INSECTS 



a pair of silk-producing glands which open through a hol- 

 low spine on the lower lip beneath the mouth (Fig. 155). 

 The silk is used by the caterpillars during the feeding 

 part of their lives in various ways, but it serves particu- 

 larly for the construction of the cocoon. The most 

 highly perfected instinct of the caterpillar is that which 

 impels it to build the cocoon, often an intricately woven 

 structure, just before the time of its transformation to 

 the pupa. The caterpillar spins the cocoon around itself, 

 then sheds its skin, which is thrust into the rear end of 

 the cocoon as a crumpled wad. Plate 1 1 shows the cater- 

 pillar of a small moth that infests apple trees constructing 

 its cocoon, finally inclosing itself within the latter, and 

 there transforming to the pupa. 



The larvae of the wasps and bees likewise inclose them- 

 selves within cocoons formed inside the cells of the comb 

 in which thev have been reared. The cocoon is made of 

 threads, but the material is soft, and the freshly spun 

 strands run together into a sheet that dries as a parch- 

 mentlike lining of the cell. The larvae of many of the 

 wasplike parasitic insects that feed within the bodies of 

 other living insects leave their hosts when ready for trans- 

 formation, and spin cocoons either near the deserted host 

 or on its bod v. 



The maggots, or larvae, of the flies have adopted an- 

 other method of acquiring protection during the pupal 

 stage. Instead of shedding the loosened cuticula previ- 

 ous to the transformation, the maggot transforms within 

 the skin, and the latter then shrinks and hardens until it 

 becomes a tough oval capsule inclosing the larva (Fig. 

 182 E). The capsule is called a puparium. It appears, 

 however, that the larva within the puparium undergoes an- 

 other molt before it actually becomes a pupa, for, when the 

 pupa is formed, it is found to be surrounded by a delicate 

 membranous sheath inside the hard wall of the puparium, 

 and when the adult fly issues it leaves this sheath and a 

 thin pupal skin behind in the puparial shell. 



[252] 



