Page:Insects - Their Ways and Means of Living.djvu/302



a pair of silk-producing glands which open through a hol- Iow spine on the Iower lip beneath the mouth (Fig. 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 higlly perfected instinct of the caterpillar is that which impels it t, build the cocoon, often an intricately woven strt,ctt,re, .just before the rime of its transformation to the pt,pa. "Fhe caterpillar spins the cocoon around itself, then sheds its skin, which is thrust into the ear end of the cocoon as a crt, mpled wad. Plate l  shows the cater- pillar of a small moth that infests apple trees constructing its cocoon, finally inclosing itself within the latter, and there transformig to the pt,pa. The larvae of the wasps a,d becs likewise inclose them- selves 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 sort, and the freshly spun strands rt, n 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 v«hen ready for trans- formation, and spin cocoons either near the deserted host or on i ts bodv. The magg«ts, or larvae, of the flies have adopted an- other method of acqt,iring protection during the pupal stage, lnstead of shedding the loosened cuticula previ- ous to the transformation, the maggot transforms within the skin, and the latter then shrinks and hardens until becomes a tough oral capsule inclosing the larva (Fig. 82 E). The capsule is called a p«pari«». It appears, however, that the larva within the puparitm undergoes an- other molt before it actt,ally becomes a pupa, for, when the pupa is formed, it is found to be st,rrounded by a delicate mem[)ranous 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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