58 INSECT TRANSFORMATION 



air, forms silken thread of great value to many caterpillars in 

 their various life-activities, being used for the construction of 

 protective webs on which some larvae live, and more frequently 

 for cocoons or supports during the pupal or resting stage. Small 

 spiny lobes on the front aspect of the tongue in various cater- 

 pillars have been detected 1 and identified with maxillulae. 



Behind the head come the three segments of the thorax, each 

 with a pair of jointed chitinous legs, short indeed when com- 

 pared with those of the butterfly or moth, but with the haunch, 

 thigh, and shin regions fully recognizable, while the unsegr 

 mented foot carries a single claw. The caterpillar's prothorax 

 is often protected dorsally by a strongly chitinized plate, the 

 presence of which distinguishes that segment. A spiracle is con- 

 spicuous on either side of the prothorax, but it is noteworthy that 

 these air-openings are not present on the mesothorax and meta- 

 thorax, the segments which in the adult bear the wings. 



The caterpillar's abdomen is elongate, being made up of 

 ten distinguishable segments, of which the first, second, 

 seventh, eighth and ninth are always limbless, while the third, 

 fourth, fifth, sixth, and tenth usually carry each a pair of 

 short, stout, cylindrical appendages known as prolegs. These 

 are unjointed, covered with thin, flexible cuticle like that of 

 the caterpillar's body generally, and provided at the extremity 

 with rows of short spines arranged either in a circle around 

 the circumference of the prolegs, or in a crescentic ridge along 

 its inner edge. The effect of this distribution of limbs in the 

 caterpillar is to afford support at either end of the body by 

 means of the thoracic legs in front and the tail-prolegs or 

 " claspers " (these on the tenth abdominal segment) behind, 

 and also in the middle region of the body by means of the 

 prolegs on the third to the sixth abdominal segments inclusive. 

 Such a provision for support is clearly advantageous to a worm- 

 like creature that has to crawl along a twig or to balance itself 

 on the edge of a leaf, eating as it goes. In many caterpillars, 

 modification of the typical arrangement of prolegs here 

 described is to be found, notably through a reduction in the 

 number of pairs of these limbs which necessitates a modifica- 

 tion in the method of locomotion. On each of the first eight 



1 J. J. De Gryse: "The Hypopharynx of Lepidopterous Larvae," 

 Proc. Entom. Soc. Washington, XVII. 1915. 



