INSECTS
a pair of silk-producing glands which open through a hollow 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 particularly 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 11 shows the caterpillar 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 themselves within cocoons formed inside the cells of the comb in which they 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 parchmentlike 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 transformation, and spin cocoons either near the deserted host or on its body.
The maggots, or larvae, of the flies have adopted another method of acquiring protection during the pupal stage. Instead of shedding the loosened cuticula previous 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 another 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.
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