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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



When the wandering caterpillar finally chooses a place for her 

 cocoon she proceeds first to weave against the bark a thin oval mat 

 of silk, about as long as her own body when fully stretched out. She 

 thus puts down her carpet first and afterwards builds her house 

 over it. 



A caterpillar's silk is formed in a pair of long, tubular glands 

 within the body which open by a single duct through a hollow spine 

 on the tip of the under lip. This spine is the spinneret (figs. 2 

 and 12, Spn). The fresh silk is a gummy liquid which sticks tightly 

 to whatever object it touches and, by contact with the air, hardens 

 almost immediately to a solid substance. To spin this liquid silk 



into a thread the caterpillar touches 

 the point of the spinneret to the de- 

 sired support and draws back its 

 head. The viscid raw silk, adhering 

 to the point touched, pulls out of 

 the spinneret as a delicate filament 

 which at once sets into a tough, flex- 

 ible, inelastic thread. 



Most silk structures made by cat- 

 erpillars are woven from a continu- 

 ous thread laid on in a multitude of 

 irregular figure-8 loops as the 

 worker rythmically swings her head 

 from side to side with a forward 

 and backward twist at each turn. 

 The caterpillar thus weaves toward 

 fig. 12.— Mouth parts of the Buccu- herself, all the while retreating 



latrix caterpillar. A, upper lip or lab- ■ 7 . 



rum (Lm) and edge of head with up- slowly before tne advancing edge of 

 per hinges of mandibles (a, a) ; b, her fabric. In this way Bucculatrix 



jaws or mandibles ; C, part of lower < i • • 



side of head with under lip or labium lays down her carpet, beginning at 



(Lb) and spinneret (Spn), maxillae one en( J an( J working toward the 

 (Mx), antennae (Ant), and eyes (•)..,,,.,,■, . n ,. 



middle, then reversing and starting 

 the other end as far as she can reach with her hind toes at the end 

 where the weaving was begun. In this way the caterpillar fits the 

 carpet to her own length, and all her subsequent work is based on this 

 one measurement. 



After the carpet is finished, the next thing in order is the con- 

 struction of the stockade. This, when completed, ordinarily forms 

 a symmetrical oval (pi. 3, H) ; but some caterpillars are not expert 

 surveyors, so one often sees stockades of very irregular shapes. One 

 caterpillar, whose procedure the writer followed from beginning to 

 end, eventually placed her pickets in a very good oval but had diffi- 

 culty in making a proper start. She first set up six palisades in an 



