 in the Transactions of Entomological iSociety of London, 1885, p. 322 ; ibid., 

 1886, p. 156 ; ibid., 1887, p. 295 ; Report of British Association, 1887, p. 765 ; 

 and his excellent book, The Colours of Animals. 



t Archie filr Mikroakojiischc Anatomic, 1855. 



