28 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I43 



out from the spinneret. Others spin a web on the smooth surface of 

 a leaf on which they are feeding to obtain a better foothold. Still 

 others construct a shelter or retreat by drawing leaves together and 

 securing them by strands of silk. Then there are the casemakers and 

 bagworms that enclose themselves in a close-fitting jacket or a bag 

 by weaving together bits of leaf. The webworms and tent caterpillars 

 make those large silken domiciles in trees, in which a whole family 

 hatched from one batch of eggs lives a communal life. As the tent 

 caterpillars go out on the limbs to forage, they leave a trail of silk 

 to guide them back to the nest where they spend the night. When 

 ready to pupate, tent caterpillars simply jump off the tree. Caterpillars 

 of butterflies spin a small web mat against a support from which they 

 can hang awaiting pupation. Some secure themselves with a silken 

 girdle around the thorax which remains as a suspensorium for the 

 pupa. 



The most important use of silk would seem to be that of many 

 moth caterpillars of enclosing themselves in a closely woven cocoon, 

 within which they shed the last larval cuticle and change to the pupa. 

 Cocoon spinning evidently began among the Micropterygidae, since 

 Tillyard (1922) describes the larva of Sabatinca as spinning a cocoon 

 of rather coarse silk having a leathery appearance. However, he 

 says that "no definite spinneret apparatus could be discovered," and 

 he makes no mention of the silk-producing glands. On the other hand, 

 in Micropteryx calthella Hannemann (1956) describes and figures a 

 pair of oval glands in the thorax having a common duct that opens 

 into a salivarial chamber between the hypopharynx and the labium, 

 but these glands he says secrete saliva that is discharged on the food. 

 The leaf-mining larva of the eriocraniid Mnemonica auricyania is 

 described by Busck and Boving (1914) as having a spinneret from 

 which it spins a cocoon in the ground. Of all the insects the caterpillar 

 is the best-known silk-spinner, and the silkworm probably outranked 

 the honey bee as an insect of commercial importance until the produc- 

 tion of competing artificial fabrics. Artificial honey has not yet been 

 invented. 



Though the faculty of producing and spinning silk was evidently 

 not acquired by the ancestral caterpillars for any specific purpose, yet 

 the evolution of their modern spinning apparatus involved a very 

 considerable reconstruction of their mouth parts, and the conversion 

 of the salivary glands into silk-producing organs. 



The ordinary salivary glands of insects lie in the thorax ; their ducts 

 unite in a common outlet duct that opens into the space between the 

 hypopharynx and the labium, known as the salivarium. On the walls 

 of the salivarium near the mouth of the duct are attached opposing 



