LARVA 



325 



Fig. 166.— Front view of head of 

 a caterpillar, with tlie jaws 

 partially opened. «, Labruni ; 

 /), mandible ; r. antenna ; il, 

 ocelli ; e, maxilla ; /, lingua ; ij, 

 spinneret ; h, labial palp. 



fewer, transparent, somewhat prominent, little spaces jjlaced on 



each side of the lower part of the head : they are called " ocelli," 



by Landois '"' ocelli compositi." Under each of these external 



facets there are placed percipient a 



structures, apparently very imperfect 



functionally, the caterpillar's sight 



being of the poorest character.^ The 



spiracles of the caterpillar are nine 



on each side, placed one on the lirst 



thoracic segment and one on each of 



the first eight alxlominal segments ; 



there are no true stigmata on the 



second and third thoracic segments, 



tliough traces of their rudiments or 



vestiges are sometimes visible. 



In the caterpillar there are no traces of the external sexual 

 organs, so that the two sexes cannot be distinguished on super- 

 ficial inspection ; it was however long ago demonstrated by 

 Herold " that the ovaries and testes exist in the youngest cater- 

 pillars, and undergo a certain amount of growth and development 

 in the larval instars ; the most important feature of which is 

 that the testes are originally separate but subsequently coalesce 

 in the middle line of the body, and become enclosed in a common 

 capsule. In a few forms — especially of Liparidae— (Lymantriidae 

 of modern authors) — the caterpillars are said to lie of different 

 colours in the two sexes. Most of what is known on this point 

 has been referred to by Hatchett Jackson.^ 



The SiLK-GLANDS of Lepidoptera are of great interest from the 

 physiological point of view, as well as from the fact that they 

 have furnished for many ages one of the most beautiful of the 

 adornments made use of by our own species. The sericteria, or 

 vessels that secrete silk, are of simple structure, and differ greatly 

 in their size in the various forms of the Order; they sometimes 

 l)ecome of great lengtli ; in the Silk-worm each of the two 

 vessels is iiearly five times as long as the Iwdy, while in 

 Boiiihyx yaiiiamai and others, even this is exceeded. They 



^ See Plateau, Bull. Ac. Bchjique, xv. 1888, p. 28 ; in reference to .structure of 

 ocelli, Blanc, THc clu Bomhyx mori . . . 1891, pp. 163, etc. ; and Landois in 

 Zcitschr. iciss. Zool. xvi. 1866, p. 27 . 



- Eiitwickclutiysijeschichtc der Schmcttcrlin<je, Cassel, 1815. 



^ Tr. Linn. Soc. London, Zool. 2nd Ser., v. 1890, pp. 147, 148. 



