326 LEPIDOPTERA CHAP. 



grow with remarkable rapidity, being in the young silk- worm 

 only 3 mm. long, in the adult 22 mm. The increase in weight is 

 still more remarkable ; when the silk-worm is thirty-one days old, 

 the sericteria weigh only 3 mgr., but when the age is fifty days 

 their weight has increased to 541 mgr., being then | of the whole 

 weight of the body. In the pupa they undergo a gradual atrophy, 

 and in the inoth they are, according to Helm, no longer to be 

 found, though earlier authors were of a contrary opinion. 1 Ac- 

 cording to Joseph,' 2 the silk-vessels begin to develop, at an ex- 

 tremely early age of the embryo, and are very different in their 

 nature from the salivary glands, the former being derivatives of 

 the' external integument (ectoderm), while the salivary glands 

 belong to the alimentary system. This view is to some extent con- 

 firmed by the observations of G-ilson as to the different manner 

 in which these two sets of glands discharge their functions. 



The chief feature in the anatomy of the larva is the great 

 size of the stomach. There is a very short oesophagus and crop ; 

 the latter becomes enlarged, spreading out so as to form the 

 stomach, a great sac occupying the larger part of the body-cavity 

 (Fig. 165). On the hinder end of this sac the Malpighian tubes 

 open ; they are similar in their disposition to those of the imago ; 

 behind the stomach the canal expands into two successive, short 

 dilatations, the first called an intestine, the second a rectum ; 

 they are connected by very short isthmuses. The dorsal vessel 

 is a simple, slender tube, extending from the eighth abdominal 

 segment to the head. The main nervous system consists of 

 supra- and infra -oesophageal ganglia, a small frontal ganglion, 

 and a ventral chain of eleven ganglia, three thoracic and eight 

 abdominal, the last of these latter being double. The sexual 

 organs are quite rudimentary, and the passages connected with 

 them very incompletely developed. 



Pupa. The pupa, which is one of the most remarkable of 

 the instars of an Insect's life, attains its highest development in 

 Lepidoptera. The Lepidopterous pupa is frequently called a " chry- 

 salis," a term originally applied to certain metallic butterfly pupae. 

 The Lepidopterous pupa differs from that of other Insects in the 

 fact that its outer skin forms a hard shell, all the appendages of 



1 For information as to the structure and function of the silk-vessels, refer to 

 Helm, Zeitschr. wiss. Zool. xxvi. 1876, p. 434 ; and Gilson, La Cellule, vi. 1890, 

 p. 116. - Jahresber. Schlesisch. Ges. Iviii. 1881, p. 116. 



