LARVA 



325 



fewer, transparent, somewhat prominent, little spaces placed 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 'SM^gii \w)i ijig/flfa^a^ '" V 



on each side, placed one on the first 



thoracic segment and one on each of ^ '' 



the first eight abdominal segments; ^ ,„„ ^ , . , , 



" o ' Fig. 166.— Front view of head of 



there are no true stigmata on the a caterpillar, witii the jaws 

 second and third thoracic segments. partially opened. «, Labrum ; 



° ' b, mandible ; c, antenna ; d, 



though traces of their rudiments or ocelli ; e, maxilla ; /, lingua ; g, 



vestiges are sometimes visible. spinneret ; a, labiai palp. 



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 ruiddle 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 be 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 maiiy 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 

 become of great length ; in the Silk-worm each of the two 

 vessels is nearly five times as long as the body, while in 

 Bomliyx yamamai and others, even this is exceeded. They 



■• See Plateau, Bull. Ac. Belgique, kv. 1888, p. 28 ; in reference to structure of 

 ocelli, Blanc, TeU du Bomhyx mori . . . 1891, pp. 163, etc. ; and Landois in 

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



^ Enhvickelungsgeschichte der Schmetterlinge^ Cassel, 1815. 



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



