60 INSECT TRANSFORMATION 



Fig. 30 st) is long and voluminous, often with sacculated walls, 

 providing an extensive area where the digestion of the abundant 

 vegetable food-stuff which the caterpillar swallows, and the 

 subsequent absorption of the extracted food-products can be 

 adequately carried on. The paired silk-glands, to which 

 reference has already been made, are long tubular organs 

 (Fig. 32 A) apparently modified salivary or spittle-glands 

 which lie in the great blood-space on either side of the food- 

 canal, folded or twisted in their course as their length, if 

 extended, would exceed that of the body. The ducts of these 

 silk-glands unite to form the median duct or tube which opens, 

 as already mentioned, at the extremity of the spinneret (Fig. 

 32 A) within the mouth. The cells (Fig. 32 C) which make 

 up the walls of the silk-glands, whose function is to secrete 

 the sticky fluid, obtaining the necessary material from the 

 surrounding blood, are remarkable on account of their large, 

 complex nuclei, each of which projects by a series of lobate 

 processes into the surrounding protoplasm of the cell. The 

 terminal region of the silk-duct, immediately before its opening 

 through the spinneret is surrounded by a thick elastic chitinous 

 wall forming the " press " by means of which the secretion of 

 the glands is forced out in the form of a prismatic or flattened 

 ribbon which solidifies to form the insect's silken thread, 

 the pressure on the emerging silk being regulated by the action 

 of suitably arranged muscles attached to the " press " (Fig. 

 32 Apr). 



In the caterpillar's nervous system, it is of importance to 

 notice the three distinct and separated thoracic ganglia (Fig. 

 30 A, i, ii, iii) and the chain of six or seven abdominal ganglia 

 (1-6). It is evident that the concentration of the nervous 

 system, so marked a feature in the structure of the butterfly, 

 is not effected in the larva. The reproductive organs ovaries 

 in the female or testes in the male have had their origin during 

 embryonic growth, and are recognizable beneath the cater- 

 pillar's dorsal abdominal wall (Fig. 30 A, T). 



In the life-period of the caterpillar, whose structure has 

 now been briefly reviewed, there is a feature that, though very 

 familiar, calls for especial emphasis. The description that 

 has just been given would fit most caterpillars at any period 

 of their lives. During the first stage, when the little larva 



